Pathophysiology of chronic cyanosis in a canine model

Pathophysiology of chronic cyanosis in a canine modelPathophysiology of chronic cyanosis in a canine model
Functional and metabolic response to global ischemia
To investigate the pathophysiology of chronic cyanosis, we subjected 14 adult mongrel dogs to diversion of the inferior vena cava to the right inferior pulmonary vein. This produced a mean oxygen tension of 42 ± 2 mm Hg and a calculated right-to-Ieft shunt of 52.0 % ± 3.9 %. These animals(Group C) and 15 normal dogs (Group N) were subjected to cardiopulmonary bypass with 20 minutes of normothermic global ischemia. Functional indices studied were rate of rise of left ventricular pressure and the end-systolic pressure/volume ratio. Metabolic status was assessed by obtaining transmural myocardial biopsy specimens for measurement of adenosine triphosphate content. Myocardial blood flow was measured with radiolabeled microspheres. There were no significant differences between Group C and Group N in either functional index or blood flow measurement prior to global ischemia. At 45 minutes after ischemia, Group N animalshad a significantly greater rate of rise of left ventricular pressure (at a left ventricular end-diastolic pressure of 0,5, 10, and 15 mm Hg, p < 0.025 to 0.05) and subendocarial perfusion (endocardial/epicardial flow ratio 0.961 ± 0.037 versus 0.815 ± 0.021, p < 0.01). At 90 minutes after ischemia, Group N animals exhibited a significantly higher end-systolic pressure/volume ratio (4.9 ± 0.7 versus 3.0 ± 0.4 mm Hg/mI, p < 0.05), rate of rise of left ventricular pressure (at an end-diastolic pressure of 0 to 20 mm Hg, p < 0.005 to 0.05), and endocardial/epicardial flow ratio (1.065 ± 0.046 versus 0.829 ± 0.059, P < 0.01). No differences in adenosine triphosphate content were found at any sampling period. The Group C left ventricles exhibited no hypertrophy but were significantly dilated compared to Group N (38.8 ± 0.3 versus 30.1 ± 0.2 mm, p < 0.05). Inferior vena cava to pulmonary vein diversion produces cyanosis with left ventricular dilatation but without hypertrophy. It is proposed that abnormal loading characteristics of the left ventricle are responsible for the functional derangements that result from global ischemia.
F. M. Lupinetti, M.D., T. H. Wareing, M.D., C. B. Huddleston, M.D., J. C. Collins, Ph.D., R. J. Boucek, Jr., M.D., H. W. Bender, Jr., M.D., and J. W. Hammon, Jr., M.D., Nashville, Tenn.
Mechanical function in the systemic ventricle of the patient with cyanotic congenital heart disease is fre­ quently abnormal. 1·3 This abnormality may persist despite operative correction.r" It would be useful to determine the pathophysiology of this dysfunction as a guide to preoperative management, timing of operative
From the Departments of Cardiac and Thoracic Surgery, Biomedical Engineering, and Pediatric Cardiology, Vanderbilt University School of Medicine, Nashville, Tenn.
Received for publication June 19, 1984.
Accepted for publication Oct. 16, 1984.
Address for reprints: John W. Hammon, Jr., M.D., Department of Cardiac and Thoracic Surgery, 338 Medical Arts Building, 1211 21st Ave. South, Nashville, Tenn. 37212.
intervention, and projection of natural history and prognosis. The mechanism of this contractile abnormal­ ity has not been adequately elucidated as yet.
Previous studies of this phenomenon have focused on four factors as potentially affecting systemic ventricular function in the presence of chronic cyanosis: (1) chronic exposure to cyanosis itself, (2) abnormal loading of the systemic ventricle, (3) abnormal loading of the pulmo­ nary ventricle, and (4) injury resulting from operation." These studies have typically employed a model of cyanosis that produces hypertrophy of one or both ventricles.13. 14 Ventricular hypertrophy alone, however, can induce physiological and metabolic disturbances, 15-21
and these effects may be difficult to isolate from those attributable to the cyanosis alone.
