Modified cannula

A surgical cannula includes a generally hollow inlet portion which can receive a fluid flow and an angled outlet portion connected in fluid communication with the inlet portion. The outlet portion is disposed at an angle of less than 180 degrees with respect to the longitudinal axis of the inlet portion. The inlet portion is provided with a structure to impart a rotational component of flow to fluid before such fluid encounters the angled outlet portion.

BACKGROUND OF THE INVENTION 
This invention is concerned with a modified cannula and is especially 
applicable to cannulae adapted for use in connecting a heart lung machine 
to a patient's aorta during open-heart surgery. 
Cannulae are devices which connect items of hardware or drainage vessels to 
a patient's body. During heart surgery, for example, a patient's blood is 
oxygenated and circulated by an artificial heart lung machine. A surgical 
incision is made into the patient's aorta wherein a cannula is surgically 
secured such that the outlet end is directed into and along the route of 
the aorta. 
The present invention finds application with cannulae generally, but 
specifically it is well suited to the modification of cannulae adapted to 
supply blood from a heart lung machine. 
With conventional such cannulae there have been flow problems associated 
with the relatively high velocity of blood into the aorta. There are also 
concerns over the possible dislodgement of fatty tissue from the vicinity 
of the aorta and its potentially serious implications. 
Problems can arise during aortic perfusion associated with cardiopulmonary 
bypass surgery. Specifically, there is concern that blood emerging at high 
velocity from cannulae could damage the aortic wall and/or dislodge 
atheromatous plaque and hence cause embolic phenomena. A secondary concern 
is that high velocities (and related high impact pressures) might disturb 
the distribution of flow to the great vessels originating from the arch. 
For cannula of the heart lung machine type, there is sometimes a bend in 
the tubing for the surgeon's convenience whilst simultaneously permitting 
flow of blood along the general route of the aorta. Commercially produced 
cannulae, e.g. of the type 3M Sarns Healthcare `soft flow` and `D4` 
cannulae, incorporate such a planar bend. 
A feature of both Soft Flow and D4 cannulae is a sharp planar bend near the 
tip. It has been found that such a bend causes skewing of the velocity 
profile, with high velocities at the outer wall of curvature and low 
velocities (and possibly flow separation and flow reversal) at the inner 
wall of curvature. 
Accordingly, it has been considered desirable to develop a new and improved 
cannula which would overcome the foregoing difficulties and others while 
providing better and more advantageous overall results. 
SUMMARY OF THE INVENTION 
In an effort to reduce the flow velocity of blood which exits from the 
cannula and minimise the force of impact on and around the internal 
surfaces of the aorta, the profile of the outlet of the cannula has been 
subjected to various modifications. One of these modifications is shown in 
FIG. 2 herein. 
A consideration of the conventional design of cannulae for heart lung 
machines and their shortcomings has led to the development of the present 
invention. It has surprisingly been found that modification of the flow 
velocity profile in a particular way before the flow encounters the bend, 
reduces the severity of the impact forces on the interior of the aorta 
wall and helps to alleviate other difficulties associated with the design 
of conventional such cannulae. 
Accordingly, the inventors proposed that, if there was to be modification 
of the cannulae with preservation of the bend, the flow should be made 
non-planar in type. Non-planar-type flow has been found to be 
characterised by swirling predominantly in one sense, strong mixing, and a 
relatively uniform distribution circumferentially of near-wall velocity. 
As a means of achieving such a flow, a twisted strip can be introduced 
into the cannulae, immediately upstream of the planar bend, the strip 
having a helical twist. Alternatively (or additionally) a swirling flow 
can be provided both in `soft flow` and `D4` cannulae by rendering the 
curvature non-planar e.g. introduce a bend in a plane different from the 
plane of the existing bend. 
According to this invention we provide a surgical cannula comprising a 
generally hollow inlet portion which can receive a fluid flow and an 
angled outlet portion connected in fluid communication with said inlet 
portion, and said outlet portion disposed at an angle of less than 
180.degree. with respect to the longitudinal axis of said inlet portion, 
characterised in that the inlet portion is provided with means to impart a 
rotational component of flow to fluid before such fluid encounters said 
angled outlet portion. 
