CEREBROVASCULAR CONSEQUENCES OF NITROGLYC ERINE INFUSIONS IN HUMANS -A METHOD COMPARISON STUDY

The cerebral haemodynamic effects of vasodilators are of clinical inte rest because a decrease in mean arterial pressure (MAP) might alter gl obal cerebral blood flow (CBF). Luxury perfusion of the brain, in cont rast, might be unfavourable in patients with reduced intracranial comp liance. Despite the widespread use of nitroglycerine (NTG), little is known about the cerebral haemodynamic consequences of NTG infusions in humans. This prospective, controlled study was designed: (1) to inves tigate the effects of NTG on CBF and cerebrovascular CO2 reactivity an d (2) to compare reference measurements of global CBF with transcrania l Doppler monitoring (TCD) of middle cerebral artery flow velocity (V- MCA). Methods. With ethical committee approval and informed patient co nsent, we investigated ten patients undergoing coronary artery bypass surgery. Measurements were performed under fentanyl/midazolam anaesthe sia prior to the start of the operation. First, during a baseline peri od, ventilation was changed in a random sequence to achieve two differ ent levels of arterial PCO2 (30 and 50 mmHg, respectively). Consequent ly, measurements were repeated during i.v. infusion of 1.5 .mu g . kg( -1). min(-1) NTG at identical PCO2 levels. Measurements of CBF were pe rformed by the Kety-Schmidt technique with argon as an indicator. Simu ltaneously, V-MCA was recorded by use of a 2-Mhz transcranial Doppler system. Cerebral perfusion pressure (CPP) was calculated from the diff erence between MAP and jugular bulb pressure. Statistical analysis was performed by two-way analysis of variance using a repeated-measures d esign to assess the effects of NTG application and respiratory changes , respectively. Results. During NTG infusion, CPP decreased slightly b y 15-17%. Because of a reduction in cerebrovascular resistance, CBF in creased at both levels of PaCO2 by 96 and 69%, respectively, However, V-MCA decreased concomitantly. Cerebrovascular CO2 reactivity did not change. Conclusions. This study demonstrates that during fentanyl/mida zolam anaesthesia NTG may cause a major increase in CBF as long as CPP does not decrease considerably. Our results further suggest that NTG causes vasodilation of basal cerebral arteries, inducing a discrepancy between relative changes in CBF and V-MCA. Consequently, TCD measurem ents during infusion of NTG should not be directly compared with prece ding measurements of MCA flow velocity.