Arteriovenous fistula: Hemodynamic effects of occlusion and exercise

Exaggerated increase in cardiac output during exercise in patients with peripheral high-flow arteriovenous malformations

PURPOSE

To test the hypothesis that cardiac output (Q) in patients with arteriovenous malformations (AVMs) is normal at rest and increases disproportionately during exercise.

MATERIALS AND METHODS

Q was measured in eight patients with large peripheral AVMs and in nine healthy subjects using a noninvasive carbon dioxide (CO₂)-based differential Fick method. Subjects were tested while seated at rest and during mild exercise (repeated leg straightening while sitting). Oxygen consumption (VO₂) was monitored as an index of the degree of exercise.

RESULTS

Average resting Q was similar between AVM patients and healthy subjects (7.40 L/min ± 3.29 vs 6.13 L/min ± 0.94, respectively, P = .29). During exercise, AVM patients showed a smaller increment in VO₂ (0.50 L/min ± 0.11 vs 0.78 L/min ± 0.26, P = .012) but with more apparent effort and shortness of breath compared with healthy subjects. The change in Q per unit change in VO₂ (ΔQ/ΔVO₂) was greater in AVM patients than in healthy subjects (16.00 L/min ± 6.50 vs 9.79 L/min ± 5.33, P < .045).

CONCLUSIONS

Exercise intolerance in AVM patients may be due to an imbalance in ΔQ/ΔVO₂ resulting from increased shunting through the AVM. Exercise provocation may increase the sensitivity of Q in the clinical evaluation of AVM patients.

Circulatory “Efficacy” during progressive aerobic exercise in children: insights from the Q: VO2 relationship

The relationship between circulatory flow (Q) and oxygen uptake (VO2) may provide insights into performance of peripheral mechanisms which govern blood flow during exercise (circulatory efficacy). This study evaluated the response of Q relative to VO2 during progressive upright cycle exercise in a group of 39 preadolescent boys (mean age 12.2 +/- SD 0.5 years). The Q-VO2 relationship was curvilinear, best described by the cubic equation Q = 3.60(VO2)(3) + 5.24(VO2)(2) + 2.40(VO2) - 0.94. Circulatory efficacy, defined as the %DeltaQ/%DeltaVO2 x 100, fell from 70.4% between the first two workloads to 49.7% at peak exercise. This decline in circulatory efficacy is consistent with other published data suggesting a decline in skeletal muscle pump function at high intensity workloads. The pattern of change in relationship of Q and VO2 during progressive exercise in these children is similar to that observed in studies of adults. This implies that performance of peripheral determinants of circulatory responses to exercise is not affected by biological maturation.

Circulatory responses to exercise: are we misreading Fick?

The Fick equation holds that oxygen uptake (Vo(2)) is the product of cardiac output and arterial venous oxygen difference. Factors limiting Vo(2) (ie, maximal Vo(2)) with exercise have therefore been traditionally sought within the determinants of cardiac function. However, such an approach ignores a large body of research evidence indicating that peripheral factors, particularly arteriolar dilatation and skeletal muscle pump function, control circulatory responses to exercise rather than central cardiac mechanisms. Efforts to understand the limiting factors for physiologic aerobic fitness are thus more appropriately directed toward characterizing these peripheral determinants of blood flow.

Hyperdynamic circulation of arteriovenous fistula preconditions the heart and limits infarct size

BACKGROUND

Chronic arteriovenous fistulae (AVF) create sustained hyperdynamic circulation. It is not known whether hyperdynamic circulation alters myocardial sensitivity to ischemia and reperfusion injury. We tested the hypothesis that AVF activate molecular responses that increase tolerance to infarction in dogs.

