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

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.

Contractile effects of adenosine A1 and A2A receptors in isolated murine hearts

The adenosine A1 receptor (A1R) inhibits β-adrenergic-induced contractile effects (antiadrenergic action), and the adenosine A2A receptor (A2AR) both opposes the A1R action and enhances contractility in the heart. This study investigated the A1R and A2AR function in β-adrenergic-stimulated, isolated wild-type and A2AR knockout murine hearts. Constant flow and pressure perfused preparations were employed, and the maximal rate of left ventricular pressure (LVP) development (+dp/d tmax) was used as an index of cardiac function. A1R activation with 2-chloro- N6-cyclopentyladenosine (CCPA) resulted in a 27% reduction in contractile response to the β-adrenergic agonist isoproterenol (ISO). Stimulation of A2AR with 2- P(2-carboxyethyl)phenethyl-amino-5′- N-ethylcarboxyamidoadenosine (CGS-21680) attenuated this antiadrenergic effect, resulting in a partial (constant flow preparation) or complete (constant pressure preparation) restoration of the ISO contractile response. These effects of A2AR were absent in knockout hearts. Up to 63% of the A2AR influence was estimated to be mediated through its inhibition of the A1R antiadrenergic effect, with the remainder being the direct contractile effect. Further experiments examined the effects of A2AR activation and associated vasodilation with low-flow ischemia in the absence of β-adrenergic stimulation. A2AR activation reduced by 5% the depression of contractile function caused by the flow reduction and also increased contractile performance over a wide range of perfusion flows. This effect was prevented by the A2AR antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-241385). It is concluded that in the murine heart, A1R and A2AR modulate the response to β-adrenergic stimulation with A2AR, attenuating the effects of A1R and also increasing contractility directly. In addition, A2AR supports myocardial contractility in a setting of low-flow ischemia.

Preload-adjusted right ventricular maximal power: concept and validation

Right ventricular (RV) maximal power (PWRmx) is dependent on preload. The objective of this study was to test our hypothesis that the PWRmx versus end-diastolic volume (EDV) relationship, analogous to the load-independent stroke work (SW) versus EDV relationship (preload-recruitable SW, PRSW), is linear, with the PWR x-axis intercept (V0PWR) corresponding to the PRSW intercept (V0SW). If our hypothesis is correct, the preload sensitivity of PWRmx could be eliminated by adjusting for EDV and V0PWR. Ten dogs were instrumented with a pulmonary flow probe, micromanometers, and RV conductance catheter. Data were obtained during bicaval occlusions under various conditions and fitted to PWRmx = a·(EDV − V0PWR)β, where a is the slope of the relationship. The PWRmx versus EDV relationship did not deviate from linearity (β = 1.09, P = not significant vs. 1), and V0PWR correlated with V0SW ( r = 0.93, P <0.0001). V0PRW was related to steady-state EDV and left ventricular end-diastolic pressure, allowing for estimation of V0PWR (V0Est) and single-beat PWRmx preload adjustment. Dividing PWRmx by the difference of EDV and V0PWR (PAMPV0PWR) eliminated preload dependency down to 50% of the baseline EDV. PWRmx adjustment using V0Est (PAMPV0Est) showed similar preload independency. Enhancing contractility increased PAMPV0PWR and PAMPV0Est from 176 ± 52 to 394 ± 205 W/ml × 104 and 145 ± 51 to 404 ± 261 W/ml × 104, respectively, accompanied by an increase of PRSW from 13.0 ± 4.5 to 29.7 ± 16.4 mmHg (all P <0.01). PAMPV0PWR and PAMPV0Est correlated with PRSW ( r = 0.85; r = 0.77; both P <0.001). Numerical modeling confirmed the accuracy of our experimental data. Thus preload adjustment of PWRmx should consider a linear PWRmx versus EDV relationship with distinct V0PWR. PAMPV0PWR is a preload-independent estimate of RV contractility that may eventually be determined noninvasively.

The role of cardiac power and systemic vascular resistance in the pathophysiology and diagnosis of patients with acute congestive heart failure

OBJECTIVE

Conventional hemodynamic indexes (cardiac index (CI), and pulmonary capillary wedge pressure) are of limited value in the diagnosis and treatment of patients with acute congestive heart failure (CHF).

PATIENTS AND METHODS

We measured CI, wedge pressure, right atrial pressure (RAP) and mean arterial blood pressure (MAP) in 89 consecutive patients admitted due to acute CHF (exacerbated systolic CHF, n=56; hypertensive crisis, n=5; pulmonary edema, n=11; and cardiogenic shock, n=17) and in two control groups. The two control groups were 11 patients with septic shock and 20 healthy volunteers. Systemic vascular resistance index (SVRi) was calculated as SVRi=(MAP-RAP)/CI. Cardiac contractility was estimated by the cardiac power index (Cpi), calculated as CIxMAP.

RESULTS AND DISCUSSION

We found that CI<2.7 l/min/m(2) and wedge pressure >12 mmHg are found consistently in patients with acute CHF. However, these measures often overlapped in patients with different acute CHF syndromes, while Cpi and SVRi permitted more accurate differentiation. Cpi was low in patients with exacerbated systolic CHF and extremely low in patients with cardiogenic shock, while SVRi was increased in patients with exacerbated systolic CHF and extremely high in patients with pulmonary edema. By using a two-dimensional presentation of Cpi vs. SVRi we found that these clinical syndromes can be accurately characterized hemodynamically. The paired measurements of each clinical group segregated into a specific region on the Cpi/SVRi diagnostic graph, that could be mathematically defined by a statistically significant line (Lambda=0.95). Therefore, measurement of SVRi and Cpi and their two-dimensional graphic representation enables accurate hemodynamic diagnosis and follow-up of individual patients with acute CHF.

Effect of acutely increased left ventricular afterload on work output from the right ventricle in conscious dogs

OBJECTIVE

To determine the effect of acute increments in left ventricular afterload on the stroke work output of the right ventricle in vivo.

METHODS

After pharmacologic attenuation of autonomic reflexes, left and right ventricular pressure-volume data were obtained in 9 conscious dogs during vena caval occlusions performed before and during aortic constriction.

RESULTS

The relationship between right ventricular stroke work and end-diastolic volume during vena caval occlusion was highly linear (r = 0.97 +/- 0.02), but the slope decreased by 20% +/- 13% during aortic constriction sufficient to increase left ventricular mean ejection pressure by 25% +/- 14% (P <.05). The volume-axis intercept remained constant. Similarly, the slope of the linear relationship between right ventricular free wall regional segment work and end-diastolic segment length declined by 22% +/- 10% during aortic constriction (P <.05), without significant change in the length-axis intercept. The reduction in both global and regional right ventricular stroke work at any given preload with increased left ventricular afterload was due entirely to decreased right ventricular stroke volume and free wall shortening, because right ventricular mean ejection pressure was unchanged. Additional experiments were performed in 5 open-chest dogs to produce a greater reduction in left ventricular free wall shortening than observed with aortic constriction by transient constriction of the left circumflex coronary artery. However, this intervention had no effect on right ventricular free wall segment work output.

CONCLUSION

Increased left ventricular afterload decreases global and regional right ventricular stroke work at any given preload, a direct, negative systolic ventricular interaction.