[Effects of different kinds of ventilation and positioning measures on blood flow velocity in the middle cerebral artery].

OBJECTIVE

Aim of the present study was to quantify the influence of varying respiratory parameters and different body positionings on transcranial Doppler sonography (TCD) values. With the approval of the local ethics committee, 34 patients without neurological deficits scheduled for an operation in supine position (n = 15) and in the so-called "rabbit-position" (RP, n = 19) were chosen for a prospective randomised investigation.

METHODS

Prior to induction of anaesthesia, during the study and after extubation, the systolic (Vs), diastolic (Vd) and mean (Vm) of blood flow velocities as well as pulsatility (Vs - Vd)/Vm of the middle cerebral artery were determined by TCD. In addition mean arterial pressure and heart rate were monitored. Pulmonary airway pressures were observed during anaesthesia and CO2-partial pressure and central venous pressure (CVP) were determined by central venous line and blood samples. Anaesthesia was initiated with 5 mg/kg thiopental and continued with 0.8-1.2 vol% isoflurane in N2O/O2 = 2:1. After positioning all variables were recorded at 10-minute intervals with an inspiratory:exspiratory (I:E) ratio of 1:2, positive endexspiratory pressures (PEEP) of 0 and 10 cm H2O and an I:E ratio of 1:1 with 0 and 10 cm H2O PEEP. The respirator was adjusted to maintain an endtidal pCO2 of 40 mmHg.

RESULTS

Neither different positioning of the patients, nor the alteration of respiratory parameters affected TCD variables in a significant way. No significant alterations were detected between the two groups of patients. Rising PEEP resulted in a minor but significant increase in CVP by an average of 3 cm H2O in both groups. The respective positions produced different respiratory pressures. These were in the "RP-group", with and without PEEP, at an approximately 5 cm H2O higher level. All other parameters did not vary significantly.

CONCLUSION

The findings of this study show that a positioning of patients in a so-called "rabbit position" during anaesthesia does not alter the rate of blood flow velocity in the middle cerebral artery. Moreover, the modification of important respiratory parameters, especially a rise of PEEP to 10 cm H2O, had no significant influence on TCD values.