The effect of some hemodynamic factors on the behaviour of the aortic valve.

To test the validity of a theoretical model of aortic valve closure, based upon the observations in a two-dimensional analogue, the effect of some hemodynamic factors on aortic valve behaviour was studied in open-chest dogs. Direct cinematography was used to record aortic valve movements. The ECG, instantaneous ascending aortic blood flow as well as left ventricular and aortic pressures were registered simultaneously. The experiments revealed that: (1) at higher stroke volumes complete valve opening was reached earlier in the cardiac cycle; (2) the higher the peak aortic flow the better a circular shape of the completely opened valve was approximated; (3) the higher the systolic aortic pressure the earlier valve closure started in the cardiac cycle; (4) initial valve closure during flow deceleration was faster in the case of a larger systolic aortic pressure drop; (5) the backflow in the aorta increased at a larger end-systolic valve orifice area; (6) a rise in heart rate does not affect the mechanism of valve closure; (7) valve behaviour was not affected significantly by fluid viscosity. The closing behaviour of the valve as determined in animal experiments under different hemodynamic circumstances showed reasonable agreement with the valve closure as predicted by theory. However, due to the simplifications assumed in the model, the theoretical description must be used tentatively, especially with large pressure changes within the valve orifice during the cardiac cycle, with low peak flows or with high Strouhal numbers INTRODUCTION Recently we have studied aortic valve motion during the cardiac cycle in model experiments as well as in open-chest dogs. Some insight into valvular closing during deceleration of the main stream has been obtained from model studies in a two-dimensional analogue of the aortic valve (van Steenhoven and van Dongen, 1979). This study showed that a simplified quasi-one-dimensional description of the flow in the main stream, and assuming constant pressure on the sinus side of the cusp, gives a good qualitative idea about the initial phase of valve closure. To verify this model, aortic valve motion during the cardiac cycle was studied in open-chest dogs, using direct high-speed cinematography (van Steenhoven et al., 1981). Some findings in this study indicated that the variations in the behaviour of the aortic valve from animal to animal could result from differences in hemodynamic conditions. These differences could also explain some discrepancies between in uivo and model results as far as the closing behaviour of the valve is concerned, because in the latter the influences of some of these hemodynamic variations are not taken into account. Finally, prosthetic valve behaviour was compared with that of the natural aortic valve and a suggestion was *Rrceired 27 April 1982; in$na/form 15 July 1982. made for the application of the aortic valve closing mechanism to the design of a heart valve prosthesis (van Steenhoven et al., 1982). The aim of the present experimental animal study was to test the validity of the previously mentioned theoretical model under various hemodynamic conditions and to investigate the influence of aortic pressure, aortic flow, heart rate and fluid viscosity on aortic valve behaviour during the cardiac cycle. Since complete valve behaviour cannot be described in one single number, the effect of each variable on valve behaviour cannot be studied independently nor analysed by multiple regression techniques. Therefore, the influence of the hemodynamic variables on valve behaviour was investigated in a comparative way. To study the influence of one factor, we compared the valvular behaviour curves together with the corresponding aortic flow curves of two experiments in which the hemodynamic factors were similar except for the one to be evaluated. Next, the experimentally determined valvular closure was compared with the results obtained in the theoretical model. METHODS AND MATERIALS Experimental procedure Experiments were performed on 15 mongrel dogs of either sex, unknown age and ranging in weight from 25 941 942 A. A. VAN STEENHOVEN, P. C. VEENSTRA and R. S. RENEMAN to 45 kg. The animals were premeditated with Hypnorm @ (1 ml/kg body weight i.m.; 1 ml Hypnorm @ contains 10 mg fluanison and 0.2 mg fentanyl base). Anesthesia was induced with sodium pentobarbital (10 mg/kg body weight iv.) and, after endotracheal intubation, was maintained with oxygen-nitrous oxide. Ventilation was