Effects of changes in heart rate and atrial filling pressure on the performance characteristics of isolated perfused pumping rat hearts.

Biomechanical indices of left ventricular performance were measured and computed by an on-line minicomputer in 13 isolated perfused working rat hearts subjected to a regimen of left atrial filling pressures and heart rates, to determine perfusion conditions for optimum heart work. We found that maximum values of most of the indices are achieved at a left atrial filling pressure of 20 to 25 cm H2O and a heart rate of 330 to 360 beats/min. Most of the performance indices when considered as functions of left atrial filling pressure display Starling-type behaviour: they rise with increasing left atrial pressure, reach a maximum at 20 to 25 cm H2O, and decline with further increases in left atrial pressure. Values for left ventricular (dP/dt)max, between 6000 to 8000 mm Hg s-1, and -(dP/dt)min, between 4000 to 6000 mm Hg s-1, are considerably higher than values previously reported. We believe this to be due to improved sampling techniques.

Medical informatics: developments in West Germany and South Africa.

Comparisons are made between medical information systems in West Germany and South Africa. In both countries the costs and importance of medical information systems dictate that a methodological approach is essential in order to obtain a solution for a particular environment. The special implications for South Africa are (i) to work from user requirements to computer system, rather than the reverse; (ii) to institute training programmes for the professionals required to organize and staff such systems; and (iii) to prepare the groundwork for medical information systems by scrutinizing the existing types, amounts, flows and uses of medical information.

An automatic device for freeze-clamping of cardiac tissue within a fraction of the contraction cycle.

An automatic quick-freeze clamping device has been developed. Opposed pneumatic pistons filled with aluminium caps previously cooled in liquid nitrogen are used to compress a portion (100 to 200 mg) of the myocardium to a 0.15 to 0.20 mm thick wafer, colling the tissue from 37 degrees C to -15 degrees C within 10 ms. The clamp is triggered electronically from the R-wave of the ECG. This tissue fixation by freezing within 10 ms is sufficiently rapid to study oscillations of myocardial metabolite levels during the contraction cycle of isolated perfused hearts of small mammals such as the rat and guinea pig whose rate is 4 to 5 beats per second.