Left ventricular end-systolic elastance is incorrectly estimated by the use of stepwise afterload variations in conscious, unsedated, autonomically intact dogs

Characterizing heart failure in the ventricular volume domain

Heart failure (HF) may be accompanied by considerable alterations of left ventricular (LV) volume, depending on the particular phenotype. Two major types of HF have been identified, although heterogeneity within each category may be considerable. All variants of HF show substantially elevated LV filling pressures, which tend to induce changes in LV size and shape. Yet, one type of HF is characterized by near-normal values for LV end-diastolic volume (EDV) and even a smaller end-systolic volume (ESV) than in matched groups of persons without cardiac disease. Furthermore, accumulating evidence indicates that, both in terms of shape and size, in men and women, the heart reacts differently to adaptive stimuli as well as to certain pharmacological interventions. Adjustments of ESV and EDV such as in HF patients are associated with (reverse) remodeling mechanisms. Therefore, it is logical to analyze HF subtypes in a graphical representation that relates ESV to EDV. Following this route, one may expect that the two major phenotypes of HF are identified as distinct entities localized in different areas of the LV volume domain. The precise coordinates of this position imply unique characteristics in terms of the actual operating point for LV volume regulation. Evidently, ejection fraction (EF; equal to 1 minus the ratio of ESV and EDV) carries little information within the LV volume representation. Thus far, classification of HF is based on information regarding EF combined with EDV. Our analysis shows that ESV in the two HF groups follows different patterns in dependency of EDV. This observation suggests that a superior HF classification system should primarily be founded on information embodied by ESV.

Assessment of dsigma*/dt (max), a load independent index of contractility, in the canine

The search for a load-independent index of myocardial contractility has been a focus for nearly 100 years. Nearly all of the parameters developed have yielded insight into cardiac function but their clinical utility has been limited. A new index, dsigma*/dt (max), has been proposed to be useful in the clinic. This parameter is expressed as the maximum time rate of change of the pressure normalized circumferential wall stress (sigma* = sigma ( theta )/P, where sigma ( theta ) is circumferential wall stress and P is pressure) for a thick walled sphere model of the left ventricle (LV). This definition for a contractility index renders dsigma*/dt (max) dependent only on LV wall volume (V (m)) and maximum time rate of change of the ventricular volume, dV/dt (max). The index dsigma*/dt (max) has been studied in patients with echocardiogram-derived volume, but up until this point its characteristics in canines have remained unknown. Validating this index in the canine will allow for a more intensive and wide-range investigation of the index that is not available with humans. The purpose of this study was to validate dsigma*/dt (max) as a load-independent measure of contractility in the canine heart with the hope that it was a noninvasive assessment of contractile function. To assess the load independence of dsigma*/dt (max), the index was estimated over a range of preloads (end diastolic volume, EDV) during a vena caval occlusion (VCO). The study was conducted in five canines under various pacing modes [right atrial (RA), right ventricular (RV), left ventricular (LV), and biventricular (BV)] at rates of 90 or 100, and 160 bpm. The animals' ventricular volume measurements were assessed by conductance catheter, calibrated with echocardiography. A 50 Hz filter was applied to the volume signal before differentiation to obtain dV/dt (max). Echocardiography was used to calculate left ventricle mass and V (m). In eight of ten cases, dsigma*/dt (max) was significantly correlated with decreasing EDV (p < 0.05). There was also a significant correlation between dsigma*/dt (max) and dP/dt (max). With a strong correlation between the values of dsigma*/dt (max), dP/dt (max), and EDV in all five subjects, dsigma*/dt (max) is not load independent in the canine heart when preload is altered by a VCO. Further evaluation of this index is required to delineate the situations where dsigma*/dt (max) can be accurately applied.