The means to impart a rotational component may be internally located e.g. 
within the inlet portion and may further be in contact with the flow of 
fluid in use. Alternatively such means could be externally located, 
providing or causing a tangential flow of fluid. Such means could be 
provided by forming a spiral twist in the body of the inlet portion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring initially to FIG. 1, a cannula of the type used for connecting a 
heart lung machine to a patient's aorta is shown. A flexible tubular inlet 
portion 1 has a hollow interior section 2 and is securely affixed by heat 
welding or adhesive to a more rigid outlet portion 3, 4, 6 which is also 
hollow and generally tubular but with a slight taper towards the outlet 
end 7 of the said portion 6. A hollow lumen extends between the inlet 
portion 1 and the outlet portion 3, 4, 6 of the cannula. The end 5 of tube 
1 overlaps part 3 of the outlet portion although other arrangements are 
possible. There is a bulbous kink 4 provided to form an abutment over 
which the surgical incision in the aorta can be repaired and fastened, to 
prevent unintentional withdrawal of the cannula in use. The outlet section 
6 of the portion is angled at approximately 120.degree. with respect to 
the longitudinal axis of the inlet 1, providing planar curvature in the 
structure. 
In use, the inlet 1 is fastened to the blood supply line from a heart lung 
machine (not shown) whereupon oxygenated blood is fed into the aorta 
through the hollow interior of the inlet and outlet portions. In doing so, 
the blood is forced at relatively high flow rates, to undergo a change in 
direction at the planar curvature which forms a `bend` in the region of 
the bulbous projection 4 between the parts 3 and 6 of the outlet portion. 
It has been found that the bend can interfere with the flow of blood 
causing it to impact upon the tissue within the aorta. There may be an 
increased tendency to propagate clots in the region of the bend. The 
present inventors have modified the cannula, in one embodiment of the 
present invention, by providing means internally of the cannula and 
`upstream` of the `bend` which causes a velocity shift to the flow of 
blood by introducing a rotational component to the flow. In one embodiment 
of such means, as illustrated in FIG. 1, an insert of spirally twisted or 
otherwise helically wound material 8 is located inside the inlet portion, 
upstream of the angled outlet section 6. Ideally such material should have 
at least two full `twists` 10 whereby the flow is caused to rotate at 
least once through 180.degree. more preferably 270.degree. and even more 
preferably through 360.degree. or more during its linear travel. 
The end 9 of the insert 8 may be linear, curved or pointed. It can be of 
suitable biocompatible materials e.g. plastics or metal known or shown to 
induce no undesirable effects on the blood flowing over it. For example it 
might be constructed from high density polyethylene, polypropylene or 
stainless steel. 
After flow of blood along the inlet 1 and past the insert 8, the flow will 
have become `twirl` or `swirl` flow and the severity of impact on exit 
from the discharge end of the outlet section 6 will have been reduced, 
with improved flow in the internal region of the bend at the bulbous 
projection 4. 
In place of the outlet section 6 shown in FIG. 1, the embodiment may be 
modified by using the outlet section shown in FIG. 2. This section shows 
an arrangement previously devised to reduce the severity of the impact of 
blood flowing from the outlet end of a fairly high pressure, high flow 
rate pump. The present invention is amenable to use in such cannulae as 
are also shown in FIG. 2. The bulbous formation 4a is an integral part of 
the outlet section 6a with a `closed` end 7a and an internal conical 
projection 11 with apex 12 and a series of four (only two of which are 
shown) discharge orifices 7b in the region of said internal conical 
projection. 
Other designs of outlet section will be possible since the invention is 
essentially concerned with modifying the velocity profile of the rapidly 
flowing blood before it encounters the planar curvature i.e. before being 
forced to turn by the angled section of the outlet portion. 
As foreshadowed earlier, some device might be fastened externally of the 
inlet 1 to confer a tangential flow, or the tubing forming the inlet could 
be at least partially twisted in the form of a spiral helix over part of 
its length before the planar curvature of the bend. 
