Ventricular haemodynamics in Python molurus: separation of pulmonary and systemic pressures
1 Department of Zoophysiology, University of Aarhus, 8000 Aarhus C,
Denmark,
2 Department of Biology, IFM, University of Linköping,
Sweden,
3 Institute of Zoology, University of Bonn, Germany
4 Department of Zoology, University of Gothenburg, Sweden
* Author for correspondence (e-mail: tobias.wang{at}biology.au.dk)
Accepted 20 August 2003
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Summary |
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Key words: Reptile, snake, Python molurus, cardiovascular, blood pressure, ventricular pressure, vascular pressure separation.
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Introduction |
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Ventricular pressure separation does not occur in the snakes Vipera,
Natrix and Thamnophis
(Johansen, 1959;
Burggren, 1977
), but the
cardiac anatomy of pythons resembles that of varanid lizards and many
descriptions of the heart from Python molurus have emphasised the
large muscular ridge and that the ventricular wall surrounding the systemic
side of the heart is much thicker than the wall surrounding the pulmonary side
(Jacquart, 1855
;
Webb et al., 1974
;
van Mierop and Kutsche, 1985
;
Farrell et al., 1998
).
Furthermore, the in situ perfused heart of Python molurus
exhibits an extraordinary degree of flow separation and the systemic side of
the ventricle can sustain higher output pressures than the pulmonary side
(Wang et al., 2002
). Given
these observations, we decided to investigate blood pressures in the systemic
and pulmonary circulations of conscious and chronically instrumented specimens
of Python molurus and to measure the intraventricular pressures of
anaesthetised animals.
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Materials and methods |
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Surgeries and experimental protocols
The experiments consisted of two phases. First, animals were instrumented
under general anaesthesia with catheters for measurements of systemic and
pulmonary arterial blood pressures in fully recovered and conscious animals.
Then, they were reanaesthetised for measurements of intraventricular
pressures.
Instrumentation for measurements in recovered animals
For anaesthesia, a small plastic bag with saturated vapours of halothane
was placed over the snake's head. When there was no longer any exhibited
response to pinching their skin, it was possible to intubate the trachea with
soft rubber tubing, so that the lungs could be artificially ventilated with
1.5% halothane mixed with air (Halothane vaporizer; Dräger, Lubeck,
Germany) at a rate of 3 breaths min1 and a tidal volume of
30 ml kg1 using a ventilator (HI 665, Harvard Apparatus
Inc., Holliston, MA, USA). A 5 cm ventrolateral incision was made immediately
above and anterior to the heart, to allow a catheter (PE60 or PE90, filled
with heparinised saline) to be placed occlusively in the cranial portion of
the vertebral artery, from where it was advanced into the right aortic arch
(RAo). The pulmonary circulation was cannulated by implanting an occlusive
catheter (PE90) into the left pulmonary artery (LPA). In Python, the
LPA is smaller than the right pulmonary artery, but this cannulation
undeniably increased the resistance of the pulmonary circulation. Both
catheters were exteriorised dorsolaterally and tied to the skin with several
sutures. The incision was closed and the snakes were artificially ventilated
with air until spontaneous lung breathing resumed. All snakes exhibited normal
behaviour within the first 2 h after surgery had begun and were left
undisturbed to recover in dark containers at room temperature
(2022°C). On the following day, the catheters were connected to
pressure transducers and blood pressures of resting undisturbed snakes were
measured over the next few hours.
To investigate whether the marked differences in pressure between the systemic and pulmonary circulation could be disrupted, we injected the vasodilator, sodium nitroprusside and the vasoconstrictor, phenylephrine via the arterial catheter (25 µg kg1; applied as a 1 ml bolus followed by an additional 1 ml of saline to flush the catheter).
Instrumentation for intraventricular measurements in anaesthetised
animals
After completing the measurements in conscious snakes, individuals were
anaesthetised by a slow infusion of 1020 mg kg1
sodium pentobarbital through the systemic catheter. When the animals no longer
responded to having their skin pinched, the trachea was exposed for intubation
and the lungs were artificially ventilated with 97% O2, 3%
CO2, prepared by a Wösthoff gas mixing pump (Bochum, Germany)
at a rate of 5 breaths min1 and a tidal volume of
approximately 50 ml kg1 (Harvard Apparatus Respirator HI
665). The heart was then exposed by a ventral incision and the pericardium
opened, so that i.v. catheters (Surflo, Terumo Medical Corporation, Elkton,
MD, USA, connected to PE90) could be inserted into the CP and the CA. These
catheters were placed directly through the ventricular wall and were secured
in place by a thin suture (50) into the ventricular muscle tissue. When
intraventricular pressures and pressures in the systemic and pulmonary
arteries had been measured simultaneously, we performed some of the same drug
injections as described above for the conscious animals. At the end of the
experiment, the snakes were killed with an overdose of nembutal (200 mg
kg1 injected through the systemic catheter) and subsequent
decapitation.
Measurements of blood pressures, data recording and statistics
Blood pressures were measured by connecting the catheters (PE60 or PE90, no
longer than 60 cm) to Baxter Edward disposable pressure transducers (model
PX600, Irvine, CA, USA). The signals from the pressure transducers were
amplified by a preamplifier built in-house, and the amplified signal was
collected at 100 Hz using a data acquisition system (MP100, Biopac, Goleta,
CA, USA). All four pressure transducers were positioned level with the
experimental animal's heart and calibrated several times a day against a
common static water column.
