Pulsatile Flow During Cardiopulmonary Bypass: Is it Beneficial?

____________ _ (}. Extra-Corpor. Techno/. 20[ 1]: p. 24-29, 60 references, Spring 1988) Pulsatile perfusion has been a topic of great controversy for many years. This paper reviews historical aspects of pulsatile perfusion, as well as current and conceptual aspects of pulsatile perfusion. The positive and negative points of pulsatile perfusion are discussed so the reader may draw conclusions in an informed way. Although organ preservation and metabolic functions point towards pulsatile perfusion positively, the current knowledge demonstrates that further research would be beneficial and also notes that pulsatile perfusion is beneficial in select cases. Introduction ____________ _ The controversy over pulsatile and nonpulsatile perfusion during cardiopulmonary bypass continues today without great prospects for resolution. This review will attempt to bring out aspects of pulsatile perfusion that may be beneficial in certain cases. The body of literature is relevant to the topic and will be followed by a short conclusion. I: The History of Pulsatile Perfusion __ _ The debate concerning the importance of the pulses started with the conflicting views of Appocrates, Aristotle, and Galen. 1 Two hundred years after the discovery by Harvey2 of the relationship of the pulses to the circulation, HameP reported one of the first laboratory studies of the pulses. The introduction of clinical cardiopulmonary bypass (CPB) stimulated the initial laboratory studies of pulsatile and nonpulsatile perfusion by Direct communications to: Martin Casper, 3491 Pointe Willow Lane, Las Vegas, Nevada 89120. Volume 20, Number 1, Spring 1988 Wesolowski and associates. · These studies showed no pronounced difference between the two forms of perfusion. Along with the clinical successes quickly accumulated with the simple and efficient nonpulsatile roller pump, the laboratory work of Wesolowski's group made nonpulsatile perfusion the "gold standard" for CPB. After the appearance of these studies, the controversy abated and pulsatile perfusion was relegated to physiology laboratories. 6 With the advent of ventricular assist devices (VAD), intra-aortic balloon pumping (IABP) and new perfusion technology, nonpulsatile CPB is being seriously challenged as the clinical standard.7 II: Physiological Hemodynamics ___ _ The study of the blood flow as a pulsatile phenomenon is scarcely new, but it has gained new impetus from three developments: l) The commercial production of reliable flowmeters, 2) the formulation of theories appropriate to oscillatory flow in blood vessels/ 3) and the increasing availability of digital computers. 8 Simple inspection of the contour of pressure and flow waves is instructive, but quantitative analysis yields additional information. There are two reasons in particular for believing that such an analysis will prove useful in clinical medicine. One is that normal function in at least some organs seems to depend on the size of the pressure and flow pulsations that reach the microcirculation. The other is that abnormal transmission of these pulse waves from the heart out to the periphery has been found in some pathologic states. Since these facts imply that disturbances of pulsatile flow may account for some manifestations of disease, they are worth considering in more detail before going on to ways of analyzing and interpreting pulsations. 8 Experiments in which constant flow has been substituted for the normal pulsatile perfusion of the kidneys over a long period of time have shown a reduction in urine volume. Similar The Journal of Extra-Corporeal Technology 25 experiments on the circulation to the brain have been accompanied by a decrease in cerebral oxygen consumption. In addition, experience with surgical procedures that require prolonged CPB suggest that the vascular bed itself functions best if exposed to pulsatile pressure. If the arterial flow provided by a mechanical pump is nonpulsatile, or nearly so, there is often a gradual rise in peripheral resistance and mortality. 8 The mechanism of these changes, if in fact they depend on the presence or absence of pulsation rather than some factor not yet identified, must reside in the microcirculation. 8 Transcapillary exchange is controlled in part by intracapillary pressure, including the magnitude of pulsation as well as mean pressure. These pulsations amount to several millimeters of mercury and can be increased or decreased by changes in the transmission properties of the arterial tree. Whether a capillary is open, closed, or partially collapsed also depends on transmural pressure, and here, too, the pulsations of pressure presumably have an influence. This effect has been demonstrated in the pulmonary circulation, and probably occurs everywhere, so that the concept of "critical closing pressure" should include pulsatile as well as mean pressure, and perhaps pulse frequency. Arterioles and venules are normally subjected to pulsatile pressures that may, because of the active response of vascular smooth muscle to stretching, influence local vascular tone. In addition to these somewhat speculative possibilities, there is the well-documented fact that'arterial pulsations provide much of the driving force for lymphatic flow. 8 Wilkins and associates suggested in 1962 that the physical properties of pulsatile flow better maintained microcirculation, lymphatic flow, and aerobic metabolism. Nonpulsatile flow was said to produce stagnation of the microcirculation thereby opening arteriovenous shunts and producing poor lymph flow and edema, culminating in a shift toward anaerobic metabolism.9 Through direct observation of the cerbral and conjunctival microcirculation, Matsumoto and colleagues noted vasodilation of venules, sludging in the microcirculation, and edema formation during nonpulsatile flow. These changes were not seen with pulsatile flow. 10 Flow in normal capillaries has been shown to be markedly pulsatile. 11 Indirect evidence indicates that more edema results from nonpulsatile bypass. Higher fluid requirements, larger transfusion volumes, and increased generalized edema have been noted during nonpulsatile CPB. 6 Taylor et al. reported no significant differences in blood counts and plasma free hemoglobin between pulsatile and nonpulsatile CPB, suggesting that pulsatile CPB is as safe as non pulsatile CPB from a standpoint of hematological dynamics. 12 26 The Journal of Extra-Corporeal Technology Dunn et al. noted that the most consistent difference between the pulsatile and nonpulsatile groups was the lower systemic vascular resistance (SVR) during CPB. 13 Pulseless blood flow alters the pattern of carotid and aortic baroreceptor impulses which can elevate SVR, increase lactate acid production, and decrease oxygen consumption, 14 whereas pulsatile perfusion significantly decreases SVR. 12 The increased clinical awareness of excessive vasoconstriction after CPB procedures has been reflected in studies concerned with the pathophysiology and treatment of the elevation in SVR. Vasoconstriction is a potentially hazardous situation in the early post-bypass period, since left ventricular work is increased and sub-endocardial perfusion may be significantly decreased. 15·17 Therefore, the use of pulsatile perfusion during CPB offers the possibility, of preventing or minimizing the potentially harmful elevation in SVR during the postbypass period. 12 III Metabolic Components ______ _