The specific embodiments compare the results obtained with an unmodified 
Soft Flow cannula with those obtained following the introduction of a 
twisted strip see FIG. 1. The strip was made of thin aluminium; had a 
length of 16 cm, a pitch of about 3 cm, and a diameter of 0.8 cm, but was 
tap red downstream, so that it could extend to within a short distance of 
the planar bend. The test fluid was water, the flow was steady at a rate 
of about 6 1/min, and the flow exited from the cannular into air. 
With the unmodified cannular, distinct jets emerged from the two outer wall 
of curvature `windows` whereas at the inner wall of curvature `windows` 
they were far less distinct, merging into a sheet. Following the 
introduction of the twisted strip, there were distinct (and similar) jets 
at both the outer and inner wall of curvature `windows`. 
With the unmodified cannula, the ratio combined flow rate at the two outer 
wall of curvature `windows`/combined flow rate at the two inner wall of 
curvature `windows` typically took a value of 1.5. Following the 
introduction of the twisted strip, the ratio typically took a value of 
1.1. 
A Pitot tube (od 4 mm, id 3 mm) was used to obtain a crude measurement of 
the impact pressure of the jet issuing from an outer wall of curvature 
`window`. With both the concave-recess cannula and the cone-type cannula, 
the relative impact pressures were about 24 units. However, the 
introduction of the twisted strip caused the relative impact pressures for 
the concave-recess cannula and the cone-type cannula to become 
respectively 16 and 12 units. 
At the same time, studies were carried out on the effect of the twisted 
strip on the flow exiting from a D4 cannula. These were qualitative 
studies. They showed that with the unmodified (planar) cannula the 
diameter of the emerging jet was constant for several cm downstream, 
whereas with the twisted strip in place, the diameter of the jet increased 
in the downstream direction. Moreover, there was evidence of swirling in 
the jet, predominantly in one sense. 
In the examples employing non-planar compound bends the same general 
methods have been employed as previously described. However, there has 
been use of a smaller Pitot tube (od 0.5 mm, id 0.3 mm) to allow 
measurement of impact pressure with improved spatial resolution. With Soft 
Flow cannulae, peak impact pressure was measured in an outer wall of 
curvature `window` and an inner wall of curvature `window` about 1 cm from 
the window. With D4 cannulae, impact pressure was measured in the jet 
about 1 cm from the cannula tip, at three stations over two orthogonal 
diameters. 
Constancy of cannula geometry upstream of the tip improved the 
reproducibility of measurements. In addition, the flow became most 
prominently non-planar in type when the upstream bend was severe and close 
to the downstream bend. Therefore, a constant upstream geometry is 
preferred and modified cannulae have been used, which possessed the 
required geometric characteristics. Results obtained with a Soft Flow 
cannula and a D4 cannula are reported separately in tables 1 and 2 
hereunder. 
TABLE 1 
______________________________________ 
Soft Flow Cannula 
Planar Non-planar 
______________________________________ 
Flow rate o.sup.a 3400 
o.sup.a 3300 
(ml/min) i.sup.b 2300 
i.sup.b 2600 
o/i 1.27 
Peak impact o.sup.c 56.5 
o.sup.c 47.9 
pressure i.sup.d 35.9 
i.sup.d 43.9 
(cm H.sub.2 O) o/i 1.57 o/i 1.09 
Peak o.sup.c 238 
o.sup.c 219 
calculated i.sup.d 189 
i.sup.d 210 
velocity o/i 1.26 
o/i 1.04 
(cm/s) 
______________________________________ 
.sup.a combined flow outer wall of curvature windows 
.sup.b combined flow inner wall of curvature windows 
.sup.c one outer wall of curvature window 
.sup.d one inner wall of curvature window 
TABLE 2 
______________________________________ 
D4 Cannula 
Planar Non-planar 
______________________________________ 
Run 1 
Flow rate 4.5 4.5 
(ml/min) 
Impact 1 42.9 (0) 
1 42.9 (1.5) 
pressure 2 38.5 (3.0) 
2 29.6 (4.5) 
(cm H.sub.2 O) 
3 19.2 (1.5) 3 25.2 (3.0) 
4 38.5 (3.0) 
5 34.0 (3.0) 
Run 2 
Flow rate 6.0 6.2 
(ml/min) 
Impact 1 50.3 (0) 
1 54.8 (0) 
pressure 2 51.8 (0) 
2 44.4 (3.0) 
(cm H.sub.2 O) 
3 36.3 (1.5) 
3 42.9 (0) 
4 51.8 (3.0) 
5 51.8 (4.5) 
Run 3 
Flow rate 6.2 
(ml/min) 
Impact 1 53.3 (4.5) 
pressure 2 45.9 (3.0) 
(cm H.sub.2 O) 
3 44.4 (4.5) 
4 56.2 (1.5) 
5 59.2 
______________________________________ 
(6.0) 
It may be noted that the impact pressure measurements were made 
symmetrically about the plane of curvature of the downstream bend. As a 
result, they adequately represent the velocity field for the case of 
non-planar geometry, because it can be expected that the secondary motion 
will then be rotated out of the plane of curvature of the downstream bend. 
