Effects of glyceryl trinitrate on cerebrovascular autoregulation
1 Zürich, Switzerland
2 Basel, Switzerland
* E-mail: lsteiner{at}uhbs.ch
Editor—We read the article by Moppett and colleagues1 with great interest. There is a paucity of data on cerebrovascular effects of vasoactive drugs in awake patients without intracranial pathology and we are grateful to the authors for providing such data. However, we would like to highlight two aspects regarding the glyceryl trinitrate (GTN) part of their study.
First, the CO2 reactivities measured in this study are very high. We would expect reactivities in the range of 15–30% per kPa.2 Such high CO2 reactivity makes the study of changes in blood flow velocity in response to changes in arterial pressure very difficult, as small changes in CO2 will have marked effects on the flow velocities. Table 1 suggests that a shift to a lower end-tidal CO2 occurred between the measurements performed before and during infusion of GTN. This would have led to improved autoregulation. Secondly, assuming intact autoregulation, the decrease in flow velocity in response to the arterial pressure decrease induced by GTN is surprisingly large. If the static rate of autoregulation3 is calculated based on the median given in Table 1, the result is 37%, that is, in contrast to the measurements of dynamic autoregulation performed by the authors, static autoregulation is clearly disturbed. This discrepancy is unexpected in the investigated group of patients.
In our opinion, based on the presented data, it cannot be concluded that GTN did not affect cerebrovascular autoregulation in this study.
Nottingham, UK
* E-mail: iain.moppett{at}nottingham.ac.uk
Editor—We thank Dr Steiner and colleagues for their interest and thoughtful comments on our study.1
In response to the end-tidal CO2 values, the individual changes in ‘resting’ end-tidal values with and without GTN were mostly <0.3 kPa, and occurred in both directions. We cannot exclude undetected changes in Paco2 due to GTN effects on pulmonary blood flow. We would therefore suggest that, within the limits of the study, any effects of changes in end-tidal CO2 were small and unlikely to affect autoregulation significantly.
We agree with Dr Steiner that the decrease in flow velocity in parallel with a GTN-induced decrease in arterial pressure can be interpreted as a reduction in static autoregulation. However, two factors suggest that this may not be the case. First, neither we1 nor other authors4 found changes in tests of dynamic autoregulation, and it would be an interesting finding to demonstrate significant uncoupling of dynamic and static tests of autoregulation. Secondly, GTN produces vasodilation of the basal cerebral arteries,5 which will result in lower flow velocities for the same cerebral blood flow. The theoretical advantage of dynamic techniques is that the confounding effects of GTN are present throughout the test thereby avoiding this problem.
We would therefore caution against claiming changes in static autoregulation without confirmatory evidence that cerebral blood flow has in fact decreased.6
References
1 Moppett IK, Sherman RW, Wild MJ, Latter JA, Mahajan RP. Effects of norepinephrine and glyceryl trinitrate on cerebral haemodynamics: transcranial Doppler study in healthy volunteers. Br J Anaesth (2008) 100:240–4.
2 Joshi S, Ornstein E, Young WL. Cerebral and spinal cord blood flow. In: Anesthesia and Neurosurgery—Cottrell JE, Smith DS, eds. (2001) St Louis: Mosby. 19–67.
3 Tiecks FP, Lam AM, Aaslid R, Newell DW. Comparison of static and dynamic cerebral autoregulation measurements. Stroke (1995) 26:1014–9.
4 Endoh H, Honda T, Ohashi S, Hida S, Shibue C, Komura N. The influence of nitroglycerin and prostaglandin E1 on dynamic cerebral autoregulation in adult patients during propofol and fentanyl anaesthesia. Anaesthesia (2001) 56:947–52.[CrossRef][Web of Science][Medline]
5 Dahl A, Russell D, Nyberg-Hansen R, Rootwelt K. Effect of nitroglycerin on cerebral circulation measured by transcranial Doppler and SPECT. Stroke (1989) 20:1733–6.
6 Willmot M, Ghadami A, Whysall B, Clarke W, Wardlaw J, Bath PMW. Transdermal glyceryl trinitrate lowers blood pressure and maintains cerebral blood flow in recent stroke. Hypertension (2006) 47:1209–15.
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