Effect of preoxygenation practices on bispectral index and the propofol induction
1 Lucknow, India
2 Belfast, N. Ireland
* E-mail: mukesh_tripathi{at}yahoo.com
Editor—Preoxygenation can be achieved by breathing 100% oxygen either by a facemask for 3 min1 or by voluntary deep breathing for 1–2 min.2 As the effort involved in eye opening is reported to change the EEG,3 we hypothesized that the voluntary deep breathing would increase the dose of propofol required to produce a bispectral index (BIS) of 60 compared with tidal breathing during preoxygenation.
After obtaining Institutional Ethical Committee approval, written informed consent was obtained from the 112 ASA I patients. The patients were monitored with an ECG, pulse oximetry (SpO2), intra-arterial pressure, and BIS (model A-2000, 3.1 software version; Aspect Medical Systems, Natick, MA, USA). Patients were preoxygenated by gently applying facemask and oxygen (10 litre min–1).
Patients were randomly assigned (sealed envelopes technique) to breath a normal tidal volume for 3 min [Group TVB (n=56)] or encouraged to take 8 deep breaths per minute for 2 min [Group DB (n=56)]. At the end of preoxygenation, a propofol infusion (200 ml h–1) was started until the endpoint (BIS-60) was achieved and the infusion was stopped. We recorded the BIS values, onset time, and propofol dose at the loss of response to verbal command, and at BIS-60. The ‘after-drop’ in BIS (from BIS-60 to its minimum level) and the percentage decrease in systolic arterial pressure (SAP) (from control to its minimum) were recorded.
The patients were similar in the two study groups for gender, age, weight, and height. Propofol dose till BIS-60 was significantly greater in Group DB [2.1 (0.4) mg kg–1] than in Group TVB [1.4 (0.3) mg kg–1] (P<0.05). The after-drop in BIS-60 in Group DB was accompanied by a greater decrease in SAP [–28 (9)%] than TVB patients [–22 (12)%] (Table 1).
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Highly significant difference (P<0.001)During preoxygenation, although the BIS values settled down in both groups, it remained higher in the DB group patients. As eye opening can affect the EEG3 and increase BIS,4 the relatively higher BIS values in the DB group may reflect the effect of deep breathing. A larger propofol dose was needed to induce anaesthesia in the DB group.
Ding and colleagues5 have also reported an increase in propofol dose during hyperventilation [E'CO2=25.9 (2.3) mm Hg]. However, PaCO2 remained higher [36 (6.2) mm Hg] in our patients. Thus, hypocarbia-induced decrease in brain circulation contributing to more propofol dose seems unlikely in our patients. The mild hypocarbia in DB patients noted by us was similar to another study.2
A study6 using a propofol infusion (20 mg kg–1 h–1) reported an after-drop from BIS-50 to 46.5 (mean) on stopping the propofol infusion at BIS-50. This correlated with the ‘residual dosecirculation effect’ of propofol on EEG and the BIS. We used a higher rate of propofol infusion (36 mg kg–1 h–1) and a greater after-drop from BIS-60 to 43 was noted. Our patients also had a decreased SAP at this time. A decrease in BIS has been reported during syncope7 and during cerebral hypoperfusion.8 Thus, the greater after-drop in BIS may not solely be due to higher residual propofol dose but also due to the effect of decreased SAP.
In conclusion, using the voluntary deep breathing manoeuvre during preoxygenation increased the induction dose of propofol. This contributed to more after-drop in the BIS value and in arterial pressure, thus emphasizing the need to interpret BIS value against the arterial pressure.
Presented in part at the 99th Annual Meeting of American Society of Anaesthesiologists, October 23–27, 2004, Las Vegas, NV, USA.
References
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7 Win NN, Kohase H, Miyamoto T, Umino M. Decreased bispectral index as an indicator of syncope before hypotension and bradycardia in two patients with needle phobia. Br J Anaesth (2003) 91:749–52.
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