Efficiency and Efficacy of Two Techniques of Preoxygenation during Modified Rapid Sequence Intubation.

BACKGROUND: Apneic mass movement of oxygen by applying continuous positive airway pressure (CPAP) is possible only when the airway is kept patent which helps to reduce the rate of desaturation. AIMS: The aim of this study was to check the efficiency of preoxygenation and apneic oxygenation by assessing the drop in partial pressure of arterial oxygen (PaO2) during apnea with and without keeping an oropharyngeal airway to maintain the patency of airway. SETTINGS AND
DESIGN: This prospective observational study was conducted at a tertiary care center.
MATERIALS AND METHODS: Sixty patients undergoing robotic and laparoscopic-assisted surgeries requiring modified rapid sequence intubation were recruited for the study. In Group A, CPAP was not applied during preoxygenation and oropharyngeal airway was not used, but oxygen was administered at 5 L/min during the apnea. In Group B, CPAP of 5 cmH2O was maintained during preoxygenation and after induction an oropharyngeal airway was inserted. Patients in both the groups were induced and paralyzed following standardized anesthesia protocol. STATISTICAL ANALYSIS USED: Chi-square test, independent t-test, and ANCOVA were used as applicable.
RESULTS: Group B showed significantly higher mean PaO2 levels after preoxygenation (525.3 ± 42.5 vs. 500.8 ± 51) and at 90 s of apnea (494.8 ± 42.6 vs. 368.6 ± 98.4) as compared to Group A. The fall in PaO2 was significantly lower in Group B. The rise in partial pressure of arterial carbon dioxide was comparable in both groups.
CONCLUSION: Preoxygenation with CPAP of 5 cmH2O followed by apneic oxygenation with CPAP keeping the airway patent with an oropharyngeal airway results in significantly higher PaO2 after preoxygenation and slower reduction in PaO2 during apnea.

INTRODUCTION

The apnea surrounding the peri-intubation period can be dangerous leading to hypoxemia, which in turn can lead to cardiac arrhythmias, hemodynamic derangements, hypoxic brain injury, and even death. Preoxygenation is practiced commonly to increase the safety during periods of apnea, which is attained by denitrogenation of the functional residual capacity to provide a reservoir of oxygen during apnea.[123] Administration of oxygen during the apneic period leads to mass movement of oxygen down a concentration gradient into the alveolus and helps in maintaining the oxygen reservoir in the lungs. This apneic mass movement of oxygen is possible only when the airway is kept patent.

The primary objective of the present study was to check the efficiency of preoxygenation and apneic oxygenation by assessing the drop in partial pressure of arterial oxygen (PaO2) during apnea following induction of anesthesia with and without keeping an oropharyngeal airway to maintain the patency of airway. The secondary objectives included assessment of efficacy of preoxygenation by these two techniques of apneic oxygenation by measuring PaO2 attained after preoxygenation, change in partial pressure of arterial carbon dioxide (PaCO2) during apnea, and tolerance of patients to these preoxygenation procedures.

MATERIALS AND METHODS

This was a prospective observational study conducted on sixty patients who were scheduled to undergo robotic and laparoscopic-assisted gastrointestinal surgeries, in which we practice a modified rapid sequence intubation (RSI). The airways of all the patients were assessed preoperatively in the anesthesia clinic as well as by the consultant anesthesiologist covering the operating theater. Patients presenting for elective laparoscopic and robotic gastrointestinal, and urological surgeries requiring modified RSI, with American Society of Anesthesiologists (ASA) physical status classes 1 to 3 and body mass index <30 were included in the study. Patients with lung disease, compromised cardiovascular status, obesity, age <18 years, pregnancy, difficult airway, and those coming for emergency surgeries were excluded from the study.

After attaching the standard monitors such as electrocardiogram, pulse oximeter, and noninvasive blood pressure, an arterial line was inserted in all the patients before the induction of anesthesia to monitor hemodynamic status and arterial blood gases (ABGs). A circle system with carbon dioxide absorber was used for the preoxygenation in both the groups. The patients were instructed to breathe normally from a tight-fitting face mask. A flow of 5 L/min of oxygen was used to preoxygenate the patients. The end-tidal oxygen (ETO2) levels was monitored to check the adequacy of preoxygenation and the preoxygenation was continued till the ETO2 levels were above 85%.

