Carotid body chemoreceptor function is impaired by vecuronium during hypoxia.

BACKGROUND: Neuromuscular blocking agents reduce the human ventilatory response to hypoxia at partial neuromuscular block. It was hypothesized that vecuronium impairs carotid body chemoreceptor function during hypoxia. METHOD: The effect of systemic administration of vecuronium on single chemoreceptor activity during hypoxia, as recorded from a single nerve fiber preparation of the carotid sinus nerve, was studied in seven mechanically ventilated New Zealand White rabbits during continuous thiopental anesthesia. During normoventilation, the isocapnic hypoxic chemosensitivity of the single carotid body chemoreceptor was measured at four levels of oxygenation; these measurements were repeated at six separate occasions: control recording before injection, after intravenous administrations of 0.1 mg and 0.5 mg of vecuronium, and then at three occasions during a 90-min recovery period. Chemoreceptor chemosensitivity during isocapnic hypoxia was expressed as a hyperbolic function: Chemoreceptor output (Hz) = a + b x PaO2(-1) (mmHg). RESULTS: Chemosensitivity was reduced after both 0.1 mg and 0.5 mg vecuronium intravenous administration compared with control measurements; the hypoxic response curve was significantly depressed after both doses (P < 0.05). Notably, there was variation in the effect of vecuronium; some chemoreceptor preparations showed only minimal impairment, whereas some showed an almost abolished response to hypoxia. The chemosensitivity remained significantly depressed at 30 and 60 min but had recovered spontaneously at 90 min after 0.5 mg vecuronium. DISCUSSION: It is concluded that vecuronium depresses carotid body chemoreceptor function to a varying extent during hypoxia and that the depression recovers spontaneously.

Vecuronium-induced depression of phrenic nerve activity during hypoxia in the rabbit.

We investigated the effects of a close carotid injection of vecuronium on changes in phrenic nerve activity during different states of oxygenation. Experiments were performed on normothermic and mechanically ventilated adult New Zealand White rabbits with a tracheostomy under continuous thiopental infusion. Carotid arteries and carotid bodies were identified bilaterally followed by glomectomy on the contralateral side and positioning of a lingual artery catheter with its tip located in the carotid bifurcation on the ipsilateral side. From the dissected ipsilateral C-4 nerve root, changes in integrated phrenic nerve activity (IPA) were recorded after isocapnic step reductions from hyperoxia to normoxia (fraction of inspired oxygen [FIO2] 0.21, normoxic challenge) and from hyperoxia to moderate hypoxia (FIO2 0.15, hypoxic challenge) immediately after a close carotid bolus injection of either normal saline or 1 or 10 microgram of vecuronium. Arterial blood gases were analyzed during each ventilatory state. Phrenic nerve response to hypoxia expressed as chemosensitivity (Sch) was computed as the relative change in phrenic nerve activity per unit decrease in arterial oxygen saturation (Sao2), that is: Sch = (IPA challenge/IPA hyperoxia) - 1/Sao2 hyperoxia - Sao2 challenge. Hypoxic challenges were associated with reduced phrenic nerve response (Sch) after injection of 1 and 10 micrograms of vecuronium compared with normal saline. During normoxic challenges, no change occurred after 1 microgram, but reduced chemosensitivity was seen after 10 micrograms of vecuronium. We conclude that vecuronium depresses phrenic nerve activity during hypoxia.

Epidural anaesthesia for caesarean section in a patient with a cerebral artery aneurysm.

A case is described of a 30-year-old insulin-dependent diabetic woman who presented at 25 weeks gestation with frontal headache. Contrast tomography revealed an aneurysm of the middle cerebral artery with no evidence of subarachnoid bleeding. Although elective caesarean section at term was planned, it was performed at gestational week 38 due to the onset of vaginal bleeding and premature labour. In contrast to previous reports, the cerebral aneurysm was not managed surgically at the same time as the delivery, but was treated conservatively. An epidural anaesthetic was performed successfully and postoperative analgesia maintained with a continuous infusion of local anaesthetic in the epidural space. Literature is reviewed and advantages of regional versus general anaesthesia are discussed.

Attenuated ventilatory response to hypoxaemia at vecuronium-induced partial neuromuscular block.

The effect of a partial neuromuscular block on the ventilatory response to hypercarbia and to hypoxaemia was studied in 11 non-anaesthetized male subjects. Respiratory frequency, tidal volume, minute volume, respiratory timing and drive were measured during air breathing and during stimulation by hypercarbia and hypoxaemia. The ventilatory response was defined as the ratio between, respectively, tidal volume and minute volume during ventilation stimulated by hypercarbia and hypoxaemia compared to measurements during air breathing. The ventilatory measurements were repeated on three separate occasions: before neuromuscular block was established, during an infusion of vecuronium aiming at a mechanical adductor pollicis train-of-four (TOF) ratio of 0.70, and after the infusion had been stopped and the neuromuscular block had spontaneously recovered to a TOF ratio of > 0.90. Resting ventilation during air breathing remained with minor variations throughout the experiment. The ventilatory response to hypercarbia was not affected at a TOF ratio of 0.70 as compared to measurements before vecuronium and at a TOF ratio of > 0.90. In contrast, the ventilatory response to hypoxaemia was markedly reduced at a TOF ratio of 0.70. We conclude that a mechanical TOF ratio of 0.70 following vecuronium may be associated with an inadequate ventilatory response to hypoxaemia.

Measurement of acute response to draught in the eye.

In order to assess the sensitivity to draught of 7 different tests, 41 volunteer subjects were exposed experimentally: 18 to 1.0 m/s and 9 to 0.5 m/s in a climate chamber: 14 to an average of 0.67 m/s in an air-conditioned car. All exposures were at 21-22 degrees C. Exposures were for 30 min indoors, 45 min in the car. Break-up Time (BUT) of the pre-corneal film after a blink was observed before and after exposure in the laboratory. There was a significant decrease after exposure to 1.0 m/s (P less than 0.01) but not to 0.5 m/s. The variance of the observed BUT increased after exposure to 0.5 m/s (P less than 0.05). The Norn Lacrimal Dilution test showed increased tear flow after the climate-chamber exposures (P less than 0.05). Self-reported BUT(S) was always several times longer than BUT. There was a significant correlation between these measures (P less than 0.05) before exposure, but not after. BUT(S), like BUT, decreased after exposure to 1.0 m/s (P less than 0.01), but not after 0.05 m/s. However, the variance of BUT(S) did not increase significantly after 0.5 m/s; it decreased significantly after 1.0 m/s (P less than 0.01). A significantly improved mucus ferning pattern was observed after draught exposure (P less than 0.005), presumably due in part to increased lacrimal flow. There was no significant effect of draught on the albumin content of tear samples taken before and after exposure. Lissamine Green staining performed before and after exposure revealed no effect on micro-damage to the conjunctival epithelium.