Glycopyrronium and hypotension following combined spinal‐epidural anaesthesia

Field: Medicine (Health Sciences)

I was interested to read the recent article on this subject (Rucklidge et al. Anaesthesia 2002; 57: 4–8). There are two well-known reasons for postspinal hypotension. One is the reduction in venous return secondary to pooling of blood and the other is arteriolar vasodilatation leading to a fall in systemic vascular resistance. Thus, the management of postspinal hypotension is based on increasing preload and/or afterload. The aim of this study was to investigate whether a tachycardia was useful to prevent or minimise postspinal hypotension. Many other investigations have explored the effect of heart rate on blood pressure [1, 2]. A reciprocal relation between heart rate and stroke volume was found from these studies. At higher rates, an insufficient filling time causes stroke volume to decrease. This situation could be even more pronounced in the presence of reduced left ventricular end-diastolic volume, as seen following spinal anaesthesia. Unless it is extreme, the changes in heart rate have little effect on blood pressure. Thus, an increased heart rate would not help to raise the blood pressure. This concept is well established and further work to study the effect of maternal tachycardia on postspinal hypotension may not be justified. Moreover, the potential harmful effects of tachycardia and other side-effects␣of␣anticholinergic drugs should not be ignored. Ephedrine is a commonly used vasopressor to treat postspinal hypotension. It acts primarily via an indirect mechanism by releasing norepinephrine from the nerve endings. It has weak agonist activity at alpha and beta adrenoceptors. An increase in systemic vascular resistance with positive inotropic action is responsible for increasing the blood pressure seen with ephedrine. The tachycardia itself should have very little effect on blood pressure unless there is a pre-existing significant bradycardia. It would be of interest to know why the authors thought that a heart rate of 80 beats.min−1 or less was a state of ‘relative bradycardia’ for pregnant women at term coming for elective Caesarean section. They included patients with ‘relative bradycardia’ on the basis of pre-anaesthetic assessment. However, many␣of␣them were not so-called ‘relatively bradycardic’ when they came to theatre and these patients did not meet their inclusion criteria. In fact, many of them were tachycardic before the commencement of the study. Post-spinal hypotension is commonly seen within 30 min of establishing spinal anaesthesia. However, this study was continued for 60 min or until the operation was finished. As a result of this, other factors (e.g. blood loss) could have contributed to the development of hypotension. Surprisingly, there was no mention on the amount of blood loss in either group. An important issue of this paper was the severity of hypotension. It is logically difficult to estimate the severity of hypotension in a clinical setting. In this report, the investigators treated their patients with fluid and ephedrine when the blood pressure dropped by 20% of the baseline value. So, it is not unexpected that the lower level of blood pressure would be nearly 80% of the baseline value, especially when the patients received prophylactic ephedrine and preloading. The pressure could go down further, if treatment with intravenous fluid and ephedrine was withheld. So, the severity of hypotension was masked by the treatment. Thus, the lower limit of blood pressure may not be an indicator of the severity of hypotension. The alternative suggested indicator of the severity of hypotension was the amount of ephedrine requirement. However, the ephedrine was not give on the basis of body weight. Tachyphylaxis following repeated doses of ephedrine has been reported. Some of the patients may have received a fixed volume of crystalloid and/or colloid irrespective of their body weight. In that case, the ephedrine requirement as an index of severity may not be true. Finally, the authors may have used an insufficient dose of glycopyrronium to cause any significant tachycardia or dry mouth. As might be expected, there is no difference between the groups.