The effects of high-frequency positive-pressure ventilation on intracranial pressure and brain surface movement in cats

Compared to traditional, low-frequency ventilation (LFV), mechanical ventilation at high frequencies (60-200 breaths/min) and low tidal volumes (HVF) is known to: 1) eliminate respiratory-synchronous variations in blood pressure; 2) minimize ventilatory effects on the cardiovascular system; 3) reduce peak airway pressures; and 4) suppress spontaneous respiratory efforts. Since these and other properties may make HFV useful in patients with acute intracranial pathology, we studied the effects of HFV on intracranial pressure (ICP) in cats. Compared with LFV (rate 11/min, tidal volume = 15 ml/kg), HFV (rate 100/min, V(T) = 3.3 ml/kg) had little effect on mean arterial pressure, heart rate, right atrial pressure, mean ICP or mean cerebral perfusion pressures, even if baseline ICP was raised using an epidural balloon. However, HFV effectively eliminated ventilator-linked fluctuations in both blood pressure and ICP, and at all levels of mean ICP studied (4.8, 15, and 30 torr), significantly reduced the peak ICP seen during a single respiratory cycle. The reduction in ICP fluctuation and peak pressure was more pronounced as intracranial compliance fell. However, the physiologic significance of such a change in the ICP pressure waveform is unknown. Because of the observed influence of HFV on ICP fluctuations, we also examined its effects on the physical movement of the exposed brain, using a non-contact, inductive displacement measuring device. During LFV, the cortical surface moved 'in and out' by 0.36 ± 0.1 (SD) mm, a distance sufficient to make microscopic focusing difficult. Changing to HFV reduced surface movement to 0.05 ± 0.01 mm, producing a very stable surface. These results suggest that HFV may play a very important role in the intraoperative management of patients undergoing certain neurosurgical procedures, particularly those requiring microsurgical techniques where reduced brain movement may facilitate surgery.