1 Department of Anaesthesia and Intensive Care, Waikato Hospital, Private Bag 3200, Hamilton, New Zealand. 2 Department of Anaesthesiology, University of Auckland, Waikato Clinical School, Private Bag 3200, Hamilton, New Zealand
Corresponding author. E-mail: vossl@waikatodhb.govt.nz LMA® is the property of Intavent Limited.
Accepted for publication: July 9, 2003
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Abstract |
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Methods. Thirty-one healthy women undergoing gynaecological surgery were allocated to groups depending on the need for airway management with tracheal intubation (n=9), a laryngeal mask (LMA, n=17) or a facemask (n=5). During general anaesthesia, we measured changes in RR interval (RR-int) and rPTT after (i) induction of anaesthesia, (ii) airway manipulation and (iii) surgical stimulus. rPTT was estimated as the interval from the peak in the R-wave to detection of the pulse oximeter waveform in the periphery.
Results. Mean baseline rPTT was 245 (SD 27) ms. Upon induction of anaesthesia, rPTT increased (by 28.2 (20.4) ms, P<0.001) in all but two patients. rPTT decreased in response to endotracheal intubation (by 43.1 (24.6) ms, P=0.001) but did not vary in response to insertion of LMA or surgical stimulus. Mean baseline RR-int was 865 (141) ms. A mean reduction in RR-int after tracheal intubation did not reach statistical significance. RR-int was unchanged with induction of anaesthesia, LMA insertion or surgical stimulus.
Conclusion. Variation in rPTT reflects autonomic responses to nociceptive stimulation and fluctuations in anaesthetic depth independently of heart rate.
Br J Anaesth 2003; 91: 6626
Keywords: anaesthesia, general; autonomic nervous system; heart, pulse transit time; receptors, nociception; sympathetic nervous system
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Introduction |
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In this pilot study, we investigated a possible alternative measure of nociceptive-induced autonomic activation during general anaesthesia, the pulse transit time (PTT). The literature refers to PTT in several ways, the two commonly used being the pPTT (reflecting the interval for pulse propagation between two different arterial sites) and the rPTT (the interval between ventricular electrical activity and the appearance of a peripheral pulse waveform, which includes the pPTT).1 2 The rPTT interval therefore includes the cardiac pre-ejection period (time from the onset of ventricular electrical activity to the beginning of ejection into the aorta), the arterial transit time (interval from aortic pulse emergence to the arrival of its foot at the monitoring site) and the pulses rise time (measured from the start of the arterial pulse waveform upstroke to the point at which pulse arrival is detected).1 This study refers to rPTT because it is easily measured using standard monitoring (the ECG and pulse oximetry on the finger) and can be monitored continuously.
The rPTT has gained prominence as a non-invasive measure of autonomic activation in obstructive sleep apnoea3 and is also promoted as a non-invasive measure of beat-to-beat changes in arterial pressure during intraoperative procedures (VSM Medtech, Vancouver, Canada). However, to our knowledge there is no published work relating rPTT to noxious stimulation and anaesthetic depth in the operating-theatre environment. The aim of the present study, therefore, was to document the variation in rPTT in response to general anaesthesia and noxious stimulation during routine elective surgery.
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Methods |
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Data monitoring and acquisition
Continuous three-lead ECG and pulse oximeter waveform (obtained from an index finger) data were downloaded directly from the anaesthetic monitor (Datex® AS/3; Instrumentarium, Helsinki, Finland). The data were digitized (AD 512; Humusoft, Novakovych, Czech Republic) at a sampling rate of 100 samples/s and stored on a PC computer in Matlab file format for later off-line analysis of rPTT and RR interval (RR-int). Data were collected from before induction to the end of surgery. Emergence from anaesthesia and recovery were not studied because movement artefact prevented accurate calculation of rPTT during this time. The end-tidal concentrations of isoflurane, nitrous oxide, oxygen, MAC and carbon dioxide were monitored continuously and were recorded manually at 5-min intervals during each case. The time and dose of all medications and duration of airway/surgical manipulations were noted. We also noted any patient movement.
Data analysis and statistics
The rPTT and RR-int were calculated off-line from the raw ECG and pulse oximeter waveform data using Matlab (version 6.0; Mathworks, Natick, MA, USA) computational data analysis software. rPTT was measured as the interval between the peak in the R-wave on the ECG to the maximum upslope of the pulse oximeter waveform from the same cardiac cycle.3 4 Plots of the detected R-wave peaks and pulse oximeter upstrokes for each cardiac cycle were superimposed on the ECG and pulse oximeter waveform traces for each subject. Visual inspection of these graphs confirmed the accuracy of the R-wave and pulse oximeter detection algorithm. Artefacts (defined as a change in rPTT in an adjacent cardiac cycle of >25 ms and in heart rate of >25 beats min1) were less than 5% of the data and were filtered out before further analysis.