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Table I. Left ventricular dPjdt (mm Hgfsec)
LVEDP (mm Hg) I Group C I Group N p Value
Before ischemia 0 1,679 ± 186 1,552 ± 220 NS 5 2,027 ± 212 2,006 ± 206 NS
10 2,124 ± 258 2,396 ± 130 NS 15 2,278 ± 260 2,562 ± 164 NS 20 2,300 ± 266 2,705 ± 196 NS
45 min after ischemia 0 1,016 ± 92 1,488 ± 144 <0.025
5 1,330 ± 175 1,891 ± 138 <0.05
10 1,469 ± 165 1,964 ± 123 <0.05 15 1,546 ± 174 2,106 ± 134 <0.05 20 1,604 ± 182 2,129 ± 162 NS
90 min after ischemia 0 1,157 ± 155 1,721 ± 173 <0.05
5 1,307 ± 195 2,008 ± 144 <0.02 10 1,566 ± 230 2,260 ± 152 <0.05 15 1,586 ± 195 2,412 ± 152 <0.01 20 1,606 ± 177 2,570 ± 198 <0.005
Legend: LVEDP. Left ventricular end-diastolic pressure.
This study examines a canine model of chronic cyanosis not associated with ventricular hypertrophy. It attempts to determine the effects of global ischemia in the presence of cyanosis on systemic ventricular function and metabolism.
Materials and methods
Chronic cyanosis was produced in 14 adult mongrel dogs (Group C). With the animal under general pento­ barbital anesthesia, a right thoracotomy was performed. The inferior vena cava was divided at its junction with the right atrium and anastomosed in an end-to-side fashion to the right inferior pulmonary vein. The azygos vein was ligated and divided. Arterial blood gases were monitored, and sodium bicarbonate was administered to maintain a pH greater than 7.35. At least 6 months elapsed between this operation and the experimental study.
The control group consisted of 15 normal adult mongrel dogs (Group N). All dogs underwent general pentobarbital anesthesia and were ventilated with 40% oxygen from a volume-cycled ventilator. A median sternotomy was performed. In Group C dogs, blood gas measurements were made on samples from the pulmo­ nary vein, aorta, superior and inferior venae cavae, and pulmonary artery. Percent shunt was calculated by the following formula:
% Shunt (PV02 - Ao02) X 100% PV02-MVBOz
The Journal of
Thoracic and Cardiovascular Surgery
where PVOz = oxygen content of the pulmonary vein, AoOz = oxygen content of the aorta, and MVBOz = ox­ ygen content of mixed venous blood."
Ultrasonic piezoelectric crystals were implanted on the subendocardium of the anterior, posterior, and apical surfaces of the left ventricle. A fourth crystal was placed on the base of the heart. These crystals provided instantaneous measurements of the left ventricular major and minor axes. A micromanometer was inserted into the left ventricle for recording of systolic and end-diastolic (LVEDP) pressures. A fluid-filled catheter was placed in the aortic arch for recording of aortic pressure and reference blood sampling during micro­ sphere injection. After systemic heparinization (3 mg/ kg), arterial and venous cannulation for cardiopulmo­ nary bypass was performed. A side branch from the arterial circuit was connected to a cannula in the left atrial appendage for volume.infusion.The extracorpore­ al circuit was primed with 1,000 ml of freshly obtained blood from a donor dog and 1,000 ml of normal saline buffered to pH 7.4.
After baseline measurements were obtained, bypass was begun and the left ventricle was vented via the left atrium. Normothermic global ischemia was maintained for 20 minutes, followed by 30 minutes of reperfusion. Bypass was terminated, and the animal was monitored for an additional hour. No pharmacologic or mechanical circulatory assistance was provided during this last period. All animals were successfully separated from bypass.
Functional measurements were performed before and 45 and 90 minutes after ischemia. The rate of rise of left ventricular pressure (dP/dt) was electronically derived. The end-systolic pressure/volume ratio" was calculated from major and minor axis measurements and the simultaneous left ventricular pressure. End-systole was defined by the dicrotic notch on the aortic pressure tracing.
Transmural myocardial biopsy specimens were obtained with a 16 gauge Tru-Cut needle before isch­ emia and at 15, 30, 45, 60, and 90 minutes after ischemia. The specimens were immediately frozen in liquid nitrogen and later analyzed for adenosine triphos­ phate (ATP) content by the bioluminescence technique of Ellis and Gardner."
