An evaluation of the plethysmographic method of measuring cranial blood flow in the new-born infant.

Cranial blood flow values obtained plethysmographically in the human new-born infant have been very variable and frequently very low. We have used a doppler ultrasound velocitymeter and skull compliance measurements to investigate whether the technique itself affects the cranial arterial inflow and whether the methods of calculation used introduce bias. Blood velocities were measured in intracranial and extracranial arteries during plethysmography, i.e. before, during and after jugular vein compressions of short duration. Skull expansion after tilting was used to estimate skull compliance. In about 10% of jugular vein compressions, arterial velocities decreased immediately, due to direct pressure on the carotid artery. In the remaining compressions a fall in arterial velocities was seen four or five heartbeats after the onset of the compression. This was probably caused by a decrease in the cranial perfusion pressure consequent on rising venous pressure. Some babies demonstrated very poor skull compliance and these babies had correspondingly low estimates of cranial blood flow. We conclude that jugular venous occlusion plethysmography is not a suitable method for use in the clinical field and is only useful as a research tool in carefully selected situations.