kept constant during the experiment with a positive pressure respirator (Pulmonat). The ECG was derived from limb leads. The chest was opened through the left fifth intercostal space and the heart was suspended in a pericardial cradle. Millar catheter-tip micromanometers (PC 470) were used to measure aortic (P,,) and left ventricular pressure (P,,). Left atria1 pressure (P,,) was measured with a saline filled catheter and an external pressure transducer (Ailtech). Aortic blood flow (qao) was measured with an electromagnetic flow probe, mounted on the ascending aorta. The probe was connected to a sine-wave electromagnetic flow meter with a carrier frequency of 600 Hz and an upper frequency response of 100 Hz-3 dB (Transflow 600). End-diastolic flow in the aorta was used as zero-reference. The flow probes werecalibrated in t’itro, previous to the experiments. Qualitatively no significant influence of the fiberscope (see below) on the flow curve could be detected, which indicates that the accuracy of the flow measurement is hardly affected by the presence of the scope. Furthermore, it is assumed that the fiberscope and aortic pressure and flow measurement do not affect valve behaviour. The variables to be measured were recorded on a multichannel physiological recorder (Schwarzer) and on an electromagnetic tape recorder (Ampex PR 2200). The upper frequency response of the recording system, Fig. 1. Diagram of the set-up and the position of the measuring devices (RA = right atrium, RV = right ventricle, LA = left atrium. LV = left ventricle; other symbols see text). which is limited by the physiological recorder, was 280 Hz-3 dB. The technique for high-speed recording of aortic valve movement has been described in detail elsewhere (van Steenhoven, 1979; van Steenhoven et al., 1981). These recordings were made at a film speed of 200 frames/s using a thin (4 mm in diameter) flexible fiberscope. This optical device was placed in front of the valve through the left carotid artery. In water the optical system has an angle of vision of 45”. The coupling of optical and electrical signals was achieved using a 50 Hz timing signal on both film and tape recorder. This technique requires the replacement of blood by a transparent liquid. Blood was replaced with two roller-pumps, the one connected to the left atrium and the other to the femoral artery. The second pump was essential to maintain peripheral arterial blood pressure at physiological levels. Free outflow occurred through a cannula in the pulmonary artery. A Tyrode solution either with (3.3 g “/o) or without gelatine was used as the transparent liquid. The fluid had a temperature of 37” and was saturated with a gas mixture consisting of 5 y0 COZ and 95 7; OZ. A schematic representation of the experimental set-up is given in Fig. 1. Data processing A sequence of film frames of five successive heart beats with a regular rhythm was chosen for analysis. An additional condition for the choice was a reasonable degree of uniformity of the instantaneous aortic flow and the aortic and left ventricular pressure curves during these five beats, as far as duration of ejection, maximum values of the parameters and the R-R interval were concerned. These frames were analysed with an analysing projector (Analector, Oude Delft) and drawings were made of the instantaneous cusp positions. From the drawings the instantaneous area of valve opening was measured with a planimeter (Ott 31). For proper evaluation of the valvular behaviour, the orifice area of the aortic valve was calculated as an instantaneous function of time and compared with the instantaneous flow within the valve. The measured aortic flow signal was shifted over about 8 ms because of the position of the flow probe with respect to the valve and the electronic delay in the flowmeter system (Z 1.5 ms). The time shift due to the position of the probe was calculated from the measured distance between flow probe and valve, assuming a pulse wave velocity of 4m/s (Nichols and McDonald, 1972). Figure 2 shows a schematic representation of the relations between aortic pressure, aortic flow and valve motion under normal conditions, where t = 0 coincides with the onset of the deceleration of aortic flow. Table 1 summarizes the hemodynamic quantities which were measured in the animals. Stroke volume was determined by integrating the electromagnetic flow signal. To reduce the stochastic errors, the influences of artefacts and the effect of physiological The etl’ect of some hemodynamic factors on the behaviour of the aortic valve 943