Myocardial blood flow before and 45 and 90 minutes after ischemia was determined by left atrial injection of 15 J.Lm radiolabeled microspheres. After extirpation of the heart and confirmation of crystal placement, the heart was weighed, and the left ventricle was carefully freed from the right ventricle, atria, valvular tissue, and
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Absolute flow (mlfminfgm}
Distribution (endojepi]
Study period
Before ischemia 45 min after ischemia 90 min after ischemia
Legend: Endo/epi, Endocardial/epicardial. *p<0.01
Group C
I Group N
Group C
I
1.178 ± 0.410 0.961 ± 0.037 1.065 ± 0.046
epicardial vessels and fat. The ventricle was divided into four concentric slices, each of which was subdivided into four sections of approximately equal size. Each section wasfurther divided into endocardial, middle, and epicar­ dial thirds. Each piece was weighed and counted in a gamma scintillation counter, along with the reference bloodsamples. Each myocardial sample weighed at least 500 mg and contained at least 400 spheres. Flows were calculated by standard methods." Total left ventricular flow and the ratio of endocardial.to epicardial flow were recorded.
Results are expressed as the mean ± the standard error of the mean. Differences between the groups were analyzed by Student's t test for unpaired data.
All animals in this study received humane care in compliance with the "Principles of Laboratory Animal Care" formulated by the National Society for Medical Research and the "Guide for the Care and Use of Laboratory Animals" prepared by the National Acade­ my of Sciences.
Results
The experimental procedure of inferior vena cava­ pulmonary vein anastomosis produced a mean partial pressure of oxygen (Po2) of 42 ± 2 mm Hg in room air and a mean P0 2 of 43 ± 1 mm Hg during ventilation at the time of study. Thus, the mean calculated right­ to-left shunt was 52.0% ± 3.9% in Group C dogs. The mean hematocrit value in Group C was 54.1% ± 2.6%, significantly greater (p < 0.05) then that of Group N, 44.4% ± 0.7%. Differences in hematocrit value were abolished by the dilutional effect of cardiopulmonary bypass. Immediately after institution of bypass, the hematocrit value fell to 32.0% ± 2.2% in Group C and 29.1% ± 1.8% in Group N. At the end of the study, the hematocrit value was 36.6% ± 1.9% in Group C and 35.0% ± 1.8% in Group N.
The mean weight of the hearts was 105 ± 5 gm in Group C and 115 ± 6 gm in Group N. The mean left ventricular weight was 86 ± 2 gm in Group C and 85 ± 1 gm in Group N. The ratio of left ventricular
weight to body weight was 4.37 ± 0.14 gm/kg in Group C and 4.88 ± 0.29 gm/kg in Group N. None of these differences is statistically significant.
The left ventricular end-diastolic diameter at an LVEDP of 0 mm Hg in the preischemic period was 38.8 ± 0.3 mm in Group C and 30.1 ± 0.2 mm in Group N. This difference is statistically significant (p < 0.05).
Mean dP/dt values are shown in Table I. No significant differences were found at any LVEDP in the preischemic measurements. By 45 minutes after isch­ emia, the dP/dt of Group N was significantly greater than that of Group C at LVEDP 0 through 15 mm Hg. By 90 minutes after ischemia, the dP/ dt was significant­ ly greater at all filling pressures.
The end-systolic pressure/volume ratio of Group C was 5.4 ± 0.6 mm Hg/ml in the preischemic period, not significantly different from that of Group N, 5.8 ± 0.5 mm Hg/ml, By 90 minutes after ischemia, the end­ systolic pressure/volume ratio of Group C fell to 3.0 ± 0.4 mm Hg/ml, which was significantly less than the 4.9 ± 0.7 mm Hg/ml in Group N (p < 0.05).
Myocardial blood flow measurements are displayed in Table II. No differences were found in absolute left ventricular blood flow at any study period. However, the endocardialjepicardial ratio of transmural flow distribu­ tion was significantly greater in Group N than in Group C both 45 and 90 minutes after ischemia.
Myocardial ATP concentrations are shown in Fig. 1. Baseline ATP content was similar in the two groups, as were the serial measurements following ischemia. No significant differences were found at any sampling period.
Discussion
The abnormal contractile function present in the systemic ventricle of the patient with cyanotic congenital heart disease is well known to clinicians dealing with this problem. The precise mechanism of this dysfunction is not well understood, however. Even after surgical cor­ rection of these congenital anomalies, contractile abnor-
2 9 4 Lupinetti et al. The Journal of
Thoracic and Cardiovascular Surgery
14
Fig. 1. Myocardial adenosine triphosphate (ATP) content. No significant differences between the cyanotic and the normal dogs were found at any sampling interval.
malities may persist. Such abnormalities have been documented following operative treatment of transposi­ tion of the great arteries (including venous," ventricu­ lar,' and arterial" levels of repair), tetralogy of Fallot,? 10
and tricuspid atresia. I I, 12
Some experimental investigations attempting to define the cause of ventricular dysfunction in chronic cyanosis have used a pulmonary artery-left atrium shunt with banding of the distal pulmonary artery. This model has been shown to produce accelerated depletion of myocardial ATP and creatine phosphates stores14 and disturbances in the geometry of left ventricular contrac­ tion." It has been proposed that the oxygen deprivation produced by this model causes the diminished levels of high-energy phosphates, thereby resulting in deperession of contractile function."
