High-fidelity, infinite time constant calibrated pressure apexcardiogram and its correlation with high-fidelity left ventricular pressure.

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Abstract
A technique of recording the pressure apexcardiogram by a high-fidelity piezoresistive transducer was developed. It permits the quantification of pressure with which the transducer is applied to the chest wall and the pressure changes during a cardiac cycle. Apexcardiographic pressures were compared with simultaneously recorded high-fidelity left ventricular pressure in 32 patients. There were no significant differences in the timing of the left ventricular systolic upstroke and the O point. Peak dP/dt [rate of pressure change] of the apexcardiogram occurred (10 .+-. 13 ms) significantly earlier than the intraventricular pressure recordings. Though there was a correlation between developed diastolic pressure (end-diastolic minus early diastolic pressure) measured by the apexcardiogram (20 .+-. 14 mmHg) and intraventricular pressure recordings (11 .+-. 6 mmHg), the former was significantly higher. Pressure overshoots in early and late diastole contributed to this overestimation by the apexcardiogram. The relative contribution of each phase varied with the patient. There was a similar correlation between systolic pressures, but this index tended to be underestimated by the apexcardiogram. The apical impulse may not be solely caused by the passive transmission of left ventricular pressure. This method of measuring application and developed pressure at the apex permits investigation of the impulse origin and its alteration in pathological states.