oratory results as they apply to aviation medi-
cine.

Tests of Altitude Tolerance. More useful than
Hb analysis would be tests to evaluate the alti-
tude susceptibility of individuals whose red cells
contain HbS. As the precautions regarding ex-
posure to high altitude would apply to such
clinical tests as well as to physiological training,
such tests must be restricted to in vitro and other
measurements that can be made on blood samples.
A variety of in vitro tests, including measure-
ments of oxygen dissociation properties,® viscos-
ity changes,” and ionic balance”! under varying
oxygen concentrations, could be appropriate.

Finch and various coworkers’**> have devel-
oped an animal model that may be applicable.
Ordinarily, human red cells are rapidly destroyed
after injection into the circulation of rats, the
half-life (T14) for survival being about 15 min;
but these authors have reported increases up to
30 h in the T14 of normal human cells in rats
pretreated with ethyl palmitate, which blocks
the reticuloendothelial system, and a fraction of
cobra venom, which destroys complement C3, a
component necessary for hemolysis of the foreign
cells. Thus, it is possible to study human red
cells in vivo without risk to the subject. When.
rats treated as above are injected with %Cr-
labeled red cells from patients with SCD and
subjected to intermittent hypoxia, some of the
injected cells disappear from the rat’s blood
during periods of hypoxia, only to reappear
when normal oxygenation is resumed. Even at
normal QO, concentrations, the survival times of
SCD erythrocytes are significantly shorter than
those of normal red cells. Cyanate treatment
improves survival of the SCD cells when the rats
are kept in room air, but no advantage is con-
ferred under hypoxic conditions. Orlin, Castro,
and Finch,” using a similar model, studied the
survival of cells from five donors with sickle
trait; the survival times of these cells were un-
affected by hypoxia (10 percent O,) but were
shorter under ordinary conditions than those of
cells containing no HbS (T14: 19.6 h vs. 34.8 h).
The lack of effect on type AS cells by this de-
gree of hypoxia (equivalent to about 18,000 ft)
supports our conclusion that moderate altitudes
do not produce sickling in persons with sickle
trait.

12

Repetitions of this experiment using a large
number of donors of the AS genotype might
reveal those very few individuals susceptible to
hypoxia. The shorter-than-normal T14 of type
AS cells (not attributed by the authors’ to
hypoxia) is also interesting. Perhaps this find-
ing, and the brief survival of red cells in patients
with SCD, are related. The extension, by cya-
nate, of the survival of type SS cells at normal
O; levels but not in hypoxic conditions sup-
ports the speculation that a factor other than
HbS may influence survival. Murayama” has
proposed a second blood factor, in addition to
HDS, that influences the sickling process, but the
identity of this second factor is still unknown.

The chief obstacle to reaching any definite
conclusions about the hazards of aviation to
those with sickle trait is the lack of a clear and
distinct difference between the heterozygous and
homozygous conditions. Erythrocytes of persons
with sickle trait can become sickled in vitro if
hypoxia is severe enough, and sickled cells have
been found in the bodies of persons with sickle
trait after sudden collapse and death. In many
other inherited diseases, however, the hetero-
zygous condition is free of any signs of disease;
for example, heterozygous carriers of Christmas
disease (hemophilia B) do not suffer from even
mild hemorrhagic disorders, nor have any in-
stances of SUD associated with bleeding been
reported in such individuals.

In addition to a slight tendency toward sick-
ling, persons with sickle trait usually have hypo-
sthenuria, which also occurs in persons with
SCD. Thus, while many abnormal genes do not
express themselves at all in the heterozygous
condition, sickle trait seems to reveal some of the
effects of the Hbg* gene, possibly reflecting an
incomplete, or mixed, dominance in the AS geno-
type. In addition to the possibility of incom-
plete dominance, there may be other factors
affecting susceptibility to sickling. These are
best demonstrated by cases of extraordinarily
mild SCD.

No instances of air crashes attributable to
sickle crisis have been found in the literature,
although with a future increase in the number
of pilots of the AS genotype, such reports may
appear. If they do, the warnings of Rosenheim
and of McCormick should be recalled.?* There
is little oxygen in the blood of a cadaver. With-


associated with aggregation of « chains in the
erythrocyte.

Another interesting condition is the hereditary
persistence of fetal hemoglobin (HPFH), which
has some influence on the severity of sickle-cell
disease and will be considered in detail in an-
other section of this review.

The conditions discussed to this point repre-
sent the “quantitative” hemoglobinopathies, those
produced by the absence or persistence of entire
polypeptide chains. Another group, called
“qualitative” hemoglobinopathies, is associated
with deletions or substitutions of amino acids in
the @ or @ chain. Stamatoyannopoulos? lists 127
of these inherited conditions; some are clinically
silent, detectable only by electrophoretic or pep-
tide “finger printing” techniques, while others
produce a variety of clinical conditions includ-
ing methemoglobinemia and polycythemia. Of
these 127 qualitative mutations, 42 were asso-
ciated with a chain substitutions and 85 with 8
substitutions or deletions. Hemoglobin S (HbS)
is included in this latter group along with other
hemoglobins that may be of aeromedical signifi-
cance. For example, Hemoglobins Olympia*
and Kansas® are associated with abnormal shifts
in the O,-Hb dissociation curve; this is illustrated
in Figure 2.

The oxygen affinity of HbS is also abnormal®
and is an important factor in the sickle crisis, as
we will see.

Sickle Hemoglobin. Sickle hemoglobin and
others with altered @ chain structure are closely
related, not only because of structure and in-
heritance, but also because the severity of and
susceptibility to sickle crises in heterozygous
individuals are strongly influenced by the type
of hemoglobin that is associated with HbS. To
fully appreciate this influence, we should first
consider the homozygous condition.

Sickle-cell disease (SCD), or sickle-cell anemia,
is a condition associated with deformation
(sickling) of erythrocytes and consequent vascu-
lar insufficiency and pain, especially in the bones
and joints. According to Konotey-Ahulu,’ the
condition was recognized in West Africa in an-
cient times as a disease that “runs through
families.” Sickle-cell disease is a chronic disease
characterized by episodes of painful attacks.
Typical signs and symptoms during a crisis are
swelling of the liver and spleen (in children),

ee Pave "7
3
_q 60 9
Sat af 4
g
xe ne ; s
ea a ae eae aS OOo

pO2 (mm Ha)

Fiourr 2.—A diagrammatic comparison of the oxygen-
hemoglobin dissociation curve (ODC) of normal HbA
with Hb Olympia and Hb Kansas. Shaded areas
denote O; delivered to the tissues based on mixed
venous oxygen tension (P,O:) and arterial pO: differ-
ence, Note that Olympia, with a high Os affinity, is
unable to deliver sufficient 0; at the existing A-V
difference; compensatory erythrocytosis corrects the
deficit. Nansas has a lower-than-normal affinity but
delivers enough O; because of the Hb reserve. Patients
with Hb Kansas exhibit an asymptomatic cyanosis.

hematuria, painful and swollen extremities,
epistaxis, and sudden onset of a gnawing pain
in the bones and joints. These attacks are not
without permanent effects, which include chronic
leg ulcers, maxillary protrusion (from marrow
hyperplasia), anemia, and disturbances of
growth.

