Validation and utility of novel volume reduction technique for determination of parallel conductance

Sex-related changes in cardiac function following myocardial infarction in mice

Recent awareness of cardiovascular diseases as a number one killer of the middle-aged women has prompted interest in sex differences leading to heart failure (HF). Therefore, we evaluated cardiac function in female and male mice following myocardial infarction (MI) using the Millar pressure-volume (P-V) conductance system in vivo, at time points corresponding to early (2 wk), late compensatory hypertrophy (4 wk), and decompensation (10 wk) to HF. A significant deterioration of the load dependent and independent hemodynamic measurements occurred in both female and male mice during the early phase of hypertrophy. Later, compensatory hypertrophy was marked by a normalization of volumes to control levels in females compared with males. The most notable differences between sexes occurred in the measurements of cardiac contractility during the decompensation to HF. In females, there was a significant improvement in contractility compared with males, which was apparent in the load-independent measurements of preload recruitable stroke work (10 wk post-MI, female=48.7+/-8.0 vs. male=25.2+/-1.8 mmHg, P<0.05) and maximum dP/dt vs. maximum end-diastolic volume (10 wk post-MI, female=359+/-58 vs. male=149+/-28 mmHg.s(-1).microl(-1), P<0.05). Despite these differences, there were no differences in the heart weight to body weight ratio and infarct size between the sexes. These data demonstrate that compensatory hypertrophy is associated with an improvement in contractility and a delayed decompensation to HF in females. However, compensatory hypertrophy in males appears to be undermined by a steady decline in contractility associated with decompensation to HF.

Assessment of cardiac function with the pressure-volume conductance system following myocardial infarction in mice

Myocardial infarction (MI) is a major cause of heart failure (HF) with the progressive worsening of cardiac performance due to structural and functional alterations. Therefore, we studied cardiac function in adult mice following MI using the Millar pressure-volume (P-V) conductance catheter system in vivo during the later phase of compensatory remodeling and decompensation to HF. We evaluated load-dependent and -independent parameters in control and 2-, 4-, 6-, and 10-wk post-MI mice and integrated changes in function with changes in gene expression. Our results indicated a significant deterioration of cardiac function in post-MI mice over time, reflected first by systolic dysfunction, followed by a transient improvement before further decline in both systolic and diastolic function. Associated with the function and adaptive remodeling were transient changes in fetal gene and extracellular matrix gene expression. However, undermining the compensatory remodeling response was a continual decline in cardiac contractility, which promoted the transition into failure. Our study provided a scheme of integrated cardiac function and gene expression changes occurring during the adaptive and maladaptive response of the heart independent of systemic vascular properties during the transition to HF following MI in mice. P-V loop analysis was used to quantitatively evaluate the gradual deterioration in cardiac function post-MI. P-V loop analysis was found to be an appropriate method for assessment of global cardiac function under varying load-dependent and -independent conditions in the murine model with many similarities to data obtained from larger animals and humans.

Right and left ventricular function after chronic pulmonary artery banding in rats assessed with biventricular pressure-volume loops

In many patients with congenital heart disease, the right ventricle (RV) is subjected to abnormal loading conditions. To better understand the state of compensated RV hypertrophy, which could eventually progress to decompensation, we studied the effects of RV pressure overload in rats. In the present study, we report the biventricular adaptation to 6 wk of pulmonary artery banding (PAB). PAB resulted in an RV pressure overload to ∼60% of systemic level and a twofold increase in RV mass ( P < 0.01). Systemic hemodynamic parameters were not altered, and overt signs of heart failure were absent. Load-independent measures of ventricular function (end-systolic pressure-volume relation, preload recruitable stroke work relation, maximum first time derivative of pressure divided by end-diastolic volume), assessed by means of pressure-volume (PV) loops, demonstrated a two- to threefold increase in RV contractility under baseline conditions in PAB rats. RV contractility increased in response to dobutamine stimulation (2.5 μg·kg−1·min−1) both in PAB and sham-operated rats in a similar fashion, indicating preserved RV contractile reserve in PAB rats. Left ventricular (LV) contractility at baseline was unaffected in PAB rats, although LV volume in PAB rats was slightly decreased. LV contractility increased in response to dobutamine (2.5 μg·kg−1·min−1), both in PAB and sham rats, whereas the response to a higher dose of dobutamine (5 μg·kg−1·min−1) was blunted in PAB rats. RV pressure overload (6 wk) in rats resulted in a state of compensated RV hypertrophy with preserved RV contractile reserve, whereas LV contractile state at baseline was not affected. Furthermore, this study demonstrates the feasibility of performing biventricular PV-loop measurements in rats.