Operative contractility: A functional concept of the inotropic state

1. Initial unsuccessful attempts to evaluate ventricular function in terms of the 'heart as a pump' led to focusing on the 'heart as a muscle' and to the concept of myocardial contractility. However, no clinically ideal index exists to assess the contractile state. The aim of the present study was to develop a mathematical model to assess cardiac contractility. 2. A tri-axial system was conceived for preload (PL), afterload (AL) and contractility, where stroke volume (SV) was represented as the volume of the tetrahedron. Based on this model, 'operative' contractility ('OperCon') was calculated from the readily measured values of PL, AL and SV. The model was tested retrospectively under a variety of different experimental and clinical conditions, in 71 studies in humans and 29 studies in dogs. A prospective echocardiographic study was performed in 143 consecutive subjects to evaluate the ability of the model to assess contractility when SV and PL were measured volumetrically (mL) or dimensionally (cm). 3. With inotropic interventions, OperCon changes were comparable to those of ejection fraction (EF), velocity of shortening (Vcf) and dP/dt-max. Only with positive inotropic interventions did elastance (Ees) show significantly larger changes. With load manipulations, OperCon showed significantly smaller changes than EF and Ees and comparable changes to Vcf and dP/dt-max. Values of OperCon were similar when AL was represented by systolic blood pressure or wall stress and when volumetric or dimensional values were used. 4. Operative contractility is a reliable, simple and versatile method to assess cardiac contractility.

Ventriculoarterial coupling and left ventricular efficiency in heart transplant recipients

BACKGROUND

In heart transplants, left ventricular function may be impaired in the absence of rejection or graft atherosclerosis. Matching between left ventricle and arterial receptor, i.e., ventriculoarterial coupling, and left ventricular efficiency have never been studied.

METHODS

Left ventricular pressure-volume loops and single beat analysis were used to determine effective arterial elastance (Ea) and the slope of the end-systolic pressure-volume relation (end-systolic elastance; Ees). Left ventricular efficiency was evaluated by determination of external work (EW), pressure-volume area (PVA), coronary blood flow (continuous thermodilution), and myocardial oxygen consumption (MVO2). Measurements were made at baseline in 11 control subjects and 9 heart transplant recipients (HTX) without rejection and were repeated after phenylephrine in the latter group.

RESULTS

At baseline, Ees, Ees/Ea, and work efficiency (EW/PVA) were lower in HTX than in control subjects (2.51+/-0.87 vs. 3.70+/-1.15 mmHg/ml/m2, P<0.01; 0.96+/-0.21 vs. 1.47+/-0.31, P<0.001; and 0.53+/-0.08 vs. 0.59+/-0.09, P<0.01, respectively). Energy conversion efficiency (PVA/MVO2) and mechanical efficiency (EW/ MVO2) were higher in HTX (0.58+/-0.08 vs. 0.45+/-0.14, P<0.001; and 0.31+/-0.05 vs. 0.26+/-0.06, P<0.001, respectively). In HTX, phenylephrine infusion increased Ees, Ea, EW, PVA, and MVO2 without modifying Ees/Ea, EW/PVA, PVA/MVO2, and EW/MVO2.

CONCLUSIONS

In heart transplants, (1) left ventricular contractility is moderately depressed; (2) elevation of energy conversion efficiency compensates for the decrease in work efficiency, allowing normal mechanical efficiency; and (3) alpha 1 adrenergic stimulation does not impair ventriculoarterial coupling and mechanical efficiency.

A less invasive Emax estimation method for weaning from cardiac assistance

The maximum elastance of the ventricle (Emax) is a strong candidate for a quantitative index used for determination of the timing of weaning the patient from a left ventricular assist device (LVAD). This paper presents a new and less invasive method for deriving Emax of the left ventricle under the LVAD assistance. In this method (the CoP method), Emax can be calculated from two different end-systolic points which are produced by changing the drive phase of LVAD without any vascular clamping and any direct measurement of the left ventricular volume. Animal experiments indicated that the CoP method is useful when the measured left ventricular flow and pressure are employed. Moreover, a new technique for estimating the left ventricular flow was developed to make the CoP method less invasive without direct measurement of the flow. The technique could considerably improve the estimation accuracy of the flow in the co-pulsation mode in comparison with the previous one proposed by the authors. However, it has been revealed that the estimation accuracy of the left ventricular flow was not globally high enough to apply the CoP method to clinical cases in spite of its much less invasiveness.