Limitations
ischaemic organ damage, hypotension
COMPLICATION
Firstly, patients in our study were not randomised for anaesthetic regimen (specifically propofol versus sevoflurane). Rather, the agent for maintaining general anaesthesia was chosen by the attending anaesthesiologist. This could in theory have confounded our results, for which we attempted to control by statistical testing of differences in baseline patient characteristics.Secondly, in our study we chose to increase MAP with phenylephrine. Although cerebral conductance vessels, like the middle cerebral artery, have α- and β-adrenergic innervation,Thirdly, other potential cerebral vasomotor medication was given during surgery according to standard care. Most importantly, the use of remifentanil was overrepresented in the propofol-group in comparison to sufentanil, which may have contributed to the differences in intra-operative cerebral haemodynamic responses. One study found that in cardiac patients during remifentanil anaesthesia, moderate-dosed remifentanil had no effect on MCAFourthly, the individual lower and upper limits of CA were not measured in this study. Therefore, these findings cannot be extrapolated to the complete range of the classical CA-curve. This is especially significant for the lower limits of CA (<50 mmHg), as deep hypotension is a common complication during surgery and is associated with ischaemic organ damage, such as in kidneys, myocardium and the central nervous system.Fifthly, intracranial pressure (ICP) was not measured in this study. Strictly speaking, CA only describes the pressure−flow relationship between cerebral perfusion pressure (CPP) and CBF. If ICP is increased, there is a possibility of measuring ‘false autoregulation’.Sixthly, in an attempt to limit the use of vasopressors and vasodilators and thus prevent large swings in MAP, we decided not to randomise the MAP levels. The consequence might be that any possible confounding effect due to medication rather than the MAP itself cannot be ruled out (Table Seventhly, transfer function analysis as used here does not take directional sensitivity of dynamic CA into account. Earlier research has suggested the existence of such mechanism,Finally, measurement of CBF using TCD is indirect because it measures the blood velocity in the MCA MCA