Transmural Pressure Drop

The compressive effect is due to the transmural pressure drop P, (y)-PJj, t) across the microvessels. A negative transmural pressure drop will generate collapse of the vessel and halt the flow in the vessel. The rigidity of the vessel s wall determines its resistance to collapse and determines the transmural pressure which is associated with the collapse phenomena. The combination of all these parameters determines the well known spatial dependence of P f on the LV pressure and the coronary tonus (Bellamy, 1978 1980 Sherman, 1980). 

FIGURE 4.11 Choked flow through a flexible tube, (a) upstream pressure P , downstream pressure Pj, external pressure P flow F, ptes-suie P, cross-sectional area A. (6) A A versus transmural pressure with shapes indicated, (c) Flow versus pressure drop, assuming P, is decreased and P, is fixed. 

A graph of P plotted as a function of Q (Figure 4) has been termed a vascular function curve (Levy, 1979 Berne and Levy, 1981). From eq. 5, it is evident that the slope of the relationship between P and Q depends only on R, C and C . Changes in flow have an inverse effect on P, i.e., as 0 is increased, there will be a proportionate decrease in P. There is a limit to the reduction of P that can be produced by an increase in 0, however. At some critical maximum value of 0, sufficient fluid will be translocated from the venous to the arterial side of the circuit such that P, will drop below the ambient pressure. In a system of distensible tubes, the venous system will be collapsed by this negative transmural pressure (P minus ambient pressure). This will, of course, limit the maximum value of cardiac output regardless of the capabilities of the pump. 

ANS First of all, we have no model, in the sense that it is being used in this conference. Steal, in the usual vascular sense, only occurs in the presence of a stenosis. There are steal phenomenon which also include a collateral vessel. The principle is the same. That is, there are two resistors distal to a third stenosis upstream and those two resistors interact because there is a pressure drop across the upstream stenosis. It can cause subclavean steal, for example, where there are cerebral effects or it can cause transmural steal in the heart. That is, if there is a coronary stenosis, the epicardium steals from the endocardium and therefore the inner/outer ratio is below unity. This is thought of as a steal phenomenon. 

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