Sickle-cell disease is inherited; at least one
Hbg* gene must be present, but an individual
must also inherit an additional abnormal Hb
gene before a disease becomes manifest.’ Pheno-
types that qualify in this respect include the
hemoglobin mixtures SC, SE, S Thal," and, of
course, SS. The term “sickle-cell trait” will be
used in this paper to denote combinations of
HbS with one of the normal hemoglobins A or
A, (AS). This usage is consistent with inter-
national convention.’

At least one report has used the symbols “AS” and
“SA” to denote two different phenotypes: those of sickle
trait and the sickle-thalassemia heterozygote. Such
practice is unnecessarily confusing. I will use “AS” to
denote the trait phenotype and “S Thal” to denote the
result of a heterozygous mixture of Hbg'® with any
thalassemi¢ gene.



48,

49,

51.

56.

58.

59.

60,

6!

62.

2, Alex

S

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Buckalew, V. M,, and A. Someren; Renal Manifesta-
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Valeri, C. R.: Drugs, Hormones, and the Red Cell.
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. Boning, D., U. Meir, U. Schweigart, and M. Kunze:

Diurnal Changes of the 2,8-diphosphoglycerate Con-
centration in Human Red Cells and the Influence
of Posture, EUROP. J. APPL. PHYSIOL, 34:11-17,
1975.

. Allen, J. E., and H. Rasmussen: Human Red Blood

Cells: Prostaglandin E:,
terenol Alter Deformability,
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Johnson, M., I. Rabinowitz, A. L. Willis, and P. L.
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Epinephrine, and Isopro-
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» Roth, S., and P. Seeman: All Lipid-Soluble Anaes-

thetics Protect Red Cells, NATURE, NEW BIOL,
231 :284-285, 1971.

Rickles, F. R., and D. O'Leary: Role of Coagulation
system in Pathophysiology of Sickle Cell Disease,
ARCH. INTERN. MED., 138 :635-641, 1974.

Pirkle, H., and P. Carstens: Pulmonary Platelet
Aggregates Associated With Sudden Death in Man,
SCIENCE, 185 :1062-1064, 1974.

Vincent, J. E.: Fat Mobilization and Platelet Ag-
gregation, LANCET, 1:799-800, 1975.

Silver, M. J., W. Hoch, J. J. Kocsis, C. M. Ingerman,
and J. B. Smith: Arachidonic Acid Causes Sudden
Death in Rabbits, SCIENCE, 183 :1085-1087, 1974.
Spector, A. A., and J. C. Hoak: Fatty Acids, Plate-
lets, and Microcireulatory Obstruction, SCIENCE,
190 :490-492, 1975.

16

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67.

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. Donaldson, V. H.: Serum Inhibition of ©’l Esterase

in Health and Disease, J. LAB. CLIN, MED., 68 :369-
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Inhibition of Activated Plasma Thromboplastin
Antecedent (Factor XIa) in Pregnancy, J. LAB.
CLIN. MED., 79 :106-112, 1972.

. Ratnoff, O. D.: Some Recent Advances in the Study

of Hemostasis, CIRC. RES., 35:1-14, 1974.

Anderson, I. S., K. Yeung, D. Hillman, and L. S.
Lessin: Multiple Myeloma in a Patient With Sickle
Cell Anemia. Interacting Effects on Blood Viscosity,
AM. J. MED., 59 :568-574, 1975.

McKenzie, J. M., D, R. Celander, and M. M. Guest:
Fibrinogen Titer as an Indicator of Physiologie Sta-
bility, AM. J. PHYSIOL, 204:42-44, 1963.

McKenzie, J. M., and P. R. Fowler: Supporting Role
of the Adrenal Cortex in the Induction of Hyper-
fibrinogenemia, AM. J. PHYSIOL,, 214 :786-790, 1968.

. Kiesow, L, A., J. W. Bless, D, P. Nelson, and J. B.

Shelton: A New Method for the Rapid Determina-
tion of Oxygen-Dissociation Curves in Small Blood
Samples by Spectrophotometric Titration, CLIN.
CHIM. ACTA, 41 :128-189, 1972.

. Radice, P., L. 8. Lessin, M. Brower, and D, Hillman:

Filtration Hemolysis of Sickle (SS) Cells—a Stereo-
scan Electronic Microscope (SEM) Correlation
Study, CLIN. RES., 21:969, 1973.

. Palek, J.: Effect of Deoxygenation on Caleium Ac-

cumulation in Normal and Hemoglobin S (HbSS)
Red Cells, CLIN. RES., 21:969, 1973.

. Wright, M. C., R. A. Nelson, and S. ©. Finch: The

Effect of Cobra Venom Factor and Ethyl Palmitate
on the Prolongation of Survival of Heterologous
Erythrocytes, YALE J. BIOL. MED., 43:178-176,
1970,

. Orlin, J., O. Castro, and S, ©. Finch: Survival of

Human Sickle Trait Erythrocytes in a Heterologous
Species, CLIN. RES., 21 :969, 1973.

. Castro, O., J. Orlin, and S. C, Finch: The Effect of

Carbamylation on the Survival of Human Sickle
Cell Erythrocytes in Rats, YALE J. BIOL. MED.,
1:55-60, 1974.

. Castro, O., S. C. Finch, and G. Osbaldiston: Sickle

Cell Resistance to in

251 :620-621, 1974.

vivo Hypoxia, NATURE,


suggest that the “selective enrichment of F cells
could ameliorate sickle disease,’ which seems
reasonable, provided the number of SS cells con-
taining no HbF could be reduced significantly.
Unless this reduction could be accomplished,
persons with SCD might still suffer infarctions
caused by sickling. To the comments of these
authors it should be added that patients with
other hemoglobinopathies, notably Cooley’s ane-
mia, might also benefit from techniques that
could stimulate the production of HbF.

One other compound should be mentioned:
methemoglobin (MHb), which is significantly
increased in cases of glucose-6-phosphate de-
hydrogenase deficiency and other conditions, is
associated with a milder course of sickling dis-
ease when this enzyme deficiency occurs in per-
sons with SCD. Beutler and Mikus* induced
the formation of MHb in patients with SCD by
administering sodium nitrite or p-amino pro-
priophenone. In the blood of patients whose
MHb concentrations were at least 20 percent of
total Hb, the number of sickled cells occurring
at reduced O, concentrations was less than in
control blood samples, but the O,-Hb dissocia-
tion curve (ODC) was not materially affected.
Sodium nitrite treatments prolonged the survival
of erythrocytes in vivo, but p-amino proprio-
phenone seemed to decrease survival. Unfortu-
nately, sodium nitrite produces side effects (e.g.,
headaches) that limit its usefulness. The devel-
opment of safer techniques for inducing MHb
production in man may be a subject worthy of
further research.

