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Department of Anesthesiology and Intensive Care, Orebro University Hospital, Sweden
Address correspondence and reprint requests to Kristian Thorn, MD, Department of Anesthesiology and Intensive Care, Orebro University Hospital, 701 85 Orebro, Sweden. Address e-mail to kristian.thorn{at}orebroll.se.
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Abstract |
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Introduction |
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Methods |
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Electrocardiogram, noninvasive arterial blood pressure, Spo2, and respiratory rate were monitored in each volunteer and recorded every 5 min. Bispectral index (BIS) monitoring was used for registration of anesthesia depth. All volunteers received oxygen 2 L/min through a nasal catheter.
LES pressure (LESP) and gastric pressure were recorded, and then the barrier pressure (BrP) was calculated (BrP = LESP – gastric pressure). We did this using a Dent sleeve device, which consists of a 5-cm-long thin-walled silicon sleeve glued along a catheter (4). After inserting the catheter through the nose and into the esophagus and stomach, without using any local or topical anesthesia, the LES was identified using a pull-through technique, as described by Mittal et al. (5). Simultaneously, esophageal pressure (1 and 6 cm) and proximal and gastric pressure (1 cm) distal to the sleeve were measured with side holes. The catheter was constantly perfused with water at 0.5 mL/min per channel using a low compliance system, and the transducers were set to zero at the mid chest position and calibrated before each measurement. Pressure tracings were recorded continuously using a multiple-channel recording system.
Cricoid cartilage pressure was applied using a model of the right thumb and the two first index fingers (Fig. 1), which is in agreement with the application in clinical practice. This model applies a constant cricoid pressure of 30 N and has been developed in cooperation with the medical engineers at our hospital.
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After a stable period of 15 min of rest, we confirmed the positioning of the sleeve by letting the volunteers swallow 5 mL of water. Five minutes later, cricoid pressure was applied three times, 30 s each time, at 5-min intervals. The pressure was applied at least 30 s after any voluntary swallowing. After each application of cricoid pressure, the experience of discomfort because of the procedure was assessed using a visual analog scale (VAS; 0–10 cm). After another 5-min period of rest, an IV infusion of remifentanil 0.2 µg · kg–1 · min–1 was started. When this had infused for 20 min, the sequence of cricoid pressure applications was repeated as above. Finally, the volunteers received a bolus dose of propofol 1 mg/kg IV. Two minutes later, one last cricoid pressure was applied. This time it was only performed once because of the short time of deep anesthesia. Then the infusion of remifentanil was stopped. After another 5–10 min, the participants had awakened.
The baseline values were calculated as the average pressure during 30 s at the end of the rest period. The results during cricoid pressure are the average pressures during the periods when it was applied. The results during the infusion of remifentanil alone or together with a bolus dose of propofol are the average pressures 30 s before each sequence of cricoid pressure.
Analysis of variance followed by the Fisher Protected Least Significant Test was used for statistical analysis of the results. P < 0.05 was considered statistically significant.
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Results |
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The cricoid pressure did not induce esophageal propulsive motility, e.g., swallowing, either when awake or during remifentanil or propofol anesthesia. Remifentanil completely abolished esophageal motility, and 6 of 10 volunteers even complained of great difficulty in inducing swallowing. The discomfort from cricoid pressure measured with a VAS was 4 (3–7) during the control state and 0 (0–1) (P < 0.01) during remifentanil infusion.
Respiratory rate decreased with remifentanil, as expected, and some of the volunteers had to be reminded to continue breathing. They were all well oxygenated, and their arterial blood pressure and electrocardiogram were normal.
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Discussion |
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LESP and BrP were monitored with a Dent sleeve catheter. The sleeve has an advantage over side-hole manometry because it takes the axial movement of the LES during breathing into account. The catheter also makes it possible to study esophageal propulsive contractions, e.g., swallowing, and it is the method used for continuous-pressure monitoring (5).
In the present study, cricoid pressure did not induce swallowing. Our data also show that remifentanil abolished spontaneous esophageal motility, and some of the volunteers even experienced great difficulty in inducing swallowing. This is in accord with the studies of Noordzij et al. (7) and Mittal et al. (8) who have shown that mechanical stimulation of the laryngopharynx induces relaxation of LES without swallowing. However, our results show that remifentanil can counteract this effect. A different point of view is that remifentanil completely eliminated the experience of discomfort induced by cricoid pressure. This might be of clinical interest because anesthesiologists have varying opinions concerning when to start applying the pressure and what force to use (9). Many patients apparently experience cricoid pressure when they are still more or less awake, resulting in a decrease in BrP. Therefore, LES relaxation is probably induced by mechanical stimulation related to pain or discomfort, and these effects can be blocked by opioids.
Although uncommon, with an incidence of approximately 3–5 per 10,000 general anesthesia inductions, aspiration of gastric contents is still considered one of the major complications of anesthesia (10). The use of cricoid pressure, also known as "Sellick's maneuver" (1), has become the gold standard for preventing this. Studies by Vanner et al. (11,12) have shown that a cricoid pressure of 30–40 N is capable of preventing regurgitation. In our study, we used a pressure of 30 N because the use of 40 N may cause laryngeal distortion and thereby complicate intubation (13).
There is a correlation between LESP and reflux (14,15), but it is not possible to define LESP or BrP values below which reflux will occur (16,17). Even so, it seems logical to avoid, if possible, any anesthesiologic procedure or medication that would decrease LESP or BrP. The effects of some different drugs on LES have been studied (16,18). The opioids, morphine and pethidine, decrease LESP when given IV (19,20), but in one study, there was a slight increase in LESP when morphine was given subcutaneously (21). Further, Penagini and Bianchi (22) found that morphine decreased the number of spontaneous LES relaxations and thereby might be able to prevent reflux. We found no studies concerning remifentanil.
Because of the effects of opioids on LESP reviewed above, opioids have been used with caution when there has been an increased risk of regurgitation. That is particularly true regarding anesthesia in obstetrics (23). However, Morris and Cook (9) reported, from a national survey in the United Kingdom, that opioids were included in 76% of RSI, where fentanyl was the most widely used opioid. They believe the rationale for this is the reduction of cardiovascular stimulation and the assumption of reducing the risk of awareness (9). Consequently, we believe that studies of the effects of short-acting opioids on LESP and BrP are of clinical interest and that in the future, remifentanil might become a more widely used drug during RSI (24). It could be reasonably argued that we used rather small doses of remifentanil and propofol. Nevertheless, BIS-monitoring to evaluate the depth of anesthesia showed acceptable values, and deeper anesthesia would probably have augmented the effects of remifentanil and propofol. Further dose-response studies of the effects of remifentanil and propofol on LESP and BrP might therefore be of interest, particularly studies in humans with a full stomach.
In conclusion, cricoid pressure significantly decreased LESP and BrP when performed in awake volunteers (P < 0.05). However, this effect could not be seen during the infusion of remifentanil 0.2 µg · kg–1 · min–1. Remifentanil per se, or together with a bolus dose of propofol 1.0 mg/kg IV, did not change baseline values of LESP and BrP. Remifentanil abolished spontaneous esophageal motility and completely eliminated the experience of discomfort induced by cricoid pressure. This shows the importance of when to apply cricoid pressure during RSI.
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  J. G. Brasseur, R. Ulerich, Q. Dai, D. K. Patel, A. M. S. Soliman, and L. S. Miller Pharmacological dissection of the human gastro-oesophageal segment into three sphincteric components J. Physiol., May 1, 2007; 580(3): 961 - 975. [Abstract] [Full Text] [PDF]
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