METHODS

Twelve dogs were divided into two groups: 1) AVF group, where an AVF in the femoral region was done; and 2) sham-operated group (each n = 6). After 8 weeks, left ventricular performance was determined from stroke work/end-diastolic length relationship. Myocardial biopsy was obtained to determine heat-shock protein 70 and adenosine triphosphate (ATP) pool. Left anterior descending coronary artery was occluded for 90 minutes at 37 degrees C, followed by 4 hours of reperfusion. Coronary blood flow was determined using different colored microspheres.

RESULTS

The fistula group showed improvement of left ventricular performance (p = 0.03). The infarct size was significantly lower in the fistula group; it was 9.2+/-2.0% in the fistula group versus 28.4+/-5.2% in the sham group (p < 0.05). ATP depletion during ischemia was less in the fistula group (p = 0.02). Regional myocardial blood flow was significantly higher in the fistula group (p = 0.03).

CONCLUSIONS

Peripheral AVF improve the left ventricular performance, and decrease infarct size and ATP depletion. This protective effect is caused by the development of collaterals in the coronary circulation without expression of heat-shock protein 70.

The behavior of the Nicoladoni-Branham sign at rest and under effort

The Nicoladoni-Branham sign may change because of exercise. Both dogs with femoral AVF and patients with lower-extremity AVF showed an increase in the Nicoladoni-Branham reaction after exercise as compared with its magnitude at rest. In both groups the AVFs were located near active muscles. We did not find this behavior in 3 patients with AVFs at points distant from the muscles used in exercise.

Circulation in neonates with intracranial arteriovenous fistula and cardiac failure

Two newborn infants with severe cardiac failure caused by a large cerebral arteriovenous communication were studied with complete cardiac catheterization, indicator-dilution curves and angiography. In one infant, studied at age 10 hours, a large right to left shunt through the patent ductus was seen with retrograde aortic flow into the left carotid artery. The entire flow in the descending aorta was supplied from the ductus. The second infant, studied at age 5 days, had a 20 percent right to left shunt through the foramen ovale and the ductus was closed. Hypoxia was caused by inadequate oxygenation of pulmonary venous blood, atrial right to left shunting and possibly ductal right to left shunting. The hemodynamic findings in cases of cerebral arteriovenous fistula would seem to depend on the patient's age at the time the studies are carried out and the severity of the lesion. Cardiac output was more than twice the normal value and blood flow through the arteriovenous fistula was probably greater than 4 liters/min per m2.

Cardiopulmonary dynamics during arteriovenous perfusion: cardiac output, pulmonary arterial resistance, and right venticular stroke work

The effects of arteriovenous perfusion on myocardial performance, pulmonary circulatory dynamics, and organ preservation were studied in dogs in order to evaluate the applicability of arteriovenous extracorporeal membrane oxygenation for partial respiratory support. Studies were made on normally oxygenated dogs, on dogs made hypoxic by oxygen deprivation and ventilatory depression, and on dogs with pulmonary injury and moderate pulmonary hypertension induced by injection of oleic acid into the right ventricle. The latter two groups were included to assess the changes that occur in cardiopulmonary dynamics in the presence of hypoxia and pulmonary hypertension. A microchannel membrane oxygenator was employed to provide partial respiratory support to the group of animals with pulmonary injury. The results indicate that no deleterious hemodynamic changes occurred in any of the groups of animals with arteriovenous flow for periods of up to 5 hours at rates corresponding to 20 to 35% of the total cardiac output. Adequate systemic oxygenation was provided by the microchannel oxygenator at these rates of arteriovenous perfusion.

Measurement of mitral regurgitation in man by the upstream sampling method using continuous indicator infusions

The upstream sampling method for measuring mitral regurgitation was evaluated in 19 patients during retrograde left ventricular and transseptal left atrial catheterization. Measurements obtained by using continuous infusions of indicator were compared, in all patients, with those obtained by using sudden single injections and, in 14 patients, with semiquantitative estimates by mitral valvulography. The mean values for regurgitant flow, total flow, and the regurgitant fraction did not differ significantly for the two dilution techniques. However, measurements of forward flow following sudden injections into the left ventricle were significantly larger than those following sudden injection into the pulmonary artery, while the latter differed insignificantly from measurements during continuous infusions into the left ventricle. Although neither technique had excellent reproducibility, the continuous-infusion method was clearly superior in this respect. The authors concluded that (1) a single measurement of mitral regurgitation by upstream sampling has a probability of large error, (2) continuous infusions in place of sudden injections give more reliable estimates, but (3) continuous infusion measurements are reliable only for mean values from replicate determinations.