The Ouygen-Hemoglobin Dissociation Curve.
In his review of the problems of O, transport.
in SCD, Milner® refers to the shift-to-the-right
of the ODC in sickle-cell anemia. He cites the
in vivo ODC measurements of Bromberg and
Jensen,‘t who confirmed earlier in vitro studies
but in some cases found much lower arterial
saturations consistent. with not only a right shift,
but a slow-rising of the curve in these patients.
Milner also refers to the findings of Sproule,
Halden and Miller,*? who found indications of
arteriovenous (AV) pulmonary shunts in some
patients with SCD. Bromberg and Jensen did
not examine their patients for AV shunts, which,
if present, could result in a slow-rising of the
in vivo ODC. Milner concludes that high intra-
cellular levels of organic phosphates—such as 2,3-
diphosphoglycerate—are partly responsible for

the decreased oxygen affinity of the red cells in
SCD (young cells, prevalent in anemia, contain
large amounts of these compounds) ; but the prin-
cipal cause of the shift-to-the-right, Milner* be-
lieves, is the presence of large numbers of sickle-
damaged cells that possess high concentrations of
hemoglobin. This idea is at least consistent with
in vitro findings, such as those of Rossi-Bernardi
et al.,? who demonstrated that HbS obtained
from lysed cells may yield an almost normal
ODC, far to the left of that obtained with intact
cells. As Milner® points out, a treatment that
could produce a leftward shift in the ODC
would benefit the patient with SCD, principally
because at higher levels of oxygen saturation
the cells are less likely to sickle. Compounds
such as urea*t and cyanate** produce leftward
shifts in the ODC, and much benefit may be ob-
tained from further study of similar compounds,
hopefully derivable from dietary sources. The
B-cyanogenic glucosides (nitrilosides, CG),
found in the diets of certain native Africans,
have been associated with milder forms of SCD
in those who consume these diets in preference
to—or in the absence of—more “civilized” foods.**
The BCG compounds on ingestion yield large
amounts of thiocyanates as metabolic products,
and some successes in the clinical use of thio-
cyanates in SCD have been claimed. Thus, the
BCG compounds may deserve more intensive
study and clinical trial.7** Except for laetrile
(amygdalin), they have not received much at-
tention.

Renal Factors. Hyposthenuria—an inability
to fully concentrate urine—is found in at least
80 percent of those with sickle trait and in prob-
ably all of those with SCD. This condition is
possibly the only clinical sign common to both
genotypes; anatomical changes that may explain
it have been seen. Statius Van Eps and his
associates,*? using a microradiographie technique,
demonstrated gross lesions in the renomedullary
vessels of people with SCD. In homozygous
individuals the vasa recta (efferent arterioles of
the juxtaglomerular nephrons), which are re-
sponsible for maximum osmolar concentration,
were almost completely absent. In the kidneys
of four sickle-trait carriers, aged 28, 37, 49, and
82, the vasa recta were reduced in number and
bundle architecture was abnormal.

Findings of milder, but similar, pathological
changes in people with sickle trait may ‘be sig-


Chicago studied a population of 3,818 Black
male patients, 306 (8 percent) of whom were of
the AS genotype. Frequency of sickle trait was
essentially the same in all age groups between
ages 30-74, an indication that death rates are no
higher in individuals with sickle trait than in
those of the AA genotype. Petrakis e¢ a/.** also
found no decrease in the frequency of sickle
trait in an aging population. In response to a
request by the U.S. Department of Defense, the
National Academy of Sciences and National
Research Council (NAS/NRC) organized the
ad hoc Committee on S-hemoglobinopathies to
evaluate the importance of sickle trait and other
heterozygous states in military activities includ-
ing aviation. Its report®® refers to minor renal
abnormalities and “isolated” reports of cardio-
myopathies associated with alcoholism, as well
as some untoward effects of anesthesia,?? and
concludes that although there is some suggestion
“that sickle-cell trait is attended by minor mor-
bid consequences and some risks to health, they
are of low frequency, and sickle-cell trait is
usually benign.”

In contrast to the findings of the NAS/NRC
ad hoc committee, there have been a number of
reports of sickling phenomena in people with
sickle trait and other reports that sickling can
be a cause of death in type AS individuals.
Some of these reports can be criticized for lack
of thorough qualitative and quantitative meas-
urements of the Hb types involved. In some
cases of death a finding of sickled cells in the
cadaver was interpreted as proof of ante mortem
sickling. This interpretation was criticized by
Rosenheim,2* who reminds us “. . . it should be
realized that 8 percent. of all Negroes who die
suddenly of any cause will have AS hemoglobin
and, as McCormick has shown, all with AS
hemoglobin will have sickled cells demonstrable
at autopsy.” In most necropsies, oxygen tension
in the cadaver is well below the levels that pro-
duce sickling.

Sudden Unexpected Death. Incidents of sud-
den unexpected death (SUD) in persons with
sickle trait have been the subject of a number of
reports. (See Appendix for five cases not.
previously reported.) Some of these deaths were
associated with other suspected contributing
factors, such as acute and chronic alcoholism,

©Am, J, Med. Sei., 241 :829-835, 1961.

trauma to the head or chest, and undiagnosed
pathological conditions (e.g., pneumonia) not
related to hemoglobinopathies.| Hypotheses to
explain these deaths come readily to mind: any
condition that would tend to reduce pO:, gen-
erally or in certain tissues, might initiate the
cycle of hypoxia, sickling, greater hypoxia, ete.
There have been only a few reports of SUD in
individuals with sickle trait in which no pre-
disposing conditions could be detected. Jones
et al.* saw four cases in a population of 4,000
young Negroes during army basic training.

Many of the cases described in the literature
have exhibited certain common characteristics:
(i) The individual collapses during heavy exer-
cise or an emotional crisis. (ii) He dies soon
after collapse, or he appears to recover and then
suffers a relapse and dies within 24h. (iii) One
who dies after a number of hours may exhibit
before death signs of diffuse intravascular throm-
bosis, such as a gradual decrease in fibrinogen
concentration and an increase in prothrombin
time.* (iv) Marked vascular congestion and
erythrocytic sickling are seen at necropsy. Few
will exhibit signs of earlier crises; old infarctions
or hemosiderin deposits may be absent.

What is the cause of death in these cases?
Given the low incidence of SUD in persons
without such known predisposing conditions as
trauma, alcohol, and pneumonia, one might as-
sume that some other, waknown contributing
factors were present. Certainly, exercise and
emotion, though they seem to be initial causes
in many cases of SUD, cannot be threats to all
those with the trait. Approximately 8 percent
of the Black athletes in the U.S. National Foot-
ball League possess sickle trait.’ No incidences
of SUD are known for these athletes, although
it can be safely assumed that they experience
greater-than-ordinary physical and emotional
challenges. Green, Huntsman, and Serjeant*
quoted A. E. Romero-Herrera, who stated that

4In a letter to the author (April 8, 1975) Dr. David
Wiecking, Chief Medical Examiner for the Common-
wealth of Virginia, writes: “We have occasionally . . .
found pergons dead with considerable sickling of their
erythrocytes for reasons that are not entirely clear, It
would appear that some hypoxic episode has rendered
them susceptible to sickling. Some of these antecedent
events we believe to be undiagnosed pneumonia, severe
alcohol and/or drug intoxication, trauma to the chest,
and trauma to the head.”



In victims of SCD the erythrocytes become
deformed and stiffened when the blood pO, falls
below about 60 mm Hg. Whereas normal ery-
throcytes are highly elastic and capable of pass-
ing through capillaries as small as 3 pm,® the
sickled cell is so rigid that it is incapable of
passing through holes of much larger diameter.
Thus, sickle cells, on becoming deformed, cause
tremendous increments in resistance to blood
flow.2 As blood flow is further reduced, the
oxygen tension is lowered further, and more
cells become sickled. These events generate a
vicious cycle leading to vascular plugging and
tissue infarction.

The threshold value of pO, at which cells be-
gin to sickle is a function of Hb composition.
Figure 3, adapted from a report by Griggs and
Harris,” illustrates the differences in suscepti-
bility to hypoxia between red cells containing
mixtures of various hemoglobins. Note that
HbA confers a marked resistance to hypoxia in
cells also containing HbS.

