Zero-flow Pressure Intercept Research Articles

Instantaneous and steady-state pressure- flow relations of the coronary system in the canine beating heart

Steady-state and instantaneous pressure-flow relations were both obtained from the pump-perfused left coronary bed of the beating heart in seven mongrel dogs. The steady-state pressure-flow relation was obtained by changing flow, and measuring pressure after it reached a steady level; it showed a sigmoid shape, with flow-regulation around 70 ml . min-1 . 100 g-1, and it had an average zero-flow pressure intercept of 1.9 kPa (14 mmHg). This curve was represented by an equation, using four parameters. The quality of regulation of the coronary bed could be quantified with this equation by determining the pressure range, when flow was changed from 25% below to 25% above control level. We found this pressure range to be 8.7 +/- 2.4 kPa (65 +/- 18 mmHg) on the average. The tangent at each point of steady-state pressure-flow relation was called differential resistance. Instantaneous pressure-flow relations were obtained by superimposing stepwise changes of flow of different amplitude, at several steady-state levels of flow. Pressure followed these steps with a time-constant of 0.3 +/- 0.1 s, due to capacitive effects, then remained constant during 3 to 4 s, and thereafter changed due to regulation. Pressure was measured during the plateau, assuming it to be a regulation-free period. The instantaneous pressure-flow relations were found to be linear, and the slope was called instantaneous resistance. In the physiological range of flows, instantaneous resistance increased with flow. The ratio between instantaneous and differential resistance, the regulatory index, is suggested to quantify regulation at each point of the steady-state curve. This index was between one and zero up to the upper limit of the regulatory range; at higher flows it was negative. In the maximally vasodilated bed the instantaneous pressure-flow relations fell along the steady-state relation, and the regulatory index was thus equal to zero at all flow-levels.

Capacitive effects on canine coronary artery diastolic pressure-flow relationships.

The extent of capacitive effects on coronary diastolic pressure-flow relationships was investigated by proposing a theoretical model and by actually measuring the time constants of the coronary and systemic circulation. The dynamic pressure-flow relationship obtained by an aortic declining pressure perfusion and the static pressure-flow relationship obtained by constant pressure reservoir perfusion were compared to support the theoretical model, which revealed that the capacitive effects could be quantified by the alpha value, i.e. the ratio of the time constants of the coronary and systemic circulations. In 10 anesthetized dogs, the two independent methods disclosed that capacitive effects are limited and diastolic outflow pressure in the coronary circulation is the static zero-flow pressure intercept determined by the vascular tonus and intramyocardial pressure.

Interpretation and physiological significance of diastolic coronary artery pressure-flow relationships in the canine coronary bed.

We analyzed the relationship between diastolic coronary artery pressure and flow in the canine coronary bed, using an electrical analog model of the coronary circulation based on the theory of critical closure. The model contains a voltage-dependent nonlinear resistance and capacitance. The behavior of the resistive element was described using experimental diastolic pressure-flow curves obtained in the absence of compliance effects. Compliance free zero flow pressure intercepts (Pf0) exceeded coronary venous pressure (Pv) by 2- to 5-fold and were related to initial diastolic coronary artery pressure Pa(0) and flow F(0), and Pv by: Pf0 = 14.3 [(Pa(0) - Pv)/F(0)] + Pv + 4.0 (r = 0.93). When coronary artery pressure was suddenly lowered to values less than or equal to the compliance-free Pf0, diastolic flow abruptly decreased and, after a transient reversal, remained at zero for up to 8 seconds. In the model, zero flow pressure represents critical closing pressure and the resistance regulating flow is the difference between coronary artery and venous pressure divided by flow. Theoretically predicted pressure-flow curves were in good agreement with existing experimental data, including the effects of elevating coronary venous pressure on zero flow pressure. Differences between compliance-free pressure-flow curves and those obtained with pressure gradually decreasing were explained by a coronary arterial compliance whose magnitude varies inversely with pressure and is dependent on vasomotor tone. In conclusion, the results of this study demonstrate the existence of a diastolic pressure gradient across the canine coronary bed at zero flow which is dependent on coronary vasomotor tone. A theoretical model of the coronary circulation based on the concept of critical closure describes the observed relationship between diastolic coronary artery pressure and flow during various experimental conditions.

Steady state and instantaneous pressure-flow relationships: characterisation of the canine abdominal periphery.