Patients were divided into two groups based on the method of preoxygenation method chosen by the covering consultant. In patients of Group A (n = 30), continuous positive airway pressure (CPAP) was not applied during preoxygenation and the oropharyngeal airway was not used, but oxygen was administered at 5 L/min using the semi-closed system during the apnea. In Group B (n = 30), a CPAP of 5 cmH2O was maintained during preoxygenation and after induction of anesthesia, an oropharyngeal airway was inserted after adequate depth of anesthesia was achieved. Patency of airway was confirmed by the presence of an ETCO2 trace when the patient was breathing spontaneously. Once the patient was completely apneic as confirmed with an absent ETCO2 tracing, muscle relaxant was administered and oxygenation was continued with the semi-closed system at 5 L/min and CPAP of 10 cmH2O.

An ABG analysis was made to measure the level of PaO2 and PaCO2 in the patients at the end of preoxygenation. Patients in both the groups were induced by standard induction techniques. Oxygenation was continued in both the groups as described above. In both the groups, patients were not ventilated and muscle relaxants such as rocuronium 1 mg/kg or succinylcholine 1.5 mg/kg was administered intravenously. Apnea was documented by the absence of ETCO2. An ABG sample was drawn at the end of 90 s and then the airway was secured using the appropriate sized cuffed endotracheal tube. The patients were monitored for hemodynamic instability and desaturation throughout the study period. We also assessed the patient tolerance of the tight-fitting face mask.

After conducting an initial pilot study with ten patients in each group, it was found that the mean difference of change in PaO2 was 79.9 mmHg between the two groups. The sample estimated was five patients in each group to have 95% confidence interval and 90% power. However, we recruited sixty patients in our study. Chi-square test was used to compare the gender between the two groups and independent t-test was used to compare the PaO2 and PaCO2 as well as the change in PaCO2. ANCOVA was used to compare the fall in PaO2 between the groups by adjusting for the baseline PaO2 values. The statistical analysis was done using IBM SPSS Statistics version 20 for Windows (SPSS Inc., Chicago, IL, USA).

RESULTS

The age of patients, distribution of gender, and ASA physical status in both groups were comparable [Table 1]. The mean PaO2 levels attained after preoxygenation were significantly lower in Group A as compared to Group B (500.8 ± 51 vs. 525.3 ± 42.5 mmHg, P = 0.048). The PaO2 levels after 90 s of apnea were also significantly lower in Group A (368.6 ± 98.4 vs. 494.8 ± 42.6 mmHg, P < 0.001) [Table 2 and Figure 1]. As baseline PaO2 values were not comparable, they were taken as covariate and ANCOVA was applied for further statistical analysis. The adjusted mean values for PaO2 at 90 s were calculated and compared. In Group B, the fall in PaO2 was found to be 33.6 mmHg, whereas it was 129.1 mmHg in Group A. The difference was statistically significant with P < 0.001 [Table 3]. Baseline PaCO2 values were comparable in both groups. The rise in PaCO2 was lower in Group B when compared to Group A, but this difference was statistically insignificant [Table 2 and Figure 2]. All patients in both the groups tolerated the preoxygenation well.

Table 1

Comparison of age, gender, and American Society of Anesthesiologists physical status

Table 2

PaO2 and PaCO2 levels after preoxygenation and apnea

Figure 1

Changes in arterial partial pressure of oxygen

Table 3

Corrected PaO2 values

Figure 2

Changes in arterial partial pressure of carbon dioxide

DISCUSSION

In the present study, it was found that during modified RSI, patients preoxygenated with CPAP had significantly higher PaO2 levels after preoxygenation as compared to those who did not receive CPAP. Those patients who received CPAP during preoxygenation and continued to receive apneic oxygenation with CPAP along with an oropharyngeal airway had significantly higher PaO2 at 90 s of apnea with a significantly lower fall in PaO2 than those who received neither CPAP nor had oropharyngeal airway in place.

RSI is indicated in patients who have not fasted or are considered full stomach because of delayed gastric emptying due to various reasons. The main goals of RSI are avoidance of bag and mask ventilation, prevention of aspiration (if patient regurgitates or vomits) by applying Sellick maneuver, and rapidly securing the airway with a cuffed endotracheal tube. We have used a modified RSI without Sellick maneuver as our aim was to rapidly secure the airway and avoid bowel distension which makes laparoscopic and robotic bowel surgeries difficult.

