Yonago Acta medica 1999;42:189–192

Circadian Rhythms of the Indices in the Diffusing Function of the Lungin Healthy Men
Naoto Burioka, Hisashi Suyama, Takanori Sako, Eiji Hoshino, Yukio Matsumotoand Takao Sasaki
Third Department of Internal Medicine, Faculty of Medicine, Tottori University, Yonago 6830826, Japan
Diurnal variation in the lung diffusing function can influence the results of clinicalevaluations. We investigated circadian rhythms from diurnal variations of the indicesin the diffusing function of the lung. We measured the single breath lung diffusing
capacity for carbon monoxide (DLCO) and alveolar volume (VA) in 13 healthy men, andalso measured pulse rate at 7:00, 11:00, 15:00, 19:00 and 23:00. To identify the presence of a circadian rhythm from daily variations, data were analyzed by the groupmean cosinor method. Significant circadian rhythms were apparent for DLCO/VA, VAand pulse rate. The acrophases for these parameters occurred at 20:15, 5:26 and 18:08,respectively. The double amplitude of DLCO/VA was statistically 4.72%. Our results
suggest that diurnal variations in VA and pulse rate may contribute to the variation inDLCO/VA. Although DLCO/VA showed only a small change throughout the day, ourfindings suggest that the examination of DLCO/VA should be performed preferably at
the same time of day in an individual.
Key words: alveolar volume; circadian rhythm; cosinor analysis; diurnal variation; lung diffusion
capacity

Although the human body maintains homeostasis despite changes in internal and external
conditions, diurnal variations may occur in response to those conditions. These variations
can be periodic or non-periodic. Periodic phenomena, in particular, are considered to be important for the clarification of morbid states anddevelopment of therapeutic methods. A number of indices of respiratory function have beenfound to show diurnal variations or circadianrhythms (Hetzel and Clark, 1980; Ohga et al.,1992). The lung diffusing capacity for carbon
monoxide (DLCO) is widely used as a clinicalparameter. It has been found to show diurnal
variation in healthy individuals (Cinkotai andThomson, 1966; Panda and McHardy, 1980;
Mahajan et al., 1990), and has not (Frey et al.,1987). Diurnal variations can influence the results of evaluations in routine clinical practice.

We examined the time-course of diurnal variations of indices of the diffusing function of thelung in healthy men, and attempted to identify
circadian rhythms.
Subjects and Methods

Subjects
We studied 13 healthy men (age: 31.5 ± 4.3 years)
without cardiopulmonary diseases. There were
6 nonsmokers and 7 smokers. Smoking wasprohibited before and during examinations.
The subjects were all doctors in our respiratorylaboratory. They were made to get up before
7:00 and ate breakfast between 7:00 and 9:00,
lunch between 12:00 and 14:00, and dinner between 19:00 and 21:00. Their working was not
regulated during the study.

Abbreviations: DLCO, lung diffusing capacity for carbon monoxide; PR, pulse rate per minute; VA, alveolar
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N. Burioka et al.

Table 1. Diurnal variations in the indices of lung diffusing function and PR
Index

7:00

11:00

15:00

19:00

23:00

DLCO(mL/min/mmHg) 25.4 ± 4.1
25.4 ± 4.4
25.1 ± 4.3
26.2 ± 4.3 25.7 ± 3.4
4.29 ± 0.75
4.33 ± 0.74
4.45 ± 0.73 4.39 ± 0.65
DLCO/VA (mL/min/mmHg/L) 4.20 ± 0.71
VA(L)6.05 ± 0.56
5.95 ± 0.46
5.81 ± 0.58
5.90 ± 0.44 5.89 ± 0.53
PR(min–1)
69.4 ± 9.5
73.9 ±11.576.8 ± 8.8
76.1 ±10.8 76.4 ±10.5
Data are mean ± SD.
DLCO, lung diffusing capacity for carbon monoxide; PR, pulse rate; VA, alveolar volume.

Measurements

Data analysis

The forced expiratory curve was measured using a
Chestac 55V, a rolling seal type spirometer(Chest MI, Tokyo, Japan). The pulse rate perminute (PR) was measured in a sitting position
after resting for at least 3-min in 10 of 13 subjects before measuring lung diffusing capacity
for carbon monoxide (D L CO ) and alveolar
volume (VA ) of body temperature ambientpressure, saturated with water vapor, at 7:00,11:00, 15:00, 19:00 and 23:00. DLCO, VA andDLCO/VA were measured at 7:00, 11:00, 15:00,19:00 and 23:00 with subjects in a sitting position after at least a 3-min rest period. Duplicate
analyses were performed at each time pointwith a Pulmonet III (Gould, Bilthoven,Netherlands) or a Chestac 55V by the singlebreath diffusing capacity. In the single breath
diffusing capacity method, the subject wasforced to aspire a gas mixture containing 0.3%
CO and 10% He from residual volume up tototal lung capacity. After stopping the breathfor 10 s, the immediately expired gas was sampled, and DLCO, VA and DLCO/VA were analyzed. The same equipment was used for eachsubject.

Each subject underwent a measurement of 5
points in which measurements were done twice,and measured values were the expressed percentage value from the mean of the daily variation. The relative percentage values of each index were examined by the group mean cosinor
analysis, and the presence of circadian rhythm
was tested at a significance level of 5% (Nelsonet al., 1979). A probability value of less than
0.05 was considered statistically significant.

