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Critical incident due to a corroded expiratory valve

cardia was apparent. The lowest readings (after 10 min) were 38 bpm and 47 ⁄ 25 mmHg. Over the remaining 65 min, the systolic blood pressure varied between 81 and 108 mmHg, and the heart rate between 51 and 90 bpm. Ephedrine was administered by titration to a total of 36 mg and surgery proceeded with a low blood pressure persisting. At the completion of surgery haemodynamic variables returned rapidly to ‘normal’, heart rate 75 bpm and blood pressure 154 ⁄ 79 mmHg. Recovery was not delayed, the patient returned home later in the afternoon with no sequelae from the episode. The baroreceptor reflex is the best known of the nervous system mechanisms for the control of arterial blood pressure. The baroreceptors are concentrated in an area of the wall of each internal carotid artery above the bifurcation, the carotid sinus, and the aortic arch. Impulses pass from the carotid sinus through Hering’s nerves via the glossopharyngeal (IX) nerve and from the aortic arch through the vagus (X) nerve to the tractus solitaris of the medulla. Baroreceptors in the carotid sinus are stimulated from a systolic arterial pressure of 60 mmHg to a maximum output at 180 mmHg. The maximum number of nervous impulses is generated in the normal operating range, particularly in response to changes in arterial pressure. Baroreceptor signals inhibit the vasoconstrictor centre of the medulla and excite the vagal centre, leading to a fall of both arterial pressure and heart rate [1]. The primary function of the baroreceptor reflex is to minimise variations in systemic arterial blood pressure whether caused by postural changes, excitement, diurnal rhythm or arising spontaneously. A relatively constant arterial pressure during changes in body posture, particularly elevation of the head, is maintained. Sinoaortic denervation in dogs (stripping the aortic arch and carotid sinus areas) results in great variations in arterial blood pressure during the day with normal, hypertensive and hypotensive values being recorded and extreme lability noted [2]. On average, the postural fall of blood pressure is doubled in denervated dogs unable to rapidly increase their total peripheral resistance, with return to normal taking several minutes. In about 5% of all postural changes examined, the decrease in arterial pressure was exaggerated by a concomitant reduction in stroke volume throughout the period, leading to a much delayed return to normal values as was apparent in this patient. The previous surgery in this patient will have significantly disrupted the nervous regulatory control of blood pressure not only by the stripping of the carotid artery bilaterally at its bifurcation but also by possible interference with the vagus at the time of thyroidectomy, sufficient to explain the findings. Any future anaesthesia undergone by this patient will be informed by this experience which may be an unrecognised event in other patients or occurring to a lesser degree in some following similar surgery.