The mechanism by which HbS-containing cells
lose their elasticity at low pO, is not completely
understood. Murayama‘ has examined the ultra-
microscopic structure of sickled cells and deoxy
HbS. His studies reveal the existence in sickled
cells of microtubules formed from polymers of
HbS. These microtubules are associated in

100 ss

90 |
80 $-That
70

60 |
50

40

% Og SATURATION

30

20

10

° 1 1 a
0 10 20 30 40 50 60 70 80 90 100

pO2 (mm Hg)

Froure 8.—A graph of the normal human ODC indi-
eating the pO. values at which the red cells of various
heterozygotes begin to sickle and to confer increased
viscosity. Depending on the amounts of HbS, 2, 3-
GPD, ete. the ODC of these cells will he shifted more
or less to the right of the ODC plotted here (after
Griggs and Harris).

bundles of parallel rods of fibers about 90 nm
in diameter; these rods are thought to be re-
sponsible for the stiffness of sickled cells.”

Distribution of the Gene for HbS. Although
it is commonly thought that the sickle gene oc-
curs only in Negroes, it has also been reported
in other populations. For example, Choremis
et al2* found 12 such individuals in the Greek
town of Petromagoula (Orchomenos) near Lake
Copais; eight of these gave a positive history of
SCD. McGrew* recently reported an incidence
of 0.046-percent sickle trait in a-population of
57,665 non-Black U.S. Navy recruits. The “Afri-
can Marker,” blood group phenotype ¢De, was
not found.

It is now generally believed that the presence
of HbS in red cells confers some resistance to
malaria, especially that caused by Plasmodium
falciparum. * This resistance accounts for the
occurrence of a high Hbg* gene frequency
mostly due to a high incidence of the AS geno-
type in malarious regions. Homozygous SS
individuals were fairly short lived, especially in
earlier times, and individuals with the AA geno-
type, being more susceptible to malaria, also died
sooner than the heterozygotes. Consequently,
the frequency of sickle trait was in excess of 20
percent throughout the low-lying “belt” of
Africa before mosquito eradication programs
were initiated.

Other reports dealing with the origins and
distribution of malaria and sickle disease may
be of interest.1°1*

III. Alleged Hazards of Sickle Trait.

The high frequency of the Hbg* gene in
Africa is expected to decrease over the next
several generations as a result of mosquito eradi-
cation and improvements in housing. Ruck-
nagel,’? assuming equal fitness of individuals of
genotype AS and AA, estimates that the present
gene frequency of about 9 percent in American
Negroes will decrease to about 6.3 percent in 10
generations.

Is Rucknagel’s assumption of equal fitness of
types AS and AA a fair one? Heller” in

’Other models have been proposed by Magdoff-
Fairchild et al., Nature 239:217-219 (1972) and by
Finch et al. Proc, Natl. Acad. Sei. (US) 70:718-722
(1978).




The authors were fully aware that sequestering
of deformed cells by the spleen could have con-
cealed the sickling phenomenon and for this
reason measured urobilinogen for as long as 48 h
following exposure; no increased Hb breakdown
(as would have been indicated by increments in
urobilinogen output) was detected. The subject
with active anemia, on the other hand, exhibited
increased sickling at altitude. His blood, taken
at ground level, contained 8 percent sickled cells;
this ratio was increased to 9.8 percent at 10,000
ft and 12 percent at 16,000 ft but decreased to
7.2 percent when he began using oxygen.

IV. Contributory and Extenuating Factors
in Sickle Crises.

Conflicting reports on the susceptibility of
persons with sickle trait to hypoxia, severe exer-
cise, and severe emotional stress were noted
earlier in this paper. Although some authors
have reported sickle crises in response to these
stressors, there are apparently many individuals
with sickle trait who can tolerate mild altitudes,
compete in demanding athletic contests—even at
moderate altitude—and survive the emotional
and other challenges of contemporary life." Even
in those with SCD, some variations in the course
and severity of the disease have been noted.
Although most homozygous (SS) individuals
are not likely to survive their fifth decade, some
may experience exceptionally mild forms of the
disease. Perrine * reported a benign form of
SCD in natives of Saudi Arabia; Serjeant**
found mild forms of the disease in Jamaican
residents. It may be useful to consider some of
the factors that are known to modify the re-
sponse of HbS-containing cells to hypoxia and
to speculate on other factors that might reason-
ably be suggested as the subjects of further
investigations.

Other Hemoglobins. Singer and Singer were
probably the first to examine the effects of other
types of Hb on the gelling of HbS in the absence
of oxygen. They found that mixtures of HbS
and HbC, under a continuous flow of CO.,, gelled
at HbS concentrations lower than those required
when HbA was the diluent. Thus, both HbA

*Dr. Wiecking (loc. cit.) writes: “Why some of these
people sickle when others with equivalent sicknesses or
injuries don’t sickle, I frankly do not know. To a
degree, I suppose, our explanation of these cases is a
‘post hoc, ergo propter hoe’ type of argument.”

and HbC may participate in polymerization.
In contrast, HbF does not participate. The
mildness of SCD in people whose cells contain
high titers of HbF—for example, Perrine’s*
subjects—points up the practical value of heredi-
tary persistence of fetal hemoglobin (HPFH)
in such cases. The severity of sickling disease
in heterozygous individuals is generally consist-
ent with én vitro findings, leading Bertles,** for
example, to write of “bad” hemoglobins that
copolymerize easily with HbS and “good” hemo-
globins, such as HbF, that copolymerize weakly
or not at all. Milner ez al.%° gives two examples
of bad hemoglobin: HbD (Los Angeles) and
HbO (Arab). Individuals heterozygous for
HbS and these other hemoglobins are afflicted
with a disease very similar to that of the Hbg*
homozygote.** Those with the SC combination
are somewhat less affected, and those with sickle
trait are far less troubled by crises. Charache
and Conley* listed some Hb mixtures in order
of increasing severity of disease: AS, SF
(HPFH), SC, SD (Punjab), and SS. Possibly
the most efficient “good” hemoglobin is HbF,
which can exceed 20 percent of the total Hb in
heterozygotes for HPFH (genotype AF).** More
interesting, significant amounts of HbF are
found in the red cells of homozygous (SS) per-
sons, and Bertles** points out that HbF, which
is not uniformly distributed among these cells,
contributes to the survival of those containing
it. One reason, Bertles professes, for the high
titers of HbF in persons with SCD is that cells
devoid of HbF irreversibly sickle soon after
their release into the circulation; thus, cells con-
taining HbF survive longer and are present in
higher numbers at any given time.

Boyer et ai.*° have recently published evidence
confirming that HbF is not uniformly distrib-
uted among the red cells of normal adults (trace
amounts) and those heterozygous for HPFH.
These authors did not distinguish between two
interpretations of these findings, but both hy-
potheses are intriguing: either these HbF-
containing cells arise from perturbations in
B-chain synthesis, producing y chains—and
HbF—in cells of any stem cell lineage, or there
exist certain clones of stem cells that, time after
time, produce erythrocytes containing HbF. In
any case, the production of HbF-containing cells
is known to be “turned on” in various conditions
including leukemia and pregnancy. Boyer e¢ al.



Technical Report Documentation Page

1. Report No, 2. Government Accession No. 3. Recipient's Catalog No.
FAA- AM- 76-15

@. Title ond Subtitle 3. Report Date
THE AEROMEDICAL SIGNIFICANCE OF SICKLE-CELL
TRAIT: A REVIEW 6. Performing Organization Code

=|

Performing Organization Report No.

