Late-systolic pumping properties of the left ventricle. Deviation from elastance-resistance behavior.

Elastance-resistance [E(t)-R] representations of the left ventricle (LV) were evaluated for their ability to reproduce instantaneous pressure [P(t)] and outflow [Q(t)]. Experiments were performed in open-chest rats. P(t) and Q(t) were measured during steady-state ejecting beats and during a beat in which the aorta was suddenly clamped. The degree of clamping varied from partial to total occlusion. The total occlusion beat was considered an isovolumic beat that generated an isovolumic pressure [Piso(t)] with a characteristic time to maximal Piso(t) [Tpisomax]. In ejecting beats, 34% of stroke volume was delivered after Tpisomax. P(t) and Q(t) from the steady-state ejecting beats and Piso(t) from the clamped beat were then used to estimate parameters of an E(t)-R model. Components of P(t) and Q(t) not accounted for by E(t)-R were identified and termed extra-pressure [Pext(t)] and extra-outflow [Qext(t)]. Pext(t) and Qext(t) were near-zero valued until Tpisomax; then they became systematically positive and finally negative valued after end ejection. During partial aortic occlusion, P(t) was elevated and Q(t) was reduced. However, the time of ejection was extended, and the fraction of stroke volume delivered after Tpisomax increased as P(t) was made higher. Partial occlusion also prolonged the positive phase of Pext(t) and Qext(t). Elements possessing "active" and "deactive" properties were added to the E(t)-R model in an attempt to account for Pext(t) and Qext(t) during partial occlusion. Optional forms of these elements were considered. These expanded E(t)-R models were fitted to basal ejecting data and then asked to predict data from a partial occlusion beat. All expanded models failed to adequately predict the partial occlusion pressure and/or outflow. It was concluded that 1) late ejection was quantitatively important to LV pumping, 2) behavior during late ejection was inconsistent with E(t)-R, and 3) ad hoc modification of E(t)-R models was not likely to yield LV pumping models that could satisfactorily reproduce instantaneous P(t) and Q(t) behavior over the entire ejection period.

Pressure response to quick volume changes in tetanized isolated ferret hearts.

Observed pressure responses to quick volume changes in the isolated tetanized heart of ferrets were compared with previously reported tension responses to quick length changes in isolated cardiac muscle. Hearts were isolated from ferrets, perfused with ryanodine solution, and stimulated rapidly (50 ms between stimulations) to produce repeated 4-s intervals of tetanus. During each tetanus interval, volume increments of different amplitudes were rapidly removed and then reinfused into the left ventricular chamber. The pressure responses to these volume changes were evaluated for differences between withdrawals and infusions and for dependence on the amplitude of the volume change. It was found for both withdrawal and infusion that the response could be divided into three phases: 1) an immediate phase coincident with volume change, 2) a fast-recovery phase, and 3) a slow-recovery phase. The amplitude of the immediate phase was linearly dependent on the volume change so that a single regression line fit all the data (withdrawal and infusion). The fast recovery phase was 2.5 times faster for infusion than for withdrawal and generated a rebound effect with the pressure going below the initial pressure in the response to infusion. The pressure never went above the initial pressure in the response to withdrawal. The slow-recovery phases in infusion and withdrawal did not differ. These responses in the isolated heart bear striking similarities to tension responses to quick length changes in isolated constantly activated cardiac muscle. We concluded that muscle fiber dynamics were being faithfully transformed to left ventricular (LV) chamber dynamics without appreciable distortion because of the many intervening factors between the wall muscle fiber and the LV chamber.(ABSTRACT TRUNCATED AT 250 WORDS)

Area of the aortic pressure-flow loop measures energy storage by the proximal arteries.

We show that power measured by the area of the pressure-flow loop (APQ) is directly proportional to reactive power [Im(W)], which measures pulsatile energy storage in the proximal arteries. Analytically, Im(W) differs from APQ by K.2 pi, where K is a postulated constant of proportionality. To determine the value of K, 338 heartbeats obtained from 8 dogs under a wide range of hemodynamic conditions were analyzed. The relationship between APQ and Im(W) remained highly linear (r = 0.98), and the pooled value of K was -1.19 +/- 0.01. For the individual dogs, values of K differed at most by +/- 9% from its average value. There was also a small offset in the relationship between APQ and Im(W), probably caused by truncating the calculation of Im(W) at the 10th harmonic. Ignoring the small differences in the values of K and the offset, we found that APQ still proved to be an excellent predictor of Im(W). We also found that APQ was sensitive to changes in heart rate, left ventricular stroke output, and peripheral resistance. This reflects the dependence of pulsatile energy storage on these variables and indicates that APQ cannot serve as an index for changes in just the reactive properties of the proximal arteries.

Validation of optional elastance-resistance left ventricle pump models.

Left ventricular pressure, P(t), and outflow Q(t), data were collected in anesthetized, open-chest rats and dogs. The data were used in a three-tiered validation procedure to evaluate 14 competing forms of elastance [E(t)]-resistance (R) left ventricle (LV) pump models. Competing models arose from considering two forms of parameterization of E(t), time variation versus no time variation in LV unstretched volume (Vd) and dependence versus no dependence of R on P(t) and isovolumic P(t). A descriptive test based on the normalized root-mean-square errors in the fit to P and, separately, in the fit to Q was used to distinguish between models. The best of the competing models was the one that treated Vd as a function of time and R as a constant. Models of this form fitted the data very well and were said to be descriptively valid. The best of the competing models were then asked to predict the observed responses to changes in afterload, preload, and prior-beat history. The models did not predict these conditions well and failed to pass the test for predictive validity. Additionally, the model parameters were judged not to represent their supposed physical homologs and, thus, failed the test for explanative validity. One cause for E(t)-R model failure was an inadequate representation of events at end systole. This deficiency was apparently due to not accounting for deactivation in the model. Other features may also be needed before a comprehensive LV model can be formulated. Identical conclusions were made from data from the rat and the dog.

