REPORTS OF SCIENTIFIC MEETINGS

SOME HAEMODYNAMIC PROBLEMS IN PREGNANCY Dr. Melville Kerr, (University of Edinburgh) addressed the Society on changes in cardiac output and regional blood flow in pregnancy, with particular reference to the reduction in output which is believed to occur in the last few weeks before term. Measurements of the pressure in the inferior vena cava (I.V.C.) in late pregnancy, by means of retrograde femoral vein catheterization at Caesarean section, demonstrated high caval pressures and poorly transmitted respiratory waves in the supine position. Turning the patient on her side, manually lifting the uterus forwards, and emptying the uterus all led to a fall in caval pressure, whereas manual occlusion of the I.V.C. following delivery produced the features associated with the supine position. I.V.C. angiograms taken before and after Caesarean section indicated that the I.V.C. was occluded in the supine position in late pregnancy. These combined studies on I.V.C. pressure and flow indicated that supine caval occlusion is a physiological feature of late pregnancy. Simultaneous recording of right atrial (R.A.) pressure and I.V.C. pressure showed a reciprocal relationshipwhen I.V.C. pressure was raised, as in the supine position, R.A. pressure was reduced; turning on to the side led to a fall in I.V.C. pressure and a rise in R.A. pressure. This indicated that supine caval occlusion was associated with a reduction in right atrial diastolic filling pressure. The effects of supine caval occlusion were studied by use of an indicator-dilution technique in six healthy patients in late pregnancy. Cardiac output, heart rate and aortic pressure were recorded over twenty minute periods in both supine and lateral positions. Three patients showed a marked reduction in cardiac output in the supine position, of the order of 20-25 per cent. Despite the fall in output, the heart rate remained constant, and the arterial pressure did not fall. This indicated that there was a reciprocal increase in systemic vascular resistance, and, furthermore, that the heart rate did not reflect changes in output in these circumstances. In the other three patients, the fall in output was of the order of 4 to 10 per cent. It was noted that in these three patients with minimal changes, the foetal head was engaged, whereas in the three with marked changes, the foetal head was not engaged. It was concluded from these experiments that in the supine position in late pregnancy, and particularly if the foetal head is not engaged, cardiac output might be reduced by 20-25 per cent, but that there were usually no parallel changes in heart rate or arterial pressure. Supine hypotension is known to occur in a small minority of patients in late pregnancy. Haemodynamic studies were performed in two such patients. When these patients turned from their side on to their back, the initial circulatory response was the same as that already described in association with supine caval occlusion. However, a sudden and dramatic bradycardia was superimposed on this, and this led to an even greater fall in cardiac output which was not balanced by any rise in peripheral resistance. As a result profound hypotension occurred. This bradycardia was probably the critical factor in the aetiology of postural hypotension, and was probably vagal in origin-i.e. the supine hypotensive syndrome might be the resultant of supine caval occlusion and a vaso-vagal attack. It was postulated that this postural phenomenon might explain the fall in cardiac output in late pregnancy, because most measurements had been made in this position. Consequently, serial studies of cardiac output in the same individuals at 12, 28, and 36 weeks were made, on each occasion measuring output in both the supine and lateral position. The results were still incomplete, but they indicated that cardiac output was raised by the 12th week, and maintained at this level at the 28th week. At these times there was no appreciable postural effect. At the 36th week cardiac output in the lateral position was still maintained at mid-pregnancy values, but results in the supine position showed the expected fall. These studies indicated that the alleged reduction in cardiac output in late pregnancy did not apply to the lateral position. It was not implied that cardiac output measured in the lateral position had any more validity than cardiac output measured in the supine position. In fact it was meaningless to refer to cardiac output as an abstraction, because any value for cardiac output related only to the special experimental conditions under which the measurement was made. It had no universal applicability. The concept of supine caval occlusion as a physiological phenomenon cast some doubt on the importance of caval occlusion as a cause of placental abruption. It also explained some clinical hazards of postural hypotension. This was particularly dangerous in patients under spinal or general anaesthesia where there might be a failure of peripheral vascular resistance to compensate for the fall in output.