7. Author's)
Jess M. McKenzie, Ph.D.

9. Performing Organization Nome and Address 10. Work Unit No. (TRAIS)
FAA Civil Aeromedical Institute
P. 0. Box 25082 TT. Contract or Grant No.

Oklahoma City, Oklahoma 73125

13. Type of Report and Period Covered

12. Sponsoring Agency Name and Address

Office of Aviation Medicine OAM Report
Federal Aviation Administration
800 Independence Avenue, S.W. 14. Sponsoring Agency Code

Washington, D.C. 20591

15. Supplementary Notes

Work was performed under Tasks AM-A-75-PHY-49 and AM-C-76-PHY-49,

16. Abstract
This report presents some of the technical background necessary for understanding
the aeromedical importance of sickle-cell disease and the sickle-trait carrier,
whose erythrocytes contain mixtures of hemoglobin S and normal hemoglobin A.
This carrier state (type AS) is not, limited to Negroes; it has been found, with
lower frequency, in people exhibiting no evidence of African inheritance,
Reports of type AS people who died suddenly, exhibiting sickle cells at necropsy,
and other reports of sickling crises in these individuals at mild altitudes, have
led some authors to conclude that airmen and air passengers who are type AS
are at considerable risk. Other reports, particularly those based on the flying
experiences of large numbers of pilots with sickle trait, as well as on the
results of experimental exposures of type AS people to simulated altitude, indi-
cate that isolated instances of sudden death and altitude intolerance are
infrequent in this genotype. The author concludes that a diagnosis of sickle
trait, in the absence of a positive history of sickling crises, unusual diffi-
culties in anesthesia, or known contributing factors, is no basis for suspecting
an intolerance to moderate altitudes. Individuals with sickle trait who are
pilots should be thoroughly trained in the use of oxygen equipment and should
avoid any hypoxic experiences, as should all flying personnel.

17. Key Words 18. Distribution Statement
Stress (Physiology), Hemoglobin, Document ig available to the public
Aviation Personnel, Sickle-Cell through the National Technical Information
Disease, Sickle-Cell Trait, Medical Service, Springfield, Virginia 22151.
Certification

T9. Security Classif. (of this report) 20, Security Classif. (of this page) 21- No, of Pages | 22. Price
Unclassified Unclassified

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized



intravascular thrombosis.** This progression is
consistent with the report by Rickles and
O’Leary®® of platelet consumption during the
initial crisis, leading to the output of more sensi-
tive platelets, ete. Possibly, some patients, such
as those studied by Jones, Binder, and Dona-
who,* might benefit from the early administra-
tion of salicylates.

Vascular occlusion is more than ordinarily
hazardous to those with sickle trait. Occlusions,
which might be survived by persons without the
trait, could be complicated by the local sickling
of erythrocytes. Thus, individuals with sickle
trait might be especially susceptible to throm-
botic episodes such as those described by Pirkle
and Carstens,® who reported six individuals
(one male, five females) who died suddenly.
Three were known to have experienced some
respiratory distress before death and, at necropsy,
platelet aggregates were found in the pulmonary
‘arteries and arterioles of all six. These findings
are similar to those of Vincent,°° who described
respiratory distress and coma in rabbits after
injection of a fat-mobilizing factor isolated from
the pituitary. These results strongly suggest
that mobilization of fatty acids, which can occur
during periods of acute stress, may result in
platelet aggregation. Arachidonic acid, a pre-
cursor of PGE,, has been demonstrated to pro-
duce this effect.** It would be interesting to
examine the free fatty acid (FFA) patterns in
the blood of individuals who died suddenly.
Spector and Hoak® claim that arachidonate
comprises only a small portion of the plasma
FFA content, but this may not be the case in
individuals who die of pulmonary embolism
during times of acute severe stress.

Thrombosis and sickle crisis are often clinically
associated, especially in pregnancy,** and contra-
ceptive drugs have also been incriminated in
these two abnormalities. Hypercoagulability in
pregnancy may result from the release into the
circulation of thromboplastic substances from
the placenta, but this release is usually associated
with abnormalities (eg., abruptio placentae)
that occur in the third trimester. More likely,
thrombotic conditions may be caused by an in-
crease in the activation of Fletcher factor
(prekallikrein), because the inactivator of com-
plement C’1 esterase, also an inhibitor of Fletcher
factor, is decreased during pregnancy and in
women taking the Pill.*** Ratnoff® speculates

10

that this failure to inactivate Fletcher factor
may contribute to the increased coagulability
seen in some pregnant women, and it is tempting
to speculate further that thrombosis, leading to
microcirculatory occlusion, is partly responsible
for the increased tendency (in SCD) toward
sickle crises of pregnant women and women
taking contraceptive drugs.

Red cells containing HbS, when exposed to
hypoxic conditions, may confer an increased
viscosity to the blood even before changes in red
cell morphology become apparent. Increases in
certain plasma globulins may potentiate or exag-
gerate this effect by augmenting plasma viscosity.
For example, Anderson and coworkers® recently
reported a case of multiple myeloma in a patient
who also had SCD. Whole blood from this pa-
tient exhibited an abnormally high viscosity
when fully oxygenated, and the viscosity was
increased further when the blood was treated
with dithionite. Washed erythrocytes, placed in
plasma not containing the myeloma globulin,
conferred no increased viscosity until the oxygen
was removed with dithionite. From these and
other findings the authors conclude that an inter-
action between red cells and myeloma globulins,
resulting in dramatic increases in blood viscosity
im vivo, was responsible for the greater-than-
normal severity of SCD in this patient.

Other proteins may affect the blood viscosity.
Fibrinogen, a long-chain protein, may exert con-
siderable influence; this protein may increase as
much as 500 percent in a variety of chronic dis-
eases or other types of stress.’ Hyperfibrino-
genemia is thought to be a nonspecific reaction
to stress; in the rat, at least, the increase can be
produced by catecholamine injection: 1 mg/kg
epinephrine in oil injected subcutaneously pro-
duced, within 24 h, fibrinogen levels about 1.7
times higher than control levels. The response
is not affected by simultaneous injections of
glucocortiocoid and is not produced by corticoid
alone, but the adrenal cortex plays a permissive
role.** Elevations in the fibrinogen level, leading
to an increased blood viscosity, may well be an
important final common pathway by which
chronic stressors influence the severity of SCD
or, possibly, evoke crises in persons with sickle
trait.

Figure 4 contains a partial summary of those
mechanisms known and postulated to influence


nificant, for most authors believe these changes
to be associated with the presence of abnormal
erythrocytes ;*° but it is not necessary to postulate
an unusually severe hypoxemia in the vasa recta
of type AS individuals in order to explain their
nephropathies. Perillie and Epstein,” observing
that the sickling of HbS-containing cells was
increased in hypertonic media in vitro, proposed
that sickling—and a consequent decrease in
medullary blood flow—was most likely to occur
in the vasa recta, where the highest osmolarities
prevail. The resulting vascular insufficiency
could eventually lead to a loss of some of the
vasa recta.

