Effect of changes in aortic pressure and in coronary arterial pressure on left ventricular geometry and function Anrep vs. gardenhose effect

Non-linearity of end-systolic pressure-volume relation in afterload increases is caused by an overlay of shortening deactivation and the Frank-Starling mechanism

The linearity and load insensitivity of the end-systolic pressure-volume-relationship (ESPVR), a parameter that describes the ventricular contractile state, are controversial. We hypothesize that linearity is influenced by a variable overlay of the intrinsic mechanism of autoregulation to afterload (shortening deactivation) and preload (Frank-Starling mechanism). To study the effect of different short-term loading alterations on the shape of the ESPVR, experiments on twenty-four healthy pigs were executed. Preload reductions, afterload increases and preload reductions while the afterload level was increased were performed. The ESPVR was described either by a linear or a bilinear regression through the end-systolic pressure volume (ES-PV) points. Increases in afterload caused a biphasic course of the ES-PV points, which led to a better fit of the bilinear ESPVRs (r2 0.929 linear ESPVR vs. r2 0.96 and 0.943 bilinear ESPVR). ES-PV points of a preload reduction on a normal and augmented afterload level could be well described by a linear regression (r2 0.974 linear ESPVR vs. r2 0.976 and 0.975 bilinear ESPVR). The intercept of the second ESPVR (V0) but not the slope demonstrated a significant linear correlation with the reached afterload level (effective arterial elastance Ea). Thus, the early response to load could be described by the fixed slope of the ESPVR and variable V0, which was determined by the actual afterload. The ESPVR is only apparently nonlinear, as its course over several heartbeats was affected by an overlay of SDA and FSM. These findings could be easily transferred to cardiovascular simulation models to improve their accuracy.

Independency of myocardial stunning of endothelial stunning?

BACKGROUND AND OBJECTIVE

Vascular endothelial cells play an important role in the control of vascular tone. The reasons for coronary endothelial dysfunction are complex and may involve ischemia/reperfusion injury. We investigated whether endothelial, smooth muscle, and myocardial dysfunction are independent phenomena.

METHODS

Rabbit hearts were rapidly excised without intermittent ischemia, connected to a modified Langendorff apparatus, and perfused with a modified Krebs-Henseleit solution containing bovine erythrocytes. Normoxic control hearts (n = 16) were perfused for 125 min. Postischemic hearts (n = 15) were perfused for 45 min, submitted to global ischemia (20 min) and reperfused (60 min). Both the normoxic and the postischemic hearts were divided into three groups that received either 0.9% NaCl (placebo), or 3-morpholinosydnonimine (SIN-1; 100 microM),or substance P (SP; 5 nM).

RESULTS

After SIN-1, CBF in the normoxic hearts was increased by maximum 63% and after SP by 62%. 60 min after the onset of reperfusion, the postischemic hearts of both groups had recovered to 95% LVP(max). In the postischemic hearts, SIN-1 increased CBF still by 58%, while the endothelium-dependent vasomotion was impaired: SP improved CBF by only 9%.

SUMMARY AND CONCLUSIONS

The particular protocol permitted differentiation between myocardial and vascular stunning. The results show that, while myocardial function has already recovered, endothelial cells are more severely impaired than smooth muscle cells, and that this injury persists beyond myocardial stunning. Thus, endothelial-dependent dysfunction can still impair vasodilatation while ventricular dysfunction has already resolved.