It may also be noted that for a flow rate of 6 1/min and a typical cannula 
inner diameter of 0.7 cm, the Reynolds number was about 18,000. 
Soft Flow cannula: In tests using the planar (unmodified) cannula, the 
ratio combined flow rates at the two outer wall of curvature 
`windows`/combined flow rates at the two inner wall of curvature `windows` 
took a value of about 1.5. The twisted strip caused a reduction in the 
value of this ratio to 1.1, whereas the introduction of non-planar 
geometry caused a lesser reduction, i.e. from 1.43 to 1.27. More severe 
curvature at the upstream bend and/or bringing of the two bends closer 
together, may produce a greater reduction of the value of the ratio. 
The ratio peak impact pressure at outer `window`/peak impact pressure at 
inner `window` was not measured initially, but later found to take a value 
of about 1.6. In the initial tests the introduction of a twisted strip 
could halve the impact pressure at the outer `window`. In contrast, the 
introduction of non-planar curvature reduced peak impact pressure at the 
outer `window` by about 16%. More severe curvature at the upstream bend 
and/or the bringing of the two bends closer together, may produce a 
greater reduction of that pressure. 
The ratio peak impact pressure at outer `window`/peak impact pressure at 
inner `window` took a value of about 1.6 in the planar cannula and 1.1 in 
the non-planar cannula. The possible clinical significance of that finding 
is discussed below. 
D4 cannula: The introduction of the twisted strip and of non-planar 
curvature appeared to cause swirling predominantly in one sense in the 
exiting jet and expansion of the jet downstream of the exit orifice. 
Impact pressures were not measured in initial tests involving the twisted 
strip. However, later tests showed that impact pressures were lower at the 
centre of the jet and the inner wall of curvature of the downstream bend, 
with non-planar geometry than with the unmodified (planar) cannula (see 
Table 2 and graphs). 
Clinical significance: There have been concerns that high impact pressures 
could damage the aortic wall and/or dislodge atheromatous plaque and hence 
cause embolic phenomena. There have also been concerns that high exit 
velocities and high impact pressures in the aorta could disturb the 
distribution of flow to the great vessels originating from the arch. 
The illustrated and described embodiments demonstrate that non-planar-type 
flow can reduce both peak exit velocities and peak impact pressures. Such 
flow can be generated internally within the interior of the generally 
hollow cannula by means of a twisted strip or by rendering cannula 
geometry non-planar. The latter embodiments may be preferred because of 
greater simplicity of construction of a device and possibly its being more 
robust. 
Whilst complications could arise during cardiopulmonary bypass perfusion, 
from high velocities and high impact pressures, there may also be problems 
from low velocities and low impact pressures. The latter complications 
would be associated with low wall shear and long fluid residence times, 
and could include thrombosis and embolism. Therefore, cannulae which can 
generate a relatively uniform velocity field, such as those within the 
scope of the present invention could be commercially desirable.