Is hyposthenuria a threat, to those with sickle
trait? Considering the high frequency of the
condition in persons of this genotype and the
rarity of crises in these individuals, the danger
from hyposthenuria itself must be a minor one;
but does the condition sensitize individuals with
sickle trait to factors that may cause more serious
problems? Consider ethanol. Alcoholism has
been associated with sudden unexpected death
in persons with sickle trait. Ethanol, a diuretic,
may be a contributing factor in cases of SUD,
not only because of possible central depressant
effects, but possibly also because it contributes
toward dehydration. Being unable to efficiently
conserve water, hyposthenuric individuals may
become extremely dehydrated. Even in the ab-
sence of the concentrating mechanism of the
vasa recta, the kidney retains some minimal abil-
ity to concentrate urine, so that the renal blood
may possess the high osmotic pressures that
lower the sickling threshold.*' It is, therefore,
reasonable to speculate that in a dehydrated in-
dividual with sickle trait, the kidney might be
an important site for the initiation of a sickle
crisis.

Other evidence that osmotic stress may damage
erythrocytes is the finding by Alexander and
coworkers that in dogs restricted to 250 ml of
water per day for 5 weeks, the erythrocyte sur-
vival time was 14 percent of normal, In chronic
alcoholics, subjected to daily cycles of dehydra-
tion-rehydration, a similar damage to erythro-
cytes might result.

Drugs and Hormones. In addition to effects
on circulation and respiration and, hence, on the
sickling process, certain drugs and hormones
may produce direct changes in the red cell and

the red cell mass. Valeri®* presents an excellent
review of some of these changes. For example,
abnormally high levels of catecholamines (e.g.,
in pheochromocytoma) may cause reductions in
red cell mass, and a number of drugs are known
to produce an immunologic hemolytic anemia.

As noted earlier, the intracellular levels of
organic phosphates—particularly 2,3-diphospho-
glycerate (2,3-DPG)—are at least partly respon-
sible for the shift-to-the-right in the ODC of
individuals with sickle-cell disease. The erythro-
cyte content of 2,3-DPG can be increased by a
number of hormones* including growth hormone,
thyroxine, epinephrine, and androgens; in con-
trast, parathormone is known to effect a decrease
in 2,3-DPG. Boning ef al.** reported a diurnal
variation in the erythrocyte content of 2,3-DPG;
peak levels were seen in the afternoon and the
lowest levels, between 0100 and 0900.

Changes in erythrocyte deformability have
also been induced by drugs and hormones.
Prostaglandin E, (PGE) was reported by Allen
and Rasmussen® to reduce red-cell deformability
(measured by a filtration technique) at PGE:
concentrations as low as 10°" M; the maximum
effect was seen at 107° M. Epinephrine and
DL-isoproterenol had a similar effect, but higher
concentrations (10-? and 10’ M) were required.
The report by Johnson ef al.** that PGE, in-
duces and potentiates sickling in vitro under
hypoxie conditions may indicate a role for
prostaglandins, and perhaps catecholamines, in
the sickling crisis. Clinical trials of aspirin, an
inhibitor of PGE, synthesis, seem indicated.
Propranolol, the f-adrenergic blocking agent,
binds to erythrocytes” and might protect. them
from the effects of catecholamines.

Hemostatie Mechanisms and Plasma Proteins.
An investigation of the effects of aspirin on the
frequency and severity of sickling crises seems
doubly warranted in light of Rickles and
O’Leary’s® report that platelet consumption is
increased during the sickle crisis. This action
Jeads to the release into the circulation of young
platelets possessing an increased sensitivity to
aggregants, such as adenosine diphosphate,
thrombin, and, perhaps, PGE,. It will be re-
called that the sudden unexpected death of peo-
ple with sickle trait may involve an initial
collapse followed by partial recovery, a second
collapse, and death in coma with signs of diffuse



no crises occurred in any of the African athletes
(some with sickle trait) who participated in the
1968 Olympic games held in Mexico at an alti-
tude of 7,000 ft MSL. The NAS/NRC ad hoc
committee was unable to draw any conclusions
regarding the risk of SUD in individuals with
sickle trait, except that “there must. be other, yet
unknown contributory factors, not necessarily
related to the sickle-cell trait, that make some
persons susceptible to collapse or sudden death.”

Altitude Tolerance. Erythrocytes containing
HbS have at least one property not found in
other cells; with certain exceptions, only HbS-
containing cells become deformed at reduced
oxygen tensions. Thus, individuals whose cells
possess any amount of HbS might be expected
to have less-than-normal altitude tolerance. This
reasoning, and reports of sickle crises in flights
at various altitudes, ***° have led to the opinion
in some medical circles that individuals with
sickle trait should not be exposed to altitude
(see Reference 30 for a review of the subject).
This opinion may seem a prudent and reasonable
policy, considering these isolated instances of
crises at altitude, but the evidence on which it is
based should be reexamined carefully for a num-
ber of reasons: (i) Aviation is such a significant
means of transport today that individuals denied
its use could be seriously affected, both eco-
nomically and socially. (ii) In many of the de-
veloping nations, especially those in Africa, Hbg*
frequency is often in excess of 20 percent. These
nations have a great need for air transport;
most have national airlines and air freight com-
panies, (iii) In the United States the highest
frequency of Hbg* (about 8 percent) occurs in
the Negro segment of the population, which is
experiencing significant social and economic
changes. It is reasonable to expect an increase
in the numbers of Negro air passengers and
Negro airmen.

Another reason for the reevaluation of pre-
vailing attitudes toward the altitude tolerance
of individuals with sickle trait is the develop-
ment and recently more common use of tech-
niques for diagnosing the precise genotype of
heterozygous individuals and the recognition of
other hemoglobin type(s) that may be associated
with HbS in the red cell and may significantly
affect its response to mild hypoxia (see Figure
3). Many of the reports of sickle crises in flight
have failed to indicate the precise Hb composi-

tion of the victim’s blood and, as has been pointed
out,? not only the types of Hb present but their
amounts must be known before any judgment
can be made of the risks involved.

Reviewers of the literature on the problem of
altitude and sickle trait have largely overlooked
one aspect of an important paper that indicates
the existence of individuals who are tolerant to
hypoxia despite the fact that they possess the
Hbg* gene. Henderson and Thornell* reported
a study of Negro aviation cadets with sickle
trait (unfortunately, no quantitative Hb assays
were reported) who were in training at Tuskegee
Army Airfield from 1941 to 1945, Of more than
1,500 Negro cadets trained during that period,
none was eliminated as a result of “proved flying
deficiencies arising from the sickle trait” al-
though the incidence of trait was representative
of that in the United States Black population.
Of 312 cadets and returned combat veterans (a
United States Thunderbolt squadron operating
in Europe), 23 (7.37 percent) exhibited sickle
trait when sealed wet preparations of blood were
used as the diagnostic method (slides were
examined up to 48 h). The incidence of sickle
trait (7.14-7.17 percent) was about the same in
all stages of training. Combat veterans exhibited
an incidence of 7.7 percent. The veterans, who
had flown from 57 to 110 combat missions each,
at high and low altitudes, had not experienced
any difficulties related to sickling during these
flights. Of 48 cadets eliminated from training
for various reasons, only two exhibited sickling
by the wet preparation method. These surveys
provided evidence that people with sickle trait
may not be subject to any greater risk than
others in training and other aviation activities
including combat flying.

Henderson and Thornell also conducted an
altitude chamber experiment. Four cadets with
sickle trait and one local civilian who was known
to have sickle-cell anemia (they did not report
the results of any tests to determine exact pheno-
type; this patient might have possessed the Hb g°
gene, for example) were exposed to simulated
altitudes according to the following pattern:
15 min at 5,000 ft; 50 min at 10,000 ft; 5 min at
16,000 ft; and 5 additional min at 16,000 ft using
an oxygen mask. Henderson and Thornell, how-
ever, found no evidence of sickling in any of
their four subjects with sickle trait during the
test, nor was any evidence seen 48 h afterward.