It is unclear, however, whether these abnormalities should be attributed to the cyanosis, to the concomitant right ventricular hypertrophy that results, or to a combination of these and possibly other factors. Ventric­ ular hypertrophy in the absence of cyanosis has been shown to produce abnormal ATP metabolism.l- 16
increased sensitivity to global ischemia," and distur­ bances in regional myocardial blood flow.15, 18-20 The presence of right ventricular hypertrophy may be a
factor in the production of left ventricular hypertrophy as well."
Inferior vena cava-pulmonary vein bypass, as shown in the present study, provides a moderately severe degree of cyanosis but without ventricular hypertrophy. This model may therefore be preferable for investigation of the pathophysiology of chronic cyanosis. This absence of ventricular hypertrophy may explain the normal ATP levels in the cyanotic animals in this investigation.
Both end-systolic pressure/volume ratio, a sensitive indicator of ventricular contractile function independent of preload and afterload, and dP /dt were markedly diminished in the cyanotic animals after global isch­ emia. If these functional abnormalities cannot be corre­ lated with diminished myocardial energy stores, an alternative hypothesis is warranted.
Polycythemia has been shown to contribute to an abnormal myocardial blood flow distribution resulting in subendocardial underperfusion." For this reason, in the present study a greater degree of hemodilution was permitted in the cyanotic animals. The equalization of hematocrit values in the two groups after initiation of bypass eliminates polycythemia as a factor in the dysfunction exhibited by the cyanotic group.
The cyanotic animals in the investigation had a marked diminution of endocardial/epicardial blood flow distribution. This relative subendocardial ischemia may explain the diminished contractile function. Further­ more, the left ventricular dilatation observed in the cyanotic animals, a possible consequence of the right­ to-left shunt, may playa role in this maldistribution of transmural flow. By Laplace's law, the wall tension is proportional to the intracavitary pressure and radius. Therefore, at equal left ventricular pressures, the cya­ notic ventricle should have greater wall tension and hence less subendocardial perfusion. It is proposed that the etiology of ventricular dysfunction in this group of cyanotic animals is related to the abnormal loading of the left ventricle, resulting in left ventricular dilatation, rather than to any metabolic derangement. Clinical data to support this hypothesis are found in the work of Graham and associates, I who discovered the degree of left ventricular dilatation to be an important correlate of abnormal function in patients with cyanotic congenital heart disease.
It should not be surprising that differences between the cyanotic and normal animals were detectable only after the stress of global ischemia had been imposed. This finding is consistent with the detailed clinical observations of Friedli and colleagues," who found abnormalities in lactate extraction in the hearts of a subgroup of cyanotic patients only when subjected to
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rapid atrial pacing. The investigations of Scheurer and associates" and of Rudolph" suggest a possibleexplana­ tion. These studies showed that, in cyanotic patients, myocardial oxygen extraction increased to compensate forthe hypoxia, and lactate extraction was maintained at normal levels. Patients with cyanotic congenital heart disease have been found to have a shift in their myocardial lactate dehydrogenase isoenzyme distribu­ tion that enhances anaerobic glycolysis.'?
In several experimental studies, acute hypoxia was shown not to depress myocardial ATP content. 31
, 32
Indeed, Park and co-workers" have demonstrated an increase in cardiac mitochondrial activity acutely and chronically in a model similar to that reported in this paper. These clinical and experimental data seem to cast doubt on the theory that chronic hypoxia per se can produce the severe functional derangements noted in the present study.
The relevance of either the cavoatrial model without hypertrophy or the pulmonary artery-left atrium model with hypertrophy to the clinical situation is open to question. Both models provide rather modest degrees of cyanosis, and the durations of cyanosis prior to study have been short. Congenital anomalies analogous to the model described in this paper have been reported in patients." All of these patients have had minimal or no symptoms, and one survived well into adulthood before a diagnosis was made.
Further studies of the physiology of cyanosis are indicated that employ a more profound degree of cyanosis observed over a longer period to time. In addition, a direct comparison of the hypertrophied and nonhypertrophied cyanotic models would help to eluci­ date the relative importance of pressure overload in the presence of cyanosis.
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