This study was performed to characterise a vascular bed in terms of pressure-flow relationships. Steady state and instantaneous relationships were obtained in the flow perfused isolated femoral beds of six mongrel dogs. The steady state pressure-flow relations were obtained by applying a series of stepwise changes of flow in random order. The relations were found to be straight and to have a zero-flow pressure intercept (P0). The slope of this relation is the differential resistance (Rd). On each steady state flow level a ramp-flow was superimposed. The pressure response was measured between 1.5 and 5 s after the start of the ramp-flow, to exclude compliance effects and (auto) regulatory effects, respectively. In this way instantaneous pressure-flow relations were obtained, the slope of this relation is the instantaneous resistance (Ri). The instantaneous resistance expresses the true physical resistance value at a working point of the steady state pressure-flow relation before the bed has performed its (auto)regulatory adaptation after a change in flow. Instantaneous resistance therefore characterises the vascular state that exists at that particular working point. After this particular vascular state has been modified by (auto)-regulation the steady state pressure-flow relation is reached again. Instantaneous resistance increases with increasing flow thereby approximating the value of the differential resistance. At the same flow a vasodilator decreases and a vasoconstrictor increases instantaneous resistance. The gain (G) of the system, that characterises the (auto)regulatory capability, was calculated as G = 1--Ri/Rd and was found to decrease with increasing flow. The (partial) reflection of travelling waves depends on both the characteristic impedance (Zc) and the instantaneous resistance rather than on differential or peripheral resistance. Furthermore it is the product of the instantaneous resistance (Ri) and vascular compliance (C) that determines the time constant of a vascular bed.

Influence of autoregulation and capacitance on diastolic coronary artery pressure-flow relationships in the dog.

SUMMARY. We studied the influence of initial coronary artery pressure on pressure-flow relationships during long diastoles in 22 closed-chest, anesthetized dogs under basal conditions and after maximum coronary vasodilation. The circumflex artery was perfused from a pressurized arterial reservoir through a cannula inserted in the carotid artery. Diastolic pressure-flow relations were obtained under two conditions: (1) when pressure was gradually decreasing (40 mm Hg/sec); and (2) when pressure and flow were constant beginning 500 msec after a rapid step decrease in pressure. Pressure-flow curves obtained with pressure gradually decreasing were linear in the basal state and during maximum coronary vasodilation [r = 0.98 ± 0.01 (SE)]. In autoregulating animals, 25 mm Hg increments in coronary artery pressure from 75 to 125 mm Hg resulted in progressive decreases in the slope (2.2 ± 0.1 to 1.2 ± 0.1 ml/min per 100 g per mm Hg, P < 0.01) and increases in the zero flow pressure intercept (37.3 ± 1.4 to 51.2 ± 2.1 mm Hg, P < 0.01) of the pressure-flow curves. These changes were not observed when autoregulation was abolished with intracoronary adenosine. Diastolic pressure-flow curves obtained under constant pressure conditions were also linear in the basal state and during maximum coronary vasodilation (r = 0.98 ± 0.01). Increasing initial coronary artery pressure from 75 to 125 mm Hg resulted in a decrease in the slope (1.6 ± 0.1 to 0.9 ± 0.0 ml/ min per 100 g per mm Hg, P < 0.01) and an increase in the zero flow pressure intercept (21.1 ± 2.1 to 37.2 ± 2.3 mm Hg, P < 0.01) of the pressure-flow curves. With maximum coronary vasodilation, the slope increased to 4.7 ± 0.1 ml/min per 100 g per mm Hg (P < 0.001) and the zero flow pressure intercept decreased to 15.2 ± 0.7 mm Hg (P < 0.001). When pressure-flow curves obtained with pressure gradually decreasing were compared to ones obtained during constant pressure perfusion, the constant pressure values for slope averaged 10-25% lower (P < 0.01), whereas values for the zero flow pressure intercept were 8-12 mm Hg lower (P < 0.01). These data indicate that diastolic coronary artery pressure-flow relations obtained when pressure is continuously changing are influenced by coronary vascular capacitance. When capacitance effects were minimized, the slope and zero flow pressure intercept of the diastolic pressure-flow curves were dependent on coronary vasomotor tone, with the zero flow pressure intercept ranging from 15 to 37 mm Hg. These data are consistent with a vascular waterfall model of diastolic flow regulation. Based on such a model, we estimate that changes in coronary vascular resistance and zero flow pressure intercept can account for 70-80% and 20-30%, respectively, of the coronary autoregulation observed over the pressure range of 75 to 125 mm Hg. (Circ Res 57: 261-270, 1982)

Diastolic coronary artery pressure-flow relations in the dog.

Conscious dogs were used to investigate the relations between aortic (Pa) pressure and coronary flow (F) during individual diastoles. When the dogs were in a semibasal state, coronary pressure-flow relations were described by a family of lines, and diastolic flow was a linear function of aortic pressure. For a given perfusion pressure, higher flows were associated with lines of progressively greater slope and lower zero flow pressure intercept (Pf=0). Zero flow pressure intercepts were estimated by extrapolation and found to vary between 20 and 50 mm Hg, depending on the magnitude of flow. The zero flow pressure may represent the height of a vascular waterfall caused by vasomotor tone with the resistance-controlling coronary flow being (P-Pf=0).F-1. Interventions that decrease vasomotor tone increase coronary flow by both decreasing vascular resistance and increasing the perfusion pressure gradient. The gradient increases because the effective coronary back pressure is the height of the vascular waterfall and the latter is reduced when vasomotor tone falls. Passive changes in vessel dimensions, arterial recruitment, and autoregulation appear to be of little importance during individual diastoles.