Hereditary Persistence
of Fetal Hb
(HPFH):

HbF (agyo)

B-Thalassemia
HbA2(a282) Elevated

Normal: HbF (a yo)

a- Thalassemia: Hb Barts (yq)

(homozygote)

Normal Adult:

HbA (ap Bp) 98 %
HbAg(az82) 2%
HbF (agyo) Trace

(heterozygote)

a- Thalassemia:
HbH (Bq)

Ficure 1,—A diagrammatic summary of some of the hemoglobin types found in normal
adults and fetuses (circled), In the hereditary persistence of fetal hemoglobin

(HPFH) and in the majority of thalassemias.
ral hemoglobin types are found in heterozygotes and
Often the erythrocytes of thalassemic persons contain greater-than-

and varying mixtures of sev
homozygotes.

The latter conditions are complex,

normal amounts of HbF; this is not necessarily associated with HPFH.

renal function, the kidneys being second to the
CNS in sensitivity to hypoxia. Thus, for any
tissue and metabolic state, a certain level of O.
supply is required; below this level, a condition
of insufficiency is said to exist. The Os supply
to most tissues is normally well above this criti-
cal level, as demonstrated by a resting average
arteriovenous O, difference of 0.041 ml O./ml
blood, that can increase to more than 0.096 ml
0./ml blood during periods of increased met-
abolic demand or decreased blood flow. Part of
this ability to extract more or less ©, from the
blood is due to the peculiar shape of the O,-Hb
dissociation curve; Hb releases more O, as the
pO, and pH decrease and as pCO, increases.

Variety and Ontogeny of Hb Types. Figure
1 presents a review of this subject. In all Hb
types the heme moiety is associated with two
pairs of polypeptide chains (denoted here by
Greek letters). In normal individuals fetal Hb
(HbF) decreases to about 2 percent after birth.
The @ polypeptides, found in HbF, persist in the
normal adult hemoglobins (HbA and HbA.),
but, instead of y chains, HbA and HbA, contains
B or 8 chains respectively. An inherited failure
of « chain synthesis may result in fetal death in
the homozygous condition or «-Thalassemia in
heterozygous individuals. In Cooley’s anemia or
B-Thalassemia, an insufficient synthesis of B
polypeptide chains results in a severe anemia


out the presence of other significant findings
indicating ante mortem sickling—and a history
of mild hypoxia is not one of them—a finding
of post mortem sickling is of no more diagnostic
value than is hemoglobin electrophoresis, and the
examiner must content himself with the Scot’s
verdict: not proved. Thus, in the absence of
positive history of altitude intolerance or of
known contributory factors to such intolerance,
persons with sickle trait are probably no more
susceptible to hypoxia than are other individuals.

Keeping in mind the possibility that hetero-
zygous individuals may exhibit varying degrees
of Hb,* gene expression, how may we interpret
the conflicting reports reviewed so far? One
difference in these reports is apparent: Reports
that moderate hypoxia is no threat to persons
with sickle trait are based on surveys of large
numbers of these individuals who fly, on repeated
in vitro tests of their erythrocytes, or on com-
plete resistance of test subjects to moderate alti-
tudes. Reports of altitude intolerance, on the
other hand, are based on isolated instances of
collapse during or shortly after flight. Readers
of these anecdotal reports may be provoked to
inquire: How many others with sickle trait who
were not affected were aboard the same aircraft;
what types and amounts of hemoglobin were
present in the victim’s red cells; and, most im-
portant, how many other individuals of this
genotype have flown under similar conditions
without incident? Persons of the AA genotype
may survive hypoxic conditions that lead to un-
consciousness; once normal oxygen levels are
restored, consciousness is regained and in most
cases there is no apparent lasting effect of the
hypoxic experience; but in the individual whose
cells contain sufficient HbS, an otherwise surviv-
able exposure to hypoxia, if it also Jeads to sick-
ling, could induce a “vicious cycle” (see Figure
4) of sickling that could lead to death. It must
be remembered, however, that, in the SA geno-
type, initial sickling does not occur (Figure 3)
until the oxygen tension approaches 10 mm Hg.

Vv. Summary and Conclusions.

1. Sickle trait is not a causal factor with re-
spect to altitude intolerance. On the other hand,
it may be an important factor in the reversibil-
ity of hypoxic collapse.

13

2. The presence of sickle trait is not, of itself,
a basis for aeromedical disqualification, nor is
sickling in the cadaver conclusive evidence that
a sickle crisis was the primary cause of death
or of a fatal accident.

3. It is likely that other factors (e.g., preg-
nancy, chronic disease, trauma, drug and alcohol
abuse) might potentiate the effects of hypoxia
in people with sickle trait, and the aviation
medical examiner should consider these factors
in evaluating individuals with sickle trait for
aeromedical qualification.

4. Pilots of the SA genotype should be in-
formed of the probable combined effects of
hypoxia and potentiating factors and should be
advised that a good oxygen system and its proper
use is important to them, not only in preventing
sickle crises, but possibly in reversing crises,
should they occur.

5. Physiological training, especially in the use
of oxygen equipment, is of great potential use-
fulness to the airman with sickle trait. A
hypoxic experience is not necessary for this train-
ing and should be avoided. Trainees of the SA
genotype should, however, be encouraged to ob-
serve hypoxic signs in other trainees who do
participate in this phase, as these signs are valu-
able demonstrations of the insidious nature of
hypoxia.

APPENDIX

The author is grateful to Dr. Russell S. Fisher,
Chief Medical Examiner for the State of Mary-
land, for providing five examples of unexpected
collapse and death. The cases have not pre-
viously appeared in the literature. These indi-
viduals were Black, and electrophoretic exami-
nation indicated that each was of the AS
genotype:

1. A 25-year-old, somewhat obese female, en
route to the maternity clinic, collapsed while
walking up the hill near a hospital. She was
pronounced dead on arrival at the accident room.
At necropsy, she exhibited a twin pregnancy and
her erythrocytes were markedly sickled. Dr.
Fisher writes, “... her Kiipfer cells are the most
striking example of ingestion of sickle cells that
T have ever encountered.”

2. A 48-year-old female was “slapped around”
in an altercation between 2400 and 0300, She
was put to bed and her breathing seemed normal



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15

BL

35.

36.

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agement, Education, and Research, In H. Abramson,
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Anemia: Clinical Features in Adulthood and Old
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Erythrocytes (F Cells) in Normal Human Adults,
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themoglobin Formation in Sickle Cell Disease, J.
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RESPIR, DIS., 96 :400-407, 1967.

Sproule, B. J., E. R. Halden, and W. F. Miller: A
Study of Cardiopulmonary Alterations in Patients
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he

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Torrance,
Sulphocyanate
in Sickle Cell
6 :782-788, 1982.



THE AEROMEDICAL SIGNIFICANCE OF SICKLE-CELL TRAIT: A REVIEW

I. Introduction.

Travel by air is no longer an unusual luxury
to be enjoyed by favored people in a few wealthy
nations. Today it is the fastest and often the
most economical means of transportation. In
some parts of the world it is the only practical
means of travel, so that the people who now
avail themselves of it are not only the healthy,
well-nourished Americans and Europeans who
have been the subjects of so many aeromedical
investigations; today air travelers may be citi-
zens of developing nations or even patients trav-
eling to centers of treatment.

