C. Kolbitsch

Effects of normo- and hypocapnic nitrous-oxide-inhalation on cerebral blood flow velocity in patients with brain tumors.

Nitrous oxide (N2O) use during anesthesia for intracranial procedures has been a subject of controversy in the past. To date, the isolated influence of N2O on mean cerebral blood flow velocity in the middle cerebral artery (VMCA) has not been investigated during hypocapnia in patients with brain tumors. We compared VMCA during normocapnic (ETCO2: 40 mm Hg) and hypnocapnic (ETCO2: 25 mm Hg) inhalation of air and 50% nitrous oxide in oxygen N2O/O2 in eight patients with unilateral brain tumors on both the tumor side and the healthy side. Six patients completed the study. Mean VMCA increased during normocapnic inhalation of N2O/O2 (tumor side: 86 ± 16 cm sec-1; healthy side: 74 ± 17 cm sec-1) when compared with air (tumor side: 72 ± 18 cm sec-1; healthy side: 62 ± 14 cm sec-1, p < 0.01), whereas during hyperventilation VMCA decreased on both sides (p < 0.001). Mean VMCA values were quite similar during hypocapnic inhalation of 50% N2O/O2 (tumor side: 50 ± 12 cm sec-1; healthy side: 45 ± 13 cm sec-1) and air (tumor side: 51 ± 14 cm sec-1; healthy side: 45 ± 12 cm sec-1). The data of our study suggest that in patients with cerebral tumors the N,O-induced increase in mean VMCA can be completely reversed by hyperventilation.

Hypocapnia reverses the fentanyl-induced increase in cerebral blood flow velocity in awake humans.

Investigations on the effects of opioids on cerebrovascular dynamics have repeatedly demonstrated mild to moderate increases in cerebral blood flow velocity in the middle cerebral artery (CBFVMCA), cerebral blood flow, and cerebrospinal fluid pressure in humans and animals. However, the influence of hypocapnia on these fentanyl effects has not been investigated. We compared mean CBFVMCA during normo- and hypocapnia before and after administration of fentanyl (2.5 μg/kg i.v.) in 20 awake humans. During normocapnia (end-tidal carbon dioxide [ETCO2] 40 mmHg) fentanyl significantly increased mean CBFVMCA (60 ± 10 cm/s vs. 81 ± 12 cm/s [mean ± SD]; p < 0.01), whereas during hypocapnia (ETCO2 25 mmHg) mean CBFVMCA values were identical (40 ± 7 cm/s vs. 40 ± 7 cm/s) before and after fentanyl administration. These results confirm previous findings that administration of fentanyl increases CBFVMCA, but, more importantly, clearly indicate that hypocapnia reverses this potentially undesirable effect.

Nitrous oxide and cerebral haemodynamics

Sir, The study by Strebel et al. (1) evaluates the influence of nitrous oxide on cerebral hemodynamics. Although they clearly showed that N,O is a potent cerebral vasodilator during normocapnia leading to an increase in cerebral blood flow velocity (CBFV), their conclusions and recommendations are somewhat misleading. When the authors write that “in patients with an elevated ICP or a reduced intracranial compliance, the possibility that nitrous oxide may be a potent cerebral vasodilator should be considered,” they forget that these patients, who were not the subject of their study, should be hyperventilated intraoperatively, while the cerebral healthy (!) patients examined by the authors were normoventilated. A recent study (2), however, was able to show in cerebral healthy volunteers that the increase in CBFV caused by 50% N,O in oxygen can be completely reversed by hyperventilation. In our opinion it is always problematic to attempt to draw direct conclusions for the treatment of patients from findings made in healthy individuals.

Pharmacodynamics and Pharmacokinetics of Different Modes of Rocuronium Administration

Rocuronium (Org 9426), which has recently been introduced in clinical practice, is the 2-morpholino, 3-hydroxy, 16N-allyl-pyrrolidino derivative of vecuronium. In contrast to vecuronium no pharmacologically active metabolites of this compound have been found in man. Its neuromuscular blocking potency is about 7 times less than that of vecuronium [1,2]. Rocuronium has intermediate duration of action, and after several maintenance doses of rocuronium no tendency toward cumulation was shown [3,4]. Therefore, rocuronium might be suitable for prolonged administration in critically ill patients (ICU-patients). However, neuromuscular blocking agents are developed and tested for use once or twice in a life-time, usually in surgical patients. No information has been available so far on the administration of rocuronium in critically ill patients .