Esophageal pulse oximetry is more accurate and detects hypoxemia earlier than conventional pulse oximetry during general anesthesia

Abstract

The esophagus is perfused directly by prominent arteries and may provide a more consistent tissue source for pulse oximetry. The goal of this study was to evaluate the sensitivity and accuracy of an esophageal pulse oximetry probe on patients during controlled hypoxemia in comparison to measurements obtained with conventional pulse oximetry (SpulseO₂). Forty-five ASA I–II adult patients were included in this prospective observational study. Nellcor digital oximetric probes were placed on finger tips for SpulseO₂ before anesthesia. After tracheal intubation, an esophageal probe was placed in the lower segment of the esophagus for esophageal oximetric monitoring (SoesO₂). All patients were disconnected from the breathing circuit to establish a controlled hypoxemia, and were re-connected to the breathing circuit and ventilated with 100% oxygen immediately when SoesO₂ dropped to 90%. Matched SoesO₂ and SpulseO₂ readings were recorded when SoesO₂ measurements were at 100%, 95%, 90% and the lowest reading. The time for SoesO₂ and SpulseO₂ to drop from 100% to 95%, 90% and return to 100% was recorded. Oxygen saturation from arterial blood samples (SartO₂) was also measured at each time point respectively. The linear correlation coefficient of the regression analysis between SartO₂ and SoesO₂ was 0.954. The mean±2SD of the difference was 0.3%±4.3% for SoesO₂vs. SartO₂ and 6.8%±5.6% for SpulseO₂vs. SartO₂ (P<0.001). The 95% confidence interval for the absolute difference between SoesO₂ and SartO₂ was 0.3% to 0.7% and 6.2% to 7.4% between SpulseO₂ and SartO₂. The time to reach 90% saturation measured with SoesO₂ was approximately 94 seconds earlier than the SpulseO₂ (P<0.001). In conclusion, SoesO₂ is more accurate and enables earlier detection of hypoxemia when compared to conventional pulse oximetry during hypoxemia for patients undergoing general anesthesia.

Keywords

esophageal pulse oximetry / conventional pulse oximetry / hypoxemia

Introduction

The finger oximeter (SpulseO₂), providing non-invasive continuous monitoring for arterial oxygen saturation, has been one of the most significant advances in the standard of monitoring since it was invented in 1975 [1]. However, SpulseO₂ retains higher than ideal inaccuracies up to 1.12%-2.5% [2] and is subject to ambient and physiologic interferences, including motion artifact, hypo-perfusion, hypothermia, peripheral vasoconstriction and extensively burned tissue [3-5]. A previous study showed that SpulseO₂ continued to decrease for 12 to 25 s after reinstitution of manual ventilation with 100% oxygen when SpulseO₂ reached 90% in apneic children [6]. This lag time of detection caused by SpulseO₂ may lengthen treatment time and increase the incidence of complications.

The esophagus is perfused directly by prominent arteries, and could provide a more consistent source for pulse oximetry [7,8]. Some studies have suggested that the esophageal oximetry (SoesO₂) is partially derived from the adjacent descending aorta [9-12] and may provide more accurate monitoring than SpulseO₂ in critically ill patients [13,14] and in cardiothoracic surgery patients [15]. As a result we hypothesize that SoesO₂ can provide not only a more accurate, but earlier indication of arterial oxygen desaturation than conventional SpulseO₂ during general anesthesia. The goal of this study was to evaluate the sensitivity and accuracy of an esophageal pulse oximetry probe on patients during controlled hypoxemia in comparison to conventional pulse oximetry.

Methods

After obtaining approval by Hospital Research Ethics Committee and informed consent from patients at West China Hospital, Sichuan University, 45 patients (ASA physical statuses I to II) were enrolled in this prospective study (40 scheduled for elective surgeries and 5 for emergency trauma surgeries). Patients with a positive history of coagulopathy, esophageal disease or esophageal trauma were excluded.

The esophageal pulse oximeter for this study was developed by the department of anesthesiology at West China Hospital, Sichuan University (patent of utility model, No. ZL2003201155080.2). The technology is based on the mechanism of reflectance photoplethysmography using two wavelength light-emitting diodes: red and infraredlights. The light generator (light emitting diode) and detector are mounted on the esophageal probe side by side. A direction marker, facing the same direction as the light generator and detector, is located 45 cm from tip of the probe [16]. A temperature probe is also located on the tip of the probe (Fig. 1).

