Behavior of cerebral blood flow velocity in conventional ventilation and superimposed high frequency jet ventilation

Abstract

Patients with increased intracranial pressure or vasospasm after subarachnoidal haemorrhage with decreased cerebral perfusion present a special problem on developing respiratory insufficiency, since kinetic therapy or extracorporal life support are contraindicated. Superimposed High Frequency Jet Ventilation (SHFJV) has been shown to be of benefit in ventilating patients with pulmonary insufficiency. The aim of this study was to evaluate if SHFJV could be safely applied in patients with critical cerebral blood flow; if so, SHFJV might be beneficial when pulmonary insufficiency occurs concomitantly. The study was performed in 14 patients (3 with pulmonary insufficiency) applying first moderate hyperventilation (paCO2 31 to 36 mmHg) followed by increased hyperventilation (paCO2 27 to 30 mmHg) with CMV and SHFJV and measuring intracranial pressure (ICP), cerebral perfusion pressure (CPP) and blood flow velocity (BFV) of the middle cerebral artery. BFV of the middle cerebral artery which correlates closely to the cerebral blood flow, was measured continuously with transcranial Doppler ultrasound. CMV: Increased hyperventilation leads to a statistically significant increase in paO2 (121.3 to 147.2 mmHg, p < 0.05), SaO2 (98.5% to 99.2%, p < 0.05) and decrease in BFV (systole 115.9 to 89.6 cm/s, diastole 44.6 to 31.8 cm/s, p < 0.05). Heart rate, mean arterial blood pressure, ICP and ventilation parameters did not show any statistically significant differences. SHFJV: During SHFJV the parameters demonstrated similar patterns as during CMV. However, none of the changes were statistically significant (paO2 111.9 to 125.9 mmHg, SaO2 97.9 to 98.8, BFV systole 106 to 95 cm/s, diastole 52.7 to 42.4 cm/s, n.s.). After calculating the mean BFV according to the Markwalder formula to a standard paCO2 of 40 mmHg CMV and SHFJV were compared to one another. No statistical difference was seen between the two different ventilation techniques. In patients with increased ICP, pulmonary complications such as pneumonia or ARDS are frequently observed. Since there are indications that SHFJV is of benefit in pulmonary insufficiency, the study was conducted to demonstrate that SHFJV can be safely applied in patients with increased ICP.