Assessment of parallel conductance for the trans-cardiac conductance method: can we use the hypertonic saline method with pulmonary artery injections?

The trans-cardiac conductance (TCC) method provides on-line left ventricular (LV) volume signals by determining the electrical conductance of blood in the LV using central venous and epithoracic electrodes. Conductive structures outside the LV cause a 'parallel conductance' offset term (Vp) that is determined by bolus injections of hypertonic saline in the pulmonary artery (Vp(saline)). Analysis of the increased conductance signal during passage of the bolus through the LV yields Vp(saline). Since TCC signals are picked up by epithoracic electrodes, concern has been raised that hypertonic saline remaining in the lungs might lead to overestimation. The decrease in blood conductivity induced by injection of non-ionic contrast medium during a LV angiogram may also be used to determine Vp (Vp(contrast)). Since the contrast is injected directly into the LV, lung conductance should be unaltered. Thus, we compared Vp(saline) with Vp(contrast) in six anaesthetized sheep during different hemodynamic conditions. Linear regression showed that Vp(saline) = 0.99 Vp(contrast) + 2.45 ml (r2 = 0.99). Bland-Altman analysis yielded a small non-significant bias (+/-2SD) of 1.8 (+/-6.8) ml. We conclude that parallel conductance for TCC can be accurately determined with the conventional hypertonic saline method.

Single-beat estimation of right ventricular end-systolic pressure-volume relationship

Assessment of right ventricular (RV) contractility from end-systolic pressure-volume relationships (ESPVR) is difficult due to problems in measuring RV instantaneous volume and to effects of changes in RV preload or afterload. We therefore investigated in anesthetized dogs whether RV ESPVR and contractility can be determined without measuring RV volume and without changing RV preload or afterload. The maximal RV pressure of isovolumic beats (P(max)) was predicted from isovolumic portions of RV pressure during ejecting beats and compared with P(max) measured during the first beat after pulmonary artery clamping. In RV pressure-volume loops obtained from RV pressure and integrated pulmonary arterial flow, end-systolic elastance (E(es)) was assessed as the slope of P(max)-derived ESPVR, pulmonary artery effective elastance (E(a)) as the slope of end-diastolic to end-systolic relation, and coupling efficiency as the E(es)-to-E(a) ratio (E(es)/E(a)). Predicted P(max) correlated with observed P(max) (r = 0.98 +/- 0.02). Dobutamine increased E(es) from 1.07 to 2.00 mmHg/ml and E(es)/E(a) from 1.64 to 2.49, and propranolol decreased E(es)/E(a) from 1.64 to 0.91 (all P < 0.05). After adrenergic blockade, preload reduction did not affect E(es), whereas hypoxia and arterial constriction markedly increased E(a) and somewhat increased E(es) due to the Anrep effect. Low preload did not affect E(es)/E(a) and high afterload decreased E(es)/E(a). In conclusion, in the right ventricle 1) P(max) can be calculated from normal beats, 2) P(max) can be used to determine ESPVR without change in load, and 3) P(max)-derived ESPVR can be used to assess ventricular contractility and ventricular-arterial coupling efficiency.

Hypertonic saline method accurately determines parallel conductance for dual-field conductance catheter

Conversion of conductance catheter data to absolute ventricular volumes requires assessment of parallel conductance (G(P)). We determined the accuracy of GP obtained by the hypertonic saline method (G(P)saline) compared with angiographically derived GP (G(P)Angio) and quantified the variabilities of GP for the dual-field conductance catheter method in nine anesthetized sheep studied at baseline, treated with dobutamine, and subjected to volume loading and beta-blockade. G(P)saline and G(P)Angio showed an excellent linear correlation (G(P)saline = 1.002 x G(P)Angio + 0.001 Omega(-1), R2 = 0.92), and Bland-Altman analysis yielded a nonsignificant bias and narrow limits of agreement (bias +/- 2SD = 0.002 +/- 0.112 Omega(-1)). Within-animal variability of GP was very similar with both methods and was due to changes in blood conductivity rather than geometrical changes. Variability between animals was significant (26.3% of mean for G(P)saline and 25.7% for G(P)Angio) and thus warrants individual assessment. Variations during the cardiac cycle were not significantly different from zero. With biplane angiography used as gold standard, the hypertonic saline method accurately determines GP for the dual-field conductance catheter over a wide range of hemodynamic conditions.