All data are presented as means ± S.E.M. Statistical differences between systemic and pulmonary blood pressures and between values obtained on recovered and anaesthetised snakes were analysed on basis of a t-test and significant differences were accepted when P<0.05.
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Results |
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Arterial and intraventricular pressures in anaesthetised snakes
The difference between Ppul and
Psys persisted during anaesthesia, but there were
significant increases in both Ppul and
Psys compared to conscious animals
(Table 1). Anaesthesia also
elicited a large and significant increase in fH from
13.8±1.4 to 33.7±2.2 beats min1.
In all six snakes studied, the measurements of intraventricular pressure showed an overlap in the pressure profile between the CP and the pulmonary artery, while the higher pressures in the CA overlapped with the pressures recorded in the right aortic arch (Fig. 3). In all individuals, it seemed that pressure in the CA increased before pressure rose in the CP. However, because of the much lower diastolic pressures in the pulmonary artery, ventricular ejection into the pulmonary circulation preceded ejection into the systemic circulation (Fig. 3).
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Discussion |
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Although several aspects of the ventricular haemodynamics during systole
remain speculative in Varanus, interpretation of pressure
relationships is facilitated by reasonably good anatomical descriptions of
their heart (e.g. Webb et al.,
1971; Webb, 1979
).
The vertical septum is large and separates the CA from the left side of the
heart. The CV is greatly reduced and may merely form the connection between
the CA and PA. Webb (1979
)
even suggested that the vertical septum and the muscular ridge have merged to
form a single structure, so that the CV actually has become part of the CP.
While this may be an oversimplification based on nomenclature rather than
haemodynamic function (Burggren and
Johansen, 1982
), it remains very likely that the functional
separation of the CP and CA is accomplished by the free edge of the
well-developed muscular ridge making direct contact to the ventricular wall
early in systole. Detailed anatomical descriptions of the ventricle of
Python are, unfortunately, not available, but the structural
similarities between varanid lizards and ophidians have been noted numerous
times for more than a century (e.g.
Jacquart, 1855
;
Robb, 1965
; Webb et al.,
1971
,
1974
;
Webb, 1979
;
van Mierop and Kutsche, 1985
;
Farrell et al., 1998
). Thus, as
in Varanus, the ventricular wall surrounding the CA of Python
molurus is much thicker than the wall surrounding the CP and the CA is
extensively trabeculated compared to the CP
(Farrell et al., 1998
; T. Wang,
personal observation). Given their anatomical and physiological similarities,
the ventricular haemodynamics of Python and Varanus are
probably very similar.
Ventricular pressure separation is consistent with the higher pressure and
power generation of the systemic side of the in situ perfused heart
from Python molurus (Wang et al.,
2002). These differences are coupled with a considerable degree of
flow separation between the systemic and pulmonary circulations
(Wang et al., 2002
). During
in situ perfusion, right atrial inflow was predominantly directed to
the pulmonary circulation, whereas left atrial inflow was directed primarily
to the systemic circulation, suggesting that inflows from the two atria are
well separated even during diastole. During diastole, flow separation probably
results from the large right atrioventricular valve that directs right
atrial inflow across the muscular ridge to the PA, while the left
atrioventricular valve directs left atrial inflow to the base of the CA
(White, 1968
;
Webb, 1979
). Thus, it is most
likely that the cardiac shunts that exist can be attributed to `wash-out' of
the small CV during the cardiac cycle
(Heisler and Glass, 1985
),
suggesting that the magnitudes of the cardiac shunts in Python, as in
Varanus, are relative small and that it does not change appreciably
during different conditions (e.g. Berger
and Heisler, 1977
; Heisler et
al., 1983
; Ishimatsu et al.,
1988
). In Python molurus, systemic arterial oxygen levels
are high even when metabolic rate is increased during digestion
(Overgaard et al., 1999
;
Overgaard and Wang, 2002
),
which indicates that the degree of rightleft shunt is rather small.
Our study shows that pronounced ventricular pressure separation is not
restricted to the specialised group of varanid lizards, and although unlikely
to be common among squamates, it may exist in more species than previous
acknowledged. Ventricular pressure separation, however, does not occur in the
snakes Vipera berus, Natrix and Thamnophis
(Johansen, 1959;
Burggren, 1977
) and large
cardiac shunts have been reported for sea snakes and rattlesnakes
(Lillywhite and Donald, 1989
;
Wang et al., 1998
). The
development of high systemic blood pressure has often been linked to the
higher cardiac output and increased capillary density that are associated with
evolution of increased metabolic rate. To protect the lungs from oedema and
possible structural damage (e.g. Burggren,
1982
), ventricular division and pressure separation become
necessary, while the further reduction of cardiac shunts associated with
division also improves systemic oxygen delivery and augments maximal oxygen
consumption (Wang and Hicks,
2002
). As with the independent evolution of divided ventricles in
endothermic birds and mammals, ventricular separation in Varanus is
often correlated with their active life style, high exercise capabilities and
high metabolic rate (e.g. Burggren et al.,
1998
). Python is a typical inactive sit-and-wait predator
that can ingest large prey (e.g. Secor and
Diamond, 1995
), and we have proposed that functional separation of
the ventricle is related to the high oxygen consumption during digestion
(Wang et al., 2002
).
Alternatively, it is possible that ventricular pressure separation in
Python molurus is related to its use of shivering thermogenesis
during egg incubation, which results in large and prolonged metabolic
increments (Hutchison et al.,
1966
; Vinegar et al.,
1970
).
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Acknowledgments |
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References |
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