Determinants of right ventricular afterload (2013 Grover Conference series)

Abstract

Right ventricular (RV) afterload consists of both resistive and capacitive (pulsatile) components. Total afterload can be measured directly with pulmonary artery input impedance spectra or estimated, either with lumped-parameter modeling or by pressure-volume analysis. However, the inverse, hyperbolic relationship between resistance and compliance in the lung would suggest that the pulsatile components are a predictable and constant proportion of the resistive load in most situations, meaning that total RV load can be estimated from mean resistive load alone. Exceptions include elevations in left atrial pressures and, to a lesser extent, chronic thromboembolic disease. The pulsatile components may also play a more significant role at normal or near-normal pulmonary artery pressures. Measures of coupling between RV afterload and RV contractility may provide important information not apparent by other clinical and hemodynamic measures. Future research should be aimed at development of noninvasive measures of coupling.

Keywords: afterload; arterial compliance; pulmonary hypertension; pulmonary vascular resistance; right ventricle.

Figure 1

Example of pulmonary artery impedance spectra after Fourier transformation: modulus (amplitude of pressure divided by amplitude of flow) is plotted against frequency. Z₀ is equivalent to the ratio mean pressure/mean flow; Z₁ (impedance at the first harmonic) represents a significant amount of total blood flow and is largely influenced by wave reflection; Z_C (characteristic impedance) is the ratio of blood mass inertia to the proximal vessel compliance; f_MIN (frequency at the first impedance minimum) is a function of pulse wave velocity as well as the distance to the major sites of reflection.

Figure 2

Three-element Windkessel model of lumped parameters used to mimic impedance and estimate ventricular afterload. Z_C: characteristic impedance; C: compliance; R: resistance.

Figure 3

A, Right ventricular (RV) pressure-volume loop. The width of the loop is stroke volume (dashed line). RV afterload can be described as the sum of RV pressures throughout ejection (gray line). B, Effective arterial elastance (E_A), a lumped measure of afterload, is the slope of the gray line connecting the points (end-systolic pressures, end-systolic volume) and (0, end-diastolic pressure). E_A is calculated as end-systolic pressure/stroke volume. PV: pulmonary valve.

Figure 4

Cartoon illustrating right ventricular pressure-volume loops at varying levels of preload decline. The slope of the gray line connecting the end-systolic pressure points is the end-systolic elastance (E_ES), a load-independent measure of contractility. The E_ES can be compared with effective arterial elastance (E_A) to assess coupling of right ventricular contractility to pulmonary vasculature load.

Figure 5

Inverse, hyperbolic relationship between pulmonary vascular resistance and pulmonary vascular compliance.

Figure 6

Shift of the pulmonary resistance-compliance curve to the left with elevations in pulmonary capillary wedge pressure (PCWP).

Figure 7

RC time (product of resistance × compliance) plotted against pulmonary vasculature resistance (PVR; A) and mean pulmonary artery pressures (mPAP; B). Groups are separated into normal and elevated PVR (A) and normal and elevated mPAP (B), with linear regression lines drawn for each group.