SpO2 is a standard of practice in anesthesia (1,2), but obtaining an adequate signal from fingers and ears may be difficult because of low perfusion or mechanical interference (3). Probes applied across the cheek (4) and nasal septum (5) are good alternatives in some patients, and there is anecdotal evidence that the tongue may be useful (6). Perhaps oximetry probes placed in highly perfused tissues will provide more accurate monitoring (6). The laryngeal mask airway (LMA) cuff sits in the highly perfused pharynx, and a prototype LMA incorporating a pulse oximeter has been described (7), but not formally tested. In the following pilot study, we compare pharyngeal SpO2 with the LMA to finger SpO2 and oxygen saturation from arterial blood samples (SaO2) in hemodynamically stable, well oxygenated, anesthetized patients.
With ethical committee approval and written, informed consent, we studied 20 patients (ASA physical status I–III, aged 18–80 yr) undergoing major orthopedic surgery in the supine position who required an arterial line. A size 5 LMA was modified by attaching a single-use pediatric pulse oximeter (Datex Medical Instrumentation, Helsinki, Finland) to the back plate with an adhesive dressing that did not cover the optical components (Figure 1). A standard anesthesia protocol was followed and routine monitoring applied. Anesthesia was induced with propofol 3 mg/kg and maintained with oxygen 33% in air and 1.5% sevoflurane. Muscle relaxation was with rocuronium 0.6 mg/kg. The LMA was inserted/fixed by an experienced LMA user (>1500 uses) following the manufacturer’s instructions. A radial arterial line was inserted into the nondominant hand and an oximeter attached to the index finger of the dominant hand. The oximeter probes and monitors (Datex AS/3; Datex Medical Instrumentation) were identical for the pharynx and finger. Both machines had been previously checked to ensure that they gave the same reading when attached to the same probe to guarantee that any differences displayed were not caused by differences in the sensitivities of each machine. Pharyngeal and finger SpO2 and SaO2 were measured during a hemodynamically stable period of anesthesia administration. Readings were allowed to stabilize for 2 min before blood was drawn for blood gas analysis. Arterial blood was collected via a vacuum container and was analyzed within 20 s of being drawn. The arterial blood gas machine (Serie 800; Chiron Diagnostics GmbH, 5020, Salzburg, Austria) was accurate to 0.01% (SaO2) and calibrated before each case. Observations were made over approximately 30 min.
Readings were taken with the cuff volume at 0, 10, 20, 30, and 40 mL with the head-neck in the neutral position. The intracuff pressure was then set at 60 cm H2O in the neutral position by using a digital cuff pressure monitor (Digital P-V Gauge™; Mallinckrodt Medical, Athlone, Ireland), and readings were taken with the head-neck maximally flexed, extended, and rotated. One set of observations was obtained in each patient at each cuff volume and head-neck position.
To provide information about the performance of the modified LMA, airway sealing pressure (maximum allowed: 40 cm H2O) and anatomic position (judged fiberoptically) were determined over the range of cuff volumes and head-neck positions used for comparing oximetry. Airway sealing pressure was measured by closing the expiratory valve of the circle system at a fixed gas flow of 3 L/min and noting the airway pressure at which the dial on the aneroid manometer reached equilibrium (8). The anatomic position of the LMA was determined fiberoptically by using the following scoring system: 4, only vocal cords visible; 3, vocal cords plus posterior epiglottis; 2, vocal cords plus anterior epiglottis; 1 = vocal cords not seen (9,10). Patients were questioned about sore throat 18–24 h postoperatively.
An unblinded anesthesiologist collected data. The levels of measurement agreement between pharyngeal and finger sensors, as well as between SaO2 and each type of probe, were calculated with the method outlined by Bland and Altman (11). The mean difference represents the average difference between each of the data points. SEM was calculated by dividing the standard deviation by n (1/2), where n = sample size. The limit of agreement represents the mean difference ± 2s (s = standard deviation of the differences). The standard error of the limits of agreement was calculated by using the formula (3s2/2)1/2 (11).