All cases were started with ASA standard monitors. Nellcor-D-20 sensors (Nellcor Inc. Pleasanton, CA, USA) were placed on the index finger of the right hand in a consistent way to increase the reproducibility. Position was confirmed by increased quality of the signals (Fig. 2). After anesthesia induction and endotracheal intubation, the SoesO₂ probe was placed in the lower segment of the esophagus (about 37 cm from incisor tooth) in the left-posterior direction guided by the direction marker to ensure the sensor and emitter were directly facing the descending aorta. The position of the probe was further adjusted by moving up and down or rotating left and right gradually until the best signal quality was acquired [10]. Both esophageal and finger pulse oximetries were monitored with the same type of module and displayed simultaneously on the same monitor (PHILIPS-150 B3, Suzhou, China, Fig. 2). A 20-gauge catheter was placed into the left radial artery on each patient for SartO₂ measurement. EEG, end-tidal carbon dioxide content and arterial blood pressure were continuously monitored during the process.

The patients were ventilated by a Datex-ohmeda anesthesia machine (Madison WI 53707-7550, USA) with 100% oxygen. After both SoesO₂ and SpulseO₂ were stabilized at 100% for 10 min, the patients were disconnected from the ventilation circuit to establish controlled hypoxemia. When SoesO₂ dropped to 90%, the patients were reconnected to the circuit and ventilated with 100% FiO₂ until both SoesO₂ and SpulseO₂ returned to 100%.

SoesO₂ and SpulseO₂ were recorded when SoesO₂ was at 100%, 95%, 90% and the lowest reading during the O₂ saturation dropping phase, and also at 90% and 100% during the O₂ saturation recovering phase for each patient. The times for SoesO₂ or SpulseO₂ to drop to 95%, 90% and the lowest reading after the disconnection of the circuit were recorded and labeled T_95, T₉₀ and T_low. The time that SoesO₂ and SpulseO₂ took to fully recover to 100% after the reconnection of the ventilation circuit was recorded and named T_rec. From the 45 patients, 87 arterial blood samples were collected when SoesO₂ or SpulseO₂ was at 100%, 95% and 90% during the O₂ saturation dropping phase, and at 90% and 100% during the O₂ saturation recovering phase. The samples were analyzed with a portable blood gas analyzer (i-Stat; Abbott Laboratories Inc., East Windsor, NJ), corrected with the corresponding core (esophagus) temperature recorded at the same time point of each blood sampling. Arterial blood samples at the lowest readings of SoesO₂ and SpulseO₂ were not collected because it was impossible to tell which reading would be the lowest at that moment due to very rapid changes in SoesO₂ and SpulseO₂.

Statistical analysis

The primary variable of interest in this study was the deviation of SoesO₂ and SpulseO₂ from SartO₂ [10]. It was estimated that a sample size of 40 would provide 80% power to detect a clinically significance at α = 0.05, using the statistical software package NQUERY ADVISOR (version 4.0).

The secondary variable of interest was the change of SoesO₂ and SpulseO₂ through time. T_95, T₉₀, T_low and T_rec were compared between SoesO₂ and SpulseO₂ with ANOVA using the SPSS software package (SPSS version 13.0, SPSS Inc., USA). Linear regression analysis was used to compare SoesO₂ with SartO₂ and SpulseO₂ with SartO₂. Bland-Altman plots were used to compare the agreement of SpulseO₂ vs. SartO₂, SoesO₂ vs. SartO_2, and SpulseO₂ vs. SoesO₂. The mean difference represents the average difference between each of the data points. Standard error of the mean difference (sem) was calculated by dividing the standard deviation byn^{\frac{\mathrm{1}}{\mathrm{2}}}, where n = sample size. The limit of agreement represents the mean difference (2SD) (SD= standard deviation of the differences) [17]. A P-value of less than 0.05 was considered to be significant.

Results

The mean±SD of patients’ age, weight and height were 41±16 years, 65±10 kg and 167±10 cm, respectively. The male/female ratio was 5∶4. The depth of esophageal probe inserted was 37.5±3.2 cm from the incisor tooth. A typical display of a SoesO₂ wave (the second row) is shown in Fig. 2.

Comparable waveforms were obtained from both SoesO₂ and SpulseO₂ during the whole study. SpulseO₂ failed for approximately 10 min on 2 patients during hypotensive episodes.