Cross-Talk Between Cardiac Muscle and Coronary Vasculature

The cardiac muscle and the coronary vasculature are in close proximity to each other, and a two-way interaction, called cross-talk, exists. Here we focus on the mechanical aspects of cross-talk including the role of the extracellular matrix. Cardiac muscle affects the coronary vasculature. In diastole, the effect of the cardiac muscle on the coronary vasculature depends on the (changes in) muscle length but appears to be small. In systole, coronary artery inflow is impeded, or even reversed, and venous outflow is augmented. These systolic effects are explained by two mechanisms. The waterfall model and the intramyocardial pump model are based on an intramyocardial pressure, assumed to be proportional to ventricular pressure. They explain the global effects of contraction on coronary flow and the effects of contraction in the layers of the heart wall. The varying elastance model, the muscle shortening and thickening model, and the vascular deformation model are based on direct contact between muscles and vessels. They predict global effects as well as differences on flow in layers and flow heterogeneity due to contraction. The relative contributions of these two mechanisms depend on the wall layer (epi- or endocardial) and type of contraction (isovolumic or shortening). Intramyocardial pressure results from (local) muscle contraction and to what extent the interstitial cavity contracts isovolumically. This explains why small arterioles and venules do not collapse in systole. Coronary vasculature affects the cardiac muscle. In diastole, at physiological ventricular volumes, an increase in coronary perfusion pressure increases ventricular stiffness, but the effect is small. In systole, there are two mechanisms by which coronary perfusion affects cardiac contractility. Increased perfusion pressure increases microvascular volume, thereby opening stretch-activated ion channels, resulting in an increased intracellular Ca2+transient, which is followed by an increase in Ca2+sensitivity and higher muscle contractility (Gregg effect). Thickening of the shortening cardiac muscle takes place at the expense of the vascular volume, which causes build-up of intracellular pressure. The intracellular pressure counteracts the tension generated by the contractile apparatus, leading to lower net force. Therefore, cardiac muscle contraction is augmented when vascular emptying is facilitated. During autoregulation, the microvasculature is protected against volume changes, and the Gregg effect is negligible. However, the effect is present in the right ventricle, as well as in pathological conditions with ineffective autoregulation. The beneficial effect of vascular emptying may be reduced in the presence of a stenosis. Thus cardiac contraction affects vascular diameters thereby reducing coronary inflow and enhancing venous outflow. Emptying of the vasculature, however, enhances muscle contraction. The extracellular matrix exerts its effect mainly on cardiac properties rather than on the cross-talk between cardiac muscle and coronary circulation.

Response of the intact canine left ventricle to increased afterload and increased coronary perfusion pressure in the presence of coronary flow autoregulation

BACKGROUND

Increased left ventricular (LV) contractile force or oxygen consumption has been documented with increased coronary arterial pressure (CAP) and flow (Gregg phenomenon). We investigated whether the increase in contractile force with increased LV afterload might be mediated by the concomitant increase in CAP when coronary autoregulation is intact.

METHODS AND RESULTS

The LV of 6 autonomically blocked open-chest dogs was perfused through the left main coronary artery by a cannula with a side gate to the aortic root. With the gate open, CAP increased from 77+/-20 to 93+/-20 mm Hg (P<0.05) with aortic constriction (AC). With the gate closed, CAP was maintained at a constant level of 100 mm Hg. A small reduction in the slope of the preload recruitable stroke work (PRSW) relationship was observed with AC, but this response was not altered by the coronary perfusion gate position. The end-systolic pressure-volume (ESPV) relationship shifted upward significantly with AC (P<0.001), but this shift was not greater with open-gate perfusion than with closed-gate perfusion. Furthermore, with coronary autoregulation intact, wide changes in CAP (between 60 and 180 mm Hg, n=5) did not alter either the PRSW or ESPV relationship. In contrast, when autoregulation was abolished with intracoronary adenosine (n=6), both indexes of contractility increased progressively with increased CAP.

CONCLUSIONS

The concomitant increase in CAP with increased afterload in the intact canine LV does not contribute to the afterload-induced increase in contractile force. Coronary perfusion pressure per se does not influence LV contractile function. Coronary perfusion pressure influences contractility only when coronary flow changes.