The mean (range) age, height, and weight were 42 (19–65) yr, 174 (151–193) cm, and 74 (53–97) kg, respectively. The male:female patient ratio was 12:8. Airway sealing pressure and fiberoptic scoring are presented in Table 1. Airway sealing pressure increased from 0–10 mL (P = 0.01) and 10–20 mL (P = 0.01) cuff volume, but did not increase thereafter. Airway sealing pressure was higher in the flexed (P < 0.01) and rotated (P < 0.001) head-neck positions compared with the extended head-neck position. Oxygen saturation values are presented in Table 2. SaO2 was the same as pharyngeal SpO2 at 20 and 30 mL cuff volume, but higher than pharyngeal SpO2 at all other cuff volumes and head-neck positions (P < 0.04). SaO2 was always higher than finger SpO2 (P < 0.01). Pharyngeal SpO2 was higher than finger SpO2 at cuff volumes 10–40 mL and in the flexed and rotated head-neck positions (all:P < 0.007), but was lower at 0 cuff volume (P < 0.0001) and similar in the extended head-neck position. There was an increase in pharyngeal SpO2 between 0 and 10 mL cuff volume (P < 0.0001), but no changes thereafter. Pharyngeal SpO2 was similar in the flexed, rotated, and extended head-neck positions. The statistical analysis is presented in Table 3. Pharyngeal SpO2 agrees more closely with SaO2 (mean difference < 0.7%) than finger SpO2 (mean difference > 1.1%) at 10–40 mL cuff volume and in head-neck flexion. The standard error of limits was identical (0.09) for both finger SpO2 and pharyngeal SpO2 if data at zero cuff volume are excluded. No patient complained of a sore throat 18–24 h postoperatively.
Our data show that pharyngeal SpO2 using the LMA is feasible in hemodynamically stable, well oxygenated, anesthetized patients. Pulse oximetry works by measuring the change in light absorption during arterial flow in a constant mass of tissue. In finger oximetry, the light emitter and sensor are aligned opposite each other, but in pharyngeal oximetry they lie side to side. The high quality signal we found with the side-to-side arrangement suggests that much of the emitted light is reflected off adjacent tissues and onto the sensor. Interestingly, the carotid artery lies in close proximity to the posterior surface of the LMA cuff.
We found that pharyngeal SpO2 readings are accurate over a clinically useful inflation range (10–40 mL) and in different head-neck positions. The readings obtained at zero cuff volume were probably inadequate because of the gap between the probe and posterior pharyngeal mucosa. The readings were most accurate (in fact, the same as SaO2) at 20–30 mL cuff volume. This suggests that at 10-mL cuff volume there is still a slight gap between the probe and mucosa and that at 40-mL cuff volume there may be a slight reduction in blood flow to the surrounding tissues. The readings for pharyngeal SpO2 were generally higher than finger SpO2. This probably reflects greater perfusion of the pharynx and adjacent structures than the finger. O’Leary et al. (4) found that buccal SpO2 was higher than finger SpO2. Our data for airway sealing pressure and fiberoptic position are similar to previous studies (12,13), suggesting that the modification did not impede performance. The overall magnitude of the limits of agreement was similar for pharyngeal and finger SpO2 but was approximately one integer smaller for pharyngeal SpO2 at 10–40 mL cuff volume and during head-neck flexion. This suggests that pharyngeal SpO2 can be more accurate than finger SpO2, provided the cuff volume is adjusted to within these limits. We placed the probes on the back plate for technical reasons, but other pharyngeal areas may also be suitable for oximetry. It may also be possible to adapt the technique to measure tracheal SpO2.
The high value for oxygen saturation at maximal cuff volume suggests that perfusion of the posterior pharyngeal wall is unimpeded with the LMA. There is a linear decrease in SpO2 with reduction in blood flow in the distal colon (14) and this is probably true in the pharynx. A recent study has shown that pharyngeal mucosal perfusion is progressively reduced in the posterior pharynx when mucosal pressure is increased from 34 to 80 cm H2O (15). However, posterior pharyngeal mucosal pressures with the LMA are generally <10 cm H2O and rarely exceed 25 cm H2O (12,16).
Pharyngeal oximetry has several potential applications. First, when adequate oximetry readings cannot be obtained from conventional sites during surgery, critical or intensive care medicine, the LMA can be used as the airway device, or placed behind the tracheal tube and used only as a monitor. Second, it can be used as a routine oximetry device in patients being managed with the LMA, and third, as a monitor of pharyngeal perfusion during routine LMA usage. Poor or absent pharyngeal SpO2 readings in the presence of good readings elsewhere might indicate that pharyngeal perfusion had been impeded by the LMA.
Pharyngeal oximetry has several limitations. First, it may not be tolerated in awake patients. Second, there may be some patients with abnormal pharyngeal anatomy in whom probe function is poor. Third, the modification we used was homemade and requires time for preparation. It would, however, be possible to produce a disposable LMA with a single-use oximeter incorporated into the back plate.
A limitation of our study was that we did not assess the accuracy of pharyngeal SpO2 over time or in low perfusion or low saturation states. However, we consider it likely that pharyngeal SpO2 will be stable over time because the readings and oximetry waveform were similar before and after the data were collected. Similarly, it is likely that pharyngeal SpO2 will be more accurate than finger oximetry during low perfusion and low saturation states because the pharynx is highly perfused and in close proximity to the carotid artery.
We conclude that pharyngeal SpO2 with the LMA is feasible and generally provides more accurate readings than finger SpO2 in hemodynamically stable, well oxygenated, anesthetized patients.
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