There are 87 time points collected for SoesO_2, SpulseO₂ and SartO₂ in this study. The correlation coefficients of the linear regression analysis were 0.954 for SartO₂ vs. SoesO₂ and 0.927 for SartO₂ vs. SpulseO₂ (P<0.05, Fig. 3, A and B). The Bland and Altman graphs for SoesO₂ vs. SartO₂, SpulseO₂ vs. SartO₂ and SpulseO₂ vs. SoesO₂ are presented in Fig. 3C, Fig. 3 D and Fig. 4B. The mean±2SD of the difference was 0.3%±4.3% for SoesO₂ vs. SartO₂, 6.8%±5.6% for SpulseO₂ vs. SartO₂, and 6.5%±7.1% for SpulseO₂ vs. SoesO_2. The 95% confidence interval (CI) for absolute difference was 0.3% to 0.7% for SoesO₂ vs. SartO₂, 6.2% to 7.4% for SpulseO₂ vs. SartO₂ and 5.8% to 7.2% for SpulseO₂ vs. SoesO_2.

SoesO₂ started to gradually drop at the time of 241±25 s after disconnected from circuit, which was significantly earlier than 333±11 s for SpulseO₂ (P<0.001). After patients were reconnected to the circuit and ventilated with 100% oxygen, SoesO₂ and SpulseO₂ still fell further. The lowest reading of SpulseO₂ was 71%±4%, which was significant lower than 82%±2% for SoesO₂ (P<0.05). Both dropping line and recovering line of SoesO₂ were parallel to those of SpulseO₂. T₉₅, T₉₀, T_low and T_rec of SoesO₂ were 286±15 s, 324±26 s, 332±26 s and 201±24 s respectively, which were significantly shorter than those of SpulseO₂ (398±28 s, 418±21 s, 521±34 s and 377±29 s, P<0.001, Fig. 4A).

Discussion

This study demonstrated that SoesO₂ is a simple non-invasive method for monitoring the arterial oxygen saturation in tracheal intubated patients. The SoesO₂ wave is similar to the digital SpulseO₂ as a single wave (Fig. 2) and SoesO₂ could provide more accurate and earlier detection of SartO₂ than conventional SpulseO₂ during hypoxemia.

In this study, we observed that both SoesO₂ and SpulseO₂ correlated well with SartO_2, however, the measurement of SoesO₂ differed from SartO₂ by only 0.3% to 0.7%, while SpulseO₂ differed from SartO₂ by 6.2% to 7.4%_. This difference indicates that compared with SpulseO₂, SartO₂ could be monitored more accurately by SoesO₂ than using a standard monitor. Furthermore, SpulseO₂ failed for approximately 10 min on 2 patients during hypotensive episodes, while SoesO₂ was still able to detect it successfully on during these failure periods. The observation suggests that SoesO₂ is a suitable method for continuous oxygen saturation monitoring, especially in hemodynamic unstable patients [13]. As described earlier, SoesO₂ signals may partially be derived from the adjacent descending aorta, while SpulseO₂ readings comes from the extremity, which could be influenced by tissue heterogeneity, significant changes in tissue blood volume and venous pulsations, particularly at low SartO₂ levels [18,19]. Also, we used a continuous side-by-side generator-detector arrangement to monitor transtracheal mixed venous oxygen saturation in this study, which could obtain higher quality signals [20] and attribute to the results.

The correlation between SoesO₂ and SartO₂ is stronger than the correlation between SpulseO₂ and SartO₂. Meanwhile, T_95, T₉₀, T_low and T_rec of SoesO₂ were significant shorter than those of SpulseO₂. These findings suggested that SoesO₂ could provide earlier detection of SartO₂ than conventional SpulseO₂ during hypoxemia. The more rapid change in oxygen saturation detection at a point closer the aorta than in the periphery may contribute this phenomenon. Also, the esophageal pulse oximeter with side-by-side generator-detector arrangement may display a faster reading than the traditional opposite arrangement. However, further studies are required to answer this question.

There were several limitations in this study. First, all non-invasive monitor have a delay time, the SpulseO₂ and SoesO₂ readings were actually reflecting the earlier values of SartO₂ during the hypoxemia period when the arterial samples were taken. Secondly, it was impossible to ensure that hypoxemia was kept in a steady-state for the monitor. Only different oxygen concentrations for ventilation may achieve such steady-states of hypoxemia. Further study is needed to address this concern.

In conclusion, the esophageal pulse oximetry provides consistent readings of arterial oxygen saturation, and is more accurate and allows earlier detection of hypoxemia than conventional pulse oximetry during hypoxemia for patients undergoing general anesthesia.

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