Minds and hearts: themes in the life of Gleb von Anrep

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The rate of force generation by the myocardium is not influenced by afterload

The influence of afterload on the rate of force generation by the myocardium was investigated using two types of preparations: the in situ dog heart (dP/dt) and isolated papillary muscle of rats (dT/dt). Thirteen anesthetized, mechanically ventilated and thoracotomized dogs were submitted to pharmacological autonomic blockade (3.0 mg/kg oxprenolol plus 0.5 mg/kg atropine). A reservoir connected to the left atrium permitted the control of left ventricular end-diastolic pressure (LVEDP). A mechanical constriction of the descending thoracic aorta allowed to increase the systolic pressure in two steps of 20 mmHg (conditions H1 and H2) above control values (condition C). After arterial pressure elevations (systolic pressure C: 119 +/- 8.1; H1: 142 +/- 7.9; H2: 166 +/- 7.7 mmHg; P < 0.01), there were no significant differences in heart rate (C: 125 +/- 13.9; H1: 125 +/- 13.5; H2: 123 +/- 14.1 bpm; P > 0.05) or LVEDP (C: 6.2 +/- 2.48; H1: 6.3 +/- 2.43; H2: 6.1 +/- 2.51 mmHg; P > 0.05). The values of dP/dt did not change after each elevation of arterial pressure (C: 3,068 +/- 1,057; H1: 3,112 +/- 996; H2: 3,086 +/- 980 mmHg/s; P > 0.05). In isolated rat papillary muscle, an afterload corresponding to 50% and 75% of the maximal developed tension did not alter the values of the maximum rate of tension development (100%: 78 +/- 13; 75%: 80 +/- 13; 50%: 79 +/- 11 g mm-2 s-1, P > 0.05). The results show that the rise in afterload per se does not cause changes in dP/dt or dT/dt.

Relaxation-systolic pressure relation. A load-independent assessment of left ventricular contractility

This contribution reviews the regulation of left ventricular pressure (LVP) fall by load and relates this regulation to left ventricular contractility. Load regulation of LVP fall has to be distinguished from neurohumoral regulation, from effects induced by arterial reflected waves and from long-term load effects on contractility. The response of LVP fall to a moderate elevation of systolic LVP is highly variable. It depends on the ratio between the actual systolic pressure and peak isovolumetric pressure, defined as "relative load". Up to a relative load of 81% to 84%, LVP fall accelerates. Above this relative load, LVP fall decelerates. Depending on the level of relative load there is a wide variety of effects ranging from moderate acceleration of LVP fall to marked deceleration of LVP fall. Acceleration of LVP fall in response to a load elevation is associated with normal cardiac function, while slowing of LVP fall is associated with impaired cardiac function. Similar but opposite effects are observed with reductions of systolic LVP. Effects of changes in systolic LVP on time constant tau reveal a fair correlation with systolic elastance (Ees), peak dP/dtmax and regional fractional shortening (or ejection fraction). There is an excellent correlation with measured isovolumetric LVP, indicating that contraction-relaxation coupling is close when contractility is expressed in terms of peak isovolumetric pressure. Assessment of contractility with systolic LVP-relaxation relation is precise and load independent and can be performed with the sole use of a high-fidelity pressure gauge positioned in the left ventricular cavity.

Haemodynamic effects of descending aortic occlusion during cardiopulmonary resuscitation

We describe two patients who both suffered a cardiac arrest whilst maintained on an intra-aortic balloon pump. In an attempt to improve coronary and cerebral blood flow during cardiopulmonary resuscitation, the intra-aortic balloon was inflated to occlude the descending aorta and preferentially direct blood to the cerebral and coronary circulation. In case 1, mean radial artery pressure rose from 71/14 mmHg (mean = 33 mmHg) to 92/24 mmHg (mean = 47 mmHg). Diastolic right atrial pressure was 16 mmHg both with the balloon deflated and inflated. In patient 2, mean radial artery pressure rose from 48/21 mmHg (mean = 25 mmHg) to 62/26 mmHg (mean = 36 mmHg). Right atrial pressure was 90/6 mmHg (mean 34 mmHg) with the balloon deflated and 104/8 mmHg (mean = 40 mmHg) with the balloon inflated. Coronary artery perfusion pressure in case 1 increased from -2 to 8 mmHg and in case 2 increased from 15 to 18 mmHg. These results suggest that occlusion of the descending aorta during cardiac massage may improve coronary and cerebral perfusion pressures. Animal studies are consistent with these findings and show that aortic occlusion may significantly improve outcome from cardiac arrest.