Results
The predicted average of forced expiratory volume in one second was 82.2 ± 6.2%. In Table 1,DLCO, DLCO/VA, VA and PR at each measuring
point for subjects are shown. No significant circadian rhythm was observed in DLCO. A significant circadian rhythm was seen in DLCO/VA ,and its acrophase was 20:15 (95% confidence
interval: 16:44–0:16) with a %amplitude of2.36% (0.54–4.21%). In VA, a significant circadian rhythm was observed, and its acrophase

Table 2. Group mean cosinor analysis of DLCO, DLCO/ VA, VA and PR in healthy mennIndex

Acrophase

Percent rhythm(%)

Percent amplitude(%)

13DLCONSNSNS
13
DLCO/VA
20:15 [16:44 – 0:16]
31.1
2.36 [0.54–4.21]
5:26 [ 3:20 – 8:12]
34.8
1.70 [0.66–2.77]
13VA
10
PR
18:08 [16:20 –20:24]
60.9
4.94 [2.50–7.45][ ], 95% confidence interval; percent rhythm, r2 × 100.
DLCO, lung diffusing capacity for carbon monoxide; NS, not significant; PR, pulse rate; VA, alveolar volume.

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was 5:26 (3:20–8:12) with a %amplitude of
1.70% (0.66–2.77%). Regarding the PR, a significant circadian rhythm was also found in theanalysis of the group of 10 subjects, and its
acrophase was 18:08 (16:20–20:24) with a%amplitude of 4.94% (2.50–7.45%) (Table 2).

Discussion
The diffusing capacity of the lung, particularlythat obtained with carbon monoxide (DLCO) has
widely been used clinically. The logical basisof the lung diffusing capacity was establishedand the measuring method of DLCO was developed by Forster and colleagues (1954). The
ratio of DLCO to VA (DLCO/VA) is also used as anindex of the diffusing function of the lung.
Lung diffusing capacity is related to gas exchange between alveolar gas and erythrocytes
in alveolar capillaries. It is therefore influenced
by the diffusion distance (thickness of the membrane), the surface area of the lung, the size ofthe capillary bed in the lung, the pulmonary
capillary blood volume, the hemoglobin content
of erythrocytes, and the uneven distribution of
the ratio of aeration to blood flow. The subject’s posture during evaluation of DLCO canalso affect measurements.
In order to clarify the cause of the diurnal
variation in detail, the diurnal variation was examined from an aspect of circadian rhythm
which has not been performed so far. We usedcosinor analysis for the purpose of identifying
circadian rhythm (Nelson et al., 1979). The
acrophase is the time of the highest point of acosine curve fitted to a time series while thebathyphase is the time of the lowest point; itdiffers 180° (12 h) from the acrophase. Although DLCO did not exhibit a circadian rhythm,DLCO/VA, VA and PR showed significant circadian rhythms. Studies have shown that VA
peaks in the morning and decreases towardevening (Panda and McHardy, 1980; Frey et al.,
1987; Mahajan et al., 1990), which is consistent
with the present study. The mechanism of thechange in VA is uncertain, but the decrease ofVA may be related to the decrease of the lungrecoil pressure (Saito et al., 1996).

Our results are different from published results (Cinkotai and Thomson, 1966; Panda andMcHardy, 1980; Mahajan et al., 1990). We hypothesize that the lack of significant circadianrhythm in DLCO may have been related to thefinding that the acrophases of VA and PR almost
occurred at opposite times. VA was high and PR
was low in the morning, and VA decreased andPR increased toward the evening. DLCO is expected to decrease when VA decreases because
the lung surface area becomes smaller and thegas-exchange area is decreased. It is consideredthat the pulmonary vascular change is one of thefactors to change the magnitude of DLCO. Whenthe PR is increased by a loading exercise test innormal subjects, pulmonary capillary blood volume increases, resulting in an increase in DLCO(Johnson et al., 1960; Rosenberg and Foster,1960). Changes in PR at rest are linked to thediurnal variation in the plasma level of norepinephrine, suggesting that acceleration of thesympathetic nervous system during the daytime
can contribute to an increase in the pulse rate.
DLCO has been found to increase in response tonorepinephrine, induced an increase in the diffusion capacity of the pulmonary membrane(Lewis et al., 1960). Therefore, although therelation is indirect, DLCO may increase when PR
increases. Since both VA and PR can affect
DLCO, the changes in these indices may havecanceled out significant changes in DLCO. In
0 (o'clock)

DLCO/VA
186VA
PR

12Fig. 1. Group mean cosinor expression on a 24-hclockface. DLCO, lung diffusing capacity for carbon
monoxide; PR, pulse rate; VA, alveolar volume.

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N. Burioka et al.

previous studies (Cinkotai and Thomson, 1966;
Panda and McHardy, 1980; Mahajan et al.,1990), the relative contributions of these 2 factors may have been different from the presentstudy.
Group mean cosinor analysis showed thatthe 95% confidence interval of the acrophase ofDLCO/V A overlapped with that of the PR andbathyphase of V A (15:20–20:12) (Fig. 1).
Therefore, the increase in DLCO/VA at 19:00
compared with 7:00 may have been due to thedecrease of VA, and an increase in pulmonary
capillary blood volume or an increase in thediffusing capacity on the pulmonary membrane.
Although the hemoglobin content of erythrocytes was not measured in the present study, it
has been found to decrease from the morning to
the evening (Renbourn, 1947) and to affect themagnitude of DLCO (Frey et al., 1987). Dailychange in hemoglobin content may also havebeen involved in the diurnal variations of theindices in the lung diffusing function.
On the basis of group mean cosinor analysis,
the maximal amplitude of an index with significant circadian rhythm is double %amplitudeat acrophase. Therefore, the maximal amplitude during diurnal variation was 4.72% forDLCO/VA (95% confidence interval: 1.08–8.42%).
Although the variation is small, our findings
suggest that DLCO/VA should be evaluated at thesame time of day when monitoring clinicalchanges in an individual whose work is not regulated.

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(Received August 4, Accepted August 24,1999)

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