Aortic bulb-aortic orifice hemodynamics in left ventricle-systemic arterial interaction.

Experiments were conducted in open-chest dogs in which ascending aortic blood flow (Q) was measured with an electromagnetic flow probe, and pressure was measured in the left ventricle (LV) and ascending aorta. The aorta just distal to the flow probe was occluded suddenly, causing the aortic bulb (AB) to be the only part of the arterial system to be in communication with the LV during clamped beats. Pressure and flow data from preocclusion, occlusion, and occlusion-release periods were analyzed to assess AB compliance (CAB) and the longitudinal impedance properties of inertance (LVO) and resistance (RVO) in the AB-aortic orifice region. In five dogs (avg wt 24.2 kg), the mean and range of values were as follows: CAB = 45 (31-55) X 10(-4) cm5/dyn; LVO = 1.86 (0.76-2.88) dyn X s2 X cm-5, RVO = 4.07 (0.66-8.55) dyn X s X cm-5. The volume-accommodating actions of CAB caused actual LV outflow to be characteristically different from Q and aortic-clamped beats to be nonisovolumic. Actions of the AB were found to be important in the estimation of arterial characteristic impedance as a component of LV afterload and in the estimation of the LV pump properties of time-varying elastance and internal resistance.

Mechanical determinants of transient changes in stroke volume.

To better define beat-to-beat regulation of stroke volume (SV), the several-beat transient response of the left ventricle (LV) to sudden changes in hydraulic loading impedance was studied. Data were collected from eight canine isolated heart-lung preparations with controlled LV loading impedance. At a selected diastolic interval, a sudden increase in hydraulic loading resistance was induced. The resulting transient response in SV, end-diastolic pressure (EDP), and end-systolic pressure (ESP) was analyzed by comparing the relative predictive capability of six competing models, each incorporating different degrees of complexity in the relationship between SV and EDP and ESP. The basic model assumed linear LV pressure-volume relationships at both end diastole and end systole. Incorporation of nonlinear, end-state interaction or coronary perfusion pressure effects into the basic model did not improve predictive performance. Models incorporating SV and ESP of the preceding beat as well as ejecting beat ESP and EDP were consistently superior to all other models. The ranking of the relative influence of the determinants of SV was 1) ejecting beat ESP, 2) preceding beat ESP, 3) ejecting beat EDP, and 4) preceding beat SV.

Informational analysis of left-ventricle/systemic-arterial interaction.

Studies of left ventricle (LV) and systemic arterial (SA) interaction can be grouped into four categories: 1) prediction of pressure and flow waveforms, 2) changes in LV/SA function with changes in SA properties, 3) identification of criteria that reveal matching between LV and SA properties, 4) definition of LV afterload. Whereas results from studies in categories 1, 2, and 3 reveal the consequences of interaction, results from studies in category 4 come closest to revealing the true character of LV/SA interaction. A useful description arising from category 4 is that of a circular feedback path connecting LV outflow, SA input-impedance, LV pressure, and LV pump properties. The identification of a node in this scheme results in the separation of LV functions into active functions and loading functions and the separation of LV/SA load into LV load and SA loading elements. The time-varying LV elastance participates in both LV active functions and LV loading functions, with the former dominating the latter. Total peripheral resistance dominates all other LV and SA loading elements in its loading effects. Although an elastance-resistance LV model coupled with a simple second-order SA load model accounts for many reported observations on LV/SA interaction, data from sudden aortic occlusion studies indicate a need to consider yet another interaction action. Evidence is presented to suggest the existence of an LV pump element that couples ejection events with relaxation and filling events.

Internal capacitance and resistance allow prediction of right ventricle outflow.

The relationship between right ventricle afterloading pressure (P) and outflow (Q) was studied in three isolated canine right ventricle (RV) preparations. Right atrial pressure was held constant while graded elevations in P were induced with stepwise occlusions of the right and left branches of the pulmonary artery. P and Q signals were collected and analyzed using a digital computer system. Data were analyzed by assuming a model structure for the RV and comparing resultant model predictions of Q with actual observations. The model structure was modified in accordance with the discrepancy between prediction and observation to improve the model's predictive capability. The initial model tested was the time-varying linear relationship between ventricular volume and pressure. Utilizing this model, accurate predictions of RV outflow in the face of varying pressure afterloads could not be made. The addition of a series resistance to this elementary model resulted in marked improvement in predictive performance. The addition of greater complexity to the model gave only marginal improvement to the model's predictive capability. It was concluded that a time-varying capacitance and series resistance adequately model internal properties of the RV that relate outflow to afterloading pressure.

Optoelectronic transducer for in vivo recording of oviductal contractile activity.

An optoelectronic instrument to record oviductal muscular activity in chronically instrumented animals was evaluated in in vitro and in vivo experiments. The intensity of red light transmitted through the oviduct was modulated by contractions of the oviductal wall producing an optical analog of the mechanical events. Accuracy of the analog was tested by Fourier analysis of signals from mechanical and optoelectronic transducers placed at the same site on the oviduct; the results validated the use of the optical device as a contraction event sensor. Contractions of the tubal mesenteries had less effect on the optical signal than on signals from extraluminal mechanical transducers. Optical and photographic recordings of luminal transport in exposed oviducts showed a correspondence of intraluminal movements to events in the optical contraction signal. This instrument does not alter tubal function, and thus it is an especially useful experimental tool to investigate the role of oviductal muscular activity in fertility.