With a greater variety and number of air
passengers and crewmembers, we should not be
surprised to see a corresponding increase in the
variety and number of medical incidents asso-
ciated with flight. Consider, for example, one
of the cases reported by Green, Huntsman, and
Serjeant :*

A 20-year-old Ghanaian nurse flying an un-
pressurized aircraft at about 10,000 ft (3,050
m) from Kumasi to Accra developed acute
abdominal pain in flight. On arrival she was
admitted to hospital with severe pain and
board-like rigidity of the whole abdomen....
She made an uneventful recovery from what
is presumed to be an infarctive incident in the
small bowel. Haemoglobin studies later
showed she was a sickle-cell trait carrier.

This is one example of many reports associat-
ing the sickle-cell gene with crises that can occur
under conditions of mild to severe hypoxia.
These reports should not be questioned in one
respect: Some individuals, whose red blood cells
were later shown by screening tests to contain
hemoglobin S, did indeed suffer some kind of
vascular incident under hypoxic conditions. But,
as we shall later see, such screening tests—or,
for that matter, more sophisticated tests, when
evaluated in an unsophisticated manner—are not
sufficient to diagnose the genotype of sickle-cell
trait.

The report by Green, Huntsman, and Serjeant*
appeared in December 1972. The following
January, interested readers began to suspect that
perhaps the risk of altitude to sickle-trait car-
riers was not so clearly defined as those authors
had indicated. Konotey-Ahulu of Accra, Ghana,
has had much clinical experience with sickle-cell
disease. His letter to the editor of the British
Medical Journal? (January 1972) criticized the
report and conclusions of Green e¢ al. as per-
petuating “the confusion which has for several
decades surrounded the use of the term sickle-
cell trait especially in relation to aviation.”
Konotey-Ahulu pointed out that a normal hemo-
globin (Hb) level in association with a positive
screening test does not preclude the coexistence
of some other abnormal Hb that could contribute
to an altitude intolerance. With reference to
the Ghanaian nurse, he warned that “by far the
most striking thing about the haemoglobin elec-
trophoresis of sickle cell-8-thalassaemia is an
‘AS? pattern with no more fetal haemoglobin
than is found in sickle-cell anaemia. ... The
jnexperienced, confronted with such an electro-
phoretic strip without the control, and without
values for A, and S, will almost certainly call it
sickle-cell trait.”

Other conflicting reports on hypoxia and
sickle trait have appeared in the literature. This
paper will review these and other reports.

II. Technical Background.

A primary function of blood is the transport
of oxygen (O,) to the other tissues. Critical
factors in this transport are (i) the blood’s O,
content, (ii) the rate of mass flow, and (iii) the
rate or facility with which the blood acquires
O, in the lungs and releases it to the tissues.
An important characteristic of any tissue is its
O, requirement, and this may vary considerably
depending on physiological state and tissue type.
It is: well known, for example, that the central
nervous system (CNS) may be irreversibly dam-
aged by low O, levels that are still adequate for





aspirin)--- - >

Circulatory disorders

MILD ‘Obesity
HYPOXEMIA Pneumonia
(propranolol) ete.
- i y s[ DECREASED J_
Bo narenersic.€Pds DEFORMABILITY PGE2

Reute

Emotional stress, INCREASED |<increased Globulins <— some diseases
Severe exercise VISCOSITY |< increased Fibrinogen <— chronic stess
MODERATE
HY POXIA Depressonts {onticoagutants)
J Pregnancy,
Elevated The “Pill
2,3-0°6——>| MODERATE
enyératin —>[_ SICKLING Thrombin
(cyonates} ~~ oe AHDF, et)
‘Alcohol SEVERE Blood
HYPOXIA Coagulation
BLOOD Piatelet

STASIS<—— Aggregation

SICKLING

Figure 4.—The sickling crisis (boxes) and some proposed contributory mechanisms

(solid arrows).

connected to the process by dotted arrows.

Proposed extenuating factors are enclosed by parentheses and

A cyclic system is emphasized here.

Not only may contributing factors drive the system toward the final “vicious cycle”
beginning with severe hypoxia, but they may. also compound the crisis, as shown by
the large circle. Regardless of the site(s) of initiation, the crisis, unless corrected
by treatment or physiological compensation, will progress from local to regional and

finally to general sickling.

the sickle crisis, Though speculative, this dia-
gram seems a reasonable synthesis of the reports
cited in this review.

Clinical Tests for HbS. Qualitative tests for
HbS are available. Most of these tests depend
on reduction of O, in blood samples or solutions
of hemoglobin; this reduction can be accom-
plished by incubating blood samples under an-
aerobic conditions (e.g., sealed cover slip method)
or by adding reducing agents, such as dithionite
salts. The end point indicating the presence of
HbS may be, for example, erythrocyte deforma-
tion or a reduction of light transmission. These
methods have their applications in mass screen-

11

ing, but they are not adequate for complete
qualitative and quantitative evaluation of Hb
composition in candidates for aeromedical cer-
tification. In terms of complexity and expense,
the next most available techniques are electro-
phoretic; but even these may not be definitive
and, as Nalbandian e¢ al. have pointed out
(JAMA. 234:832-835, 1975), may give mislead-
ing results. Therefore, it is probably most
economical and prudent to place the responsi-
bility for Hb analysis on hematologists who
have the special laboratory facilities and neces-
sary expertise. The Aviation Medical Examiner
is, of course, most capable of interpreting lab-



at this time, but she did not respond to attempts
to awaken her. She was pronounced dead at the
scene. This individual had a history of bron-
chial asthma and headaches but no record of
heart disease or other illnesses. At necropsy,
80 ce of subdural hemorrhage were found over
the left cerebral hemisphere and 20 ce under the
left temporal lobe. Although the left hemisphere
was shifted somewhat to the right, there were
no secondary lesions in the cerebral hemispheres
and no lesions of this kind or edema in the pons.
Dr. Fisher concluded that these brain changes
were not sufficient to explain death. A marked
sickling of red cells was seen in all ‘histological
sections and there was intense hemorrhage into
the splenic follicles. Dr. Fisher concluded that
hypoxia or hypotension associated with the
original trauma-induced subdural hemorrhage
was suflicient to induce a sickle crisis, “which
was the true cause of her death.”

3, A 25-year-old male was found dead. He
was a known narcotics addict and possessed re-
cent needle marks with subcutaneous hemorrhage.

A search of the body for narcotics was without
result; blood alcohol was 0.06 gm/dl. “He had
all the signs of acute sickle crisis.”

4, A 22-year-old female, a known narcotics
addict, was found dead. Analysis of the blood
revealed a level of 0.85 mg/dl of a “short acting
barbiturate.” There was histologic evidence of
sickle cris

5. A 26-year-old female exhibited signs of
having died in sickle crisis. Blood alcohol was
0.14 gm/dl. A trace of quinine (a common
heroin adulterant) was found in the urine, but
the results of tests for narcotics were negative.

Dr. Fisher, on the basis of these and other
cases, concludes: “I am certainly sufficiently con-
vinced that I would advise any AS hemoglobin
individual to avoid hypoxic anesthesia as well as
alcohol or drug abuse, or any other situation in
which there might be an episode of hypotension
or hypoxia, lest an otherwise survivable lesion be
suddenly converted to a fatal process.”

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1972,

Stamatoyannopoulos, G.:

Hemoglobin Disease, AN

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~

The Molecular Basis of
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s

Pa

s

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>

a

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