The maximum change in rPTT and RR-int during the 2-min interval after (i) induction of anaesthesia, (ii) airway manipulation and (iii) the first surgical stimulus (defined as cervical dilatation or surgical incision) was determined. Where no obvious change occurred, values 1 min after stimulus were taken. The five facemask subjects were included in the induction and surgical stimulation analyses. Mean differences were analysed with the one-sample t-test. The Pearson correlation coefficient (r) was used to quantify linear correlations of rPTT (and RR-int) with analgesic drug dose and anaesthetic depth (MAC). The rPTT and RR-int traces were inspected visually for an association between patient movements and a change in either of these variables. All data are presented as mean (SD) absolute changes. A P-value of <0.05 was considered statistically significant.
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Results |
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Somatic indication of nociceptive response
Spontaneous movements during the surgical procedures were seen in five subjects. Two patients moved in response to cervical dilatation, one to incision, one during LMA insertion and one spontaneously. One patient had a decrease in rPTT related to cervical dilatation (MAC=1.2 and alfentanil dose of 750 µg given 10 min before stimulus and movement).
Group differences
Mean baseline (preinduction) rPTT was significantly greater in the LMA group compared with the ETT group (252.1 (4.6) ms vs 228.6 (38.6) ms, P<0.05). There was no difference in baseline RR-int between groups (861.1 (137.5) ms in the ETT group and 875.2 (156.8) ms in the LMA group). Despite the younger mean age of the ETT group (Table 1), there was no significant correlation between age and baseline values of either PTT (r=0.213, P=0.277) or RR-int (r=0.163, P=0.399).
Correlations with drug dose
There were no significant correlations of the magnitude of rPTT change at induction or tracheal intubation with fentanyl dose, propofol dose or end-tidal concentrations of nitrous oxide and isoflurane.
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Discussion |
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The most striking observation was the correlation between rPTT response and the method of airway management. That is, the rPTT decreased dramatically with direct laryngoscopy and endotracheal intubation, but did not change in response to LMA insertion or incision after local anaesthesia. Direct laryngoscopy and endotracheal intubation provides greater haemodynamic stress and greater noxious stimulation compared with the insertion of a laryngeal mask.5 The longer duration of airway manipulation associated with tracheal intubation compared with laryngeal mask insertion may also have contributed to the increased rPTT response in this group. There is a direct relationship between duration of laryngoscopy and degree of cardiovascular response.6 RR-int exhibited less consistent changes than rPTT in response to corresponding levels of stimuli. The lack of rPTT or RR-int response to both LMA insertion and surgery presumably reflects adequate levels of anaesthesia for the degree of noxious stimuli associated with these procedures. The noxious effect of skin incision would have been nullified by the prior use of local anaesthesia.
The rPTT is affected by influences on the myocardium (e.g. ß sympathetic activity) and the vascular system (e.g. factors that change vascular distensibility, which include sympathetic activity).7 The reduction in rPTT observed with noxious stimulation reflect an increase in cardiac ß sympathetic tone, resulting in a shorter isovolumetric pre-ejection phase.1 There have been conflicting data relating
sympathetic activity to changes in rPTT.1 2
Patient movement during general anaesthesia in response to noxious stimuli is presumably a spinally mediated withdrawal reflex response. We found that neither RR-int nor rPTT varied consistently, if at all, in relation to these episodes of nociceptive-induced motor withdrawal. We suggest that the autonomic response to nociceptive stimuli is abolished before the withdrawal reflex during general anaesthesia. The various nociceptive-induced responses (i.e. withdrawal and autonomic activity) may be differentially affected by anaesthesia and analgesia.
Effect of anaesthetic drugs on rPTT and RR-int
The onset of general anaesthesia in the present study was associated with an increase in rPTT but no consistent change in RR-int. The initial increase in rPTT observed during the induction period is probably caused by the sympatholytic effect of propofol.8 The relative lack of reflex tachycardia in response to hypotension during the induction period may reflect inhibition of the baroreceptor sympathetic response by propofol.9 After tracheal intubation but before surgery, the rPTT again increases, which may reflect either attenuation of the autonomic activity by the volatile anaesthetic agents10 or the cessation of the noxious stimulus.
Previous studies have shown that physiological and psychological stress affect rPTT.11 rPTT decreased in response to physiological and psychological stress. While patient anxiety before induction was not determined in the present study, we postulate that an anxious patient would have a higher level of sympathetic activation and hence a relatively short rPTT. The observed lengthening of rPTT with induction may therefore reflect the patients prior emotional state. The shorter rPTT before induction observed in the ETT group may reflect a heightened level of anxiety in these comparatively younger subjects (Table 2). Greater age is associated with a shorter rPTT,4 probably because of a reduction in arterial compliance. However, no correlation between patient age and baseline rPTT was observed in our study.
Correlations with drug dose
The statistical analysis of the correlations between the haemodynamic variables and drug doses was limited in the present study by relatively low patient numbers (in the tracheal/intubation group particularly) and a narrow analgesic dose range (because of ethical and clinical considerations). Also, the lack of steady-state conditions during induction and during the early maintenance of anaesthesia confound any attempts to relate drug dose to effect on rPTT.
The present findings indicate that, during general anaesthesia, changes in rPTT relate primarily to changes in anaesthetic depth and to nociceptive stimulation. Further studies are required to clearly define the nature of these relationships and the utility of rPTT as an intraoperative monitor of autonomic activity.
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Acknowledgements |
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References |
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