Abnormal arteriovenous communications. Great variability of effects with particular reference to delayed development of cardiac failure

Clinical experiences and experimental studies provide evidence that an abnormal communication between the arterial and venous systems introduces into the circulation two circuits of flowing blood, one characterized by high arterial pressure and high peripheral resistance, and a second system bypassing part of the first, thus introducing a shorter circuit back to the heart, characterized by low pressure and low resistance, each supplied with flowing blood from the same central source, the heart. This shorter circuit acts solely as a parasitic circuit engrafted upon the normal circulation, serving no useful purpose, but capable of producing serious deleterious effects, depending upon the volume of blood diverted from the normal circulation into the shorter or parasitic circuit.

Intimately dependent upon this diversion of differing volumes of blood into the shorter circuit is the great diversity in the effects of a peripheral fistula upon the heart: the complete absence of cardiac dilatation in some cases; the rapid development of cardiac dilatation and cardiac failure within 2½ months as exemplified in Mason's case; and the long delay of 57 years in the development of cardiac failure as observed by Dorney.

Due to the high pressure in the normal circuit and the low resistance in the fistula circuit, there is a constant tendency for blood to be diverted or sequestered, as it were, in the fistulous or parasitic circuit, the factors determining the volume of blood so diverted being the size of the fistula and the distensibility of its rim or border, this, in turn, being dependent upon the character of the fibrous tissue deposited around the fistula in the course of healing. Firm, rigid, and excessive scarring permits only a small volume of blood to be sequestered in the fistula circuit, dilating only mildly the components of the parasitic circuit, whereas an elastic and distensible fistulous border permits progressive expansion of the fistulous opening with a progressive increase in the volume of blood sequestered in the fistula circuit, causing progressive dilatation of all components of this circuit including the four chambers of the heart, the aorta and artery proximal to the fistula, the fistula itself, the proximal vein, the vena cava, and the pulmonary vascular bed.

All gradations in distensibility of the fistula may occur, and hence all gradations in cardiac dilatation will occur. Also, all gradations in the rapidity of fistulous expansion may occur and, therefore, all gradations in rapidity of cardiac dilatation will occur.

This concept of a progressive expansion of a fistula of slow or rapid development permitting more and more blood to be shunted or sequestered in a shorter parasitic circuit at very rapid or very slow rates of increase provides the long-sought explanation of the rapid or long-delayed development of cardiac dilatation and failure in the presence of a peripheral fistula.

It is suggested that the same explanation is applicable to the effects of all abnormal arteriovenous communications, including intracardiac shunts and even some hemangiomata, since all are subject to the same hydrodynamic law that flowing blood like flowing water seeks the path of least resistance.

Hemodynamic Effects of Cerebral Arteriovenous Aneurysms

The hemodynamic alterations produced by large cerebral arteriovenous aneurysms are qualitatively similar to those caused by arteriovenous aneurysms elsewhere in the body. Congestive heart failure was not observed in our eight patients; it appears to be unusual in adult cases.

By comparing cardiac outputs with a control group the mean aneurysm blood flow in six patients was estimated to be 2,804 ml. per minute at rest. Isoproterenol increased cardiac output relatively more than the estimated aneurysm flow.

Implications of the present data with respect to the concept of "central" blood volume are discussed.

Progressive mental and neurologic deterioration is the rule with large cerebral arteriovenous aneurysms. Repeated bleeding episodes, which may not be obvious clinically, appear to be partially responsible.