Preoxygenation delays desaturation during periods of apnea and during apneic periods, if oxygenation without mask ventilation is allowed, the safe period of apnea can be prolonged. Apneic oxygenation results in increased peri-intubation oxygen saturation, decreased rates of desaturation, and increased success rate at intubation in the first attempt.[4] Tidal volume breathing of 100% oxygen for 3 min or 3–8 vital capacity breaths usually result in 90% ET oxygen level and is the standard recommendation in the general population.[5] In order to maximize the effectiveness of preoxygenation, a combination of breathing 100% oxygen and CPAP ventilation in reverse Trendelenburg position is advised.[6]

Loss of consciousness at the induction of anesthesia leads to loss of tone of the pharyngeal muscles and the tongue falls back on the posterior pharyngeal wall and airway obstruction is frequent.[7] In the presence of airway obstruction even if a tight face mask seal is ensured, gases will not reach the lungs. Although patency of the airway can be maintained by triple airway maneuver (neck extension, jaw thrust/chin lift, and mouth opening), easier and definite results can be obtained by inserting an oropharyngeal airway. In the present study, in Group B patients, we have ensured the patency of the airway after anesthetizing patients by using an oropharyngeal airway and then continuing the CPAP. The Group B patients had a better oxygenation at the end of the apnea of 90 s.

During apneic oxygenation, flow of gases continues to take place due to mass flow as a gradient is created by the continuous uptake of oxygen.[89] During normal respiration, a dynamic equilibrium exists between respiratory gases. Oxygen moves from alveolar space to pulmonary blood and CO2 moves vice versa. The volume of CO2 moving to the alveolar space is 80% of the volume of oxygen moving to pulmonary blood. During apnea, the rate of oxygen extraction remains unaffected at approximately 250 mL/min. However, the amount of CO2 entering the alveoli is less as CO2 is more water soluble than oxygen. Only 10% (approximately 20 mL) of the CO2 produced every minute reaches the alveolar space. Hence, the volume of gases in alveoli decreases rapidly during apnea. The gases administered will flow down the concentration gradient if the airway is kept patent into the apneic lung. This is called “apneic mass-movement oxygenation” which reduces the rate of fall in PaO2 during apnea.[1] The benefit of apneic diffusion oxygenation is dependent on achieving maximal preoxygenation, maintaining airway patency, and the existence of a high functional residual capacity–to-body weight ratio.[2]

Application of CPAP helps in recruiting collapsed alveoli,[6] which in turn helps in reducing ventilation–perfusion mismatch and the rate of desaturation. The advantage of using CPAP had been proven by the observation that a low inspired oxygen (0.5) with CPAP of 5 cmH2O resulted in improved arterial oxygenation than a high inspired fractions of oxygen (1.0) without CPAP.[10] Preoxygenation and apneic ventilation with CPAP with face mask had shown to significantly delay desaturation during apnea with significantly higher arterial partial pressure of oxygen as compared that without CPAP.[11]

Presence of a patent airway is necessary for the apneic oxygenation to be effective, as it allows delivery as well as entrainment of oxygen from the pharynx to the lungs.[12] Various techniques had been tried in this regard. Use of nasopharyngeal catheters was found to be efficacious in prolonging the apneic period compared with preoxygenation alone.[1314] Delivery of high flows of oxygen (15 L/min) via nasal prongs may help in maintaining nasopharyngeal patency.[15] Laryngeal oxygen insufflation has also shown to delay desaturation during apneic oxygenation.[16]

We have studied the effect of CPAP along with the maintenance of airway patency with an oropharyngeal airway objectively and have found that it reduces the chance of desaturation during apneic period of modified RSI. This is the strong point of our study. This simple technique enhances the efficiency and efficacy of preoxygenation when used along with CPAP.

Limitations of the study

The limitations of the study were that there was no randomization or blinding. ABGs while patients were breathing room air prior to preoxygenation were not taken into account. We have assumed that patients with an ASA physical status Class 1 or 2 would have a normal PaO2 in the range of 80–100 mmHg. The age of patients in both the groups was also comparable. The baseline ABG sample taken after preoxygenation was not comparable, Group B had a higher PaO2 due to the CPAP applied during preoxygenation, and hence further analysis was done using ANCOVA with adjusted mean values.

CONCLUSION

During RSI, preoxygenation with CPAP of 5 cmH2O followed by apneic oxygenation with CPAP keeping the airway patent with an oropharyngeal airway has a higher efficiency and efficacy than standard preoxygenation without airway and CPAP. This technique results in significantly higher PaO2 after preoxygenation and relatively slower reduction in PaO2 during periods of apnea.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.