Single Minute of Positive End-expiratory Pressure at the Time of Induction: Effect on Arterial Blood Gases and Hemodynamics in Morbidly Obese Patients Undergoing Laparoscopic Bariatric Surgery.

BACKGROUND: The effect of positive end-expiratory pressure (PEEP) has been studied in detail after induction of general anesthesia especially in obese individuals. However, sparse information can be gathered from the literature regarding its effect when applied at the time of induction and the time of onset of its effect. Thus, this study was planned to assess the effect of PEEP when applied for a single minute in morbidly obese patients.
MATERIALS AND METHODS: This was a randomized prospective study comprising seven morbidly obese patients (body mass index ≥40 kg/m2). Control group included 30 patients who received no PEEP at the time of induction. The study group consisted of thirty patients who were given a PEEP of 10 cmH2O. Serial arterial blood gas samples were taken preoperatively, at the time of intubation, 5 min after intubation and 10 min after intubation.
RESULTS: PaO2 was significantly higher in test group (242.0 ± 116.0 mmHg) than in control group (183.0 ± 107.0 mmHg) just after intubation. PaCO2 was comparable in control group (43.73 ± 6.32 mmHg) and test group (44.52 ± 6.33 mmHg) just after intubation but was significantly less in test group than in control group at 5 and 10 min thereafter. Hemodynamic parameters were comparable in both groups at all time intervals.
CONCLUSION: Application of even a single minute of PEEP at the time of induction improves oxygenation without any adverse effects on hemodynamics, in morbidly obese patients undergoing laparoscopic Bariatric surgery.

RCT Entities:   Population Interventions Outcomes

INTRODUCTION

Obesity has now been termed as globosity indicating its widespread prevalence. With it comes a load of diseases for which more and more obese patients go under the knife. General anesthesia is known to alter respiratory mechanics in normal weight, healthy adult controls within minutes of induction and preoxygenation with 100% oxygen has been recommended to reduce hypoxemia.[12] This enhances atelectasis formation during induction itself. The exaggerated effect is seen in morbidly obese patients resulting in significant impairment of pulmonary gas exchange and respiratory mechanics.[34] Thus, this patient population desaturate more rapidly than nonobese patients during apnea.[5] Recruitment maneuver along with positive end-expiratory pressure (PEEP) has been shown to be the most effective method in reducing atelectasis and improving oxygenation.[6] The effect of PEEP alone remains controversial with some studies refuting any effect on atelectasis. But if PEEP is applied at the time of induction, before atelectasis sets in, it should improve oxygenation. Maisch et al.[7] had shown that an optimal PEEP was 10 cmH2O because at this pressure level the highest compliance value in conjunction with the lowest dead space fraction indicated a maximum amount of effectively expanded alveoli. Therefore, we proposed to study the effect of PEEP of 10 cmH2O when applied at the time of induction for a single minute on arterial blood gases (ABGs) and hemodynamics.

MATERIALS AND METHODS

After taking Local Ethics Committee approval (Ethical and Research Committee, SAMC and PGI, Indore, protocol No. Saims/IEC/14/02/35, on February 25, 2014), this prospective study was conducted in the Department of Anaesthesiology and Critical Care over a period of 1 year. Seventy morbidly obese patients, American Society of Anesthesiologists physical Status II or III, aged 20–65 years with body mass index >40 kg/m2, scheduled for elective laparoscopic bariatric surgery were selected, and a written informed consent was obtained. Patients who denied consent, those undergoing emergency and/or open surgery and those requiring more than two attempts for intubation were excluded from the study.

Using a computer generated random number tables, patients were divided into two groups, study group (with PEEP) and control group (without PEEP). Arterial line was inserted preoperatively, and ABG sample was taken and hemodynamic parameter recording done while the patient was breathing room air. Both groups were preoxygenated for 3 min with 100% oxygen. Standard procedure was used for induction of anesthesia in all the patients. No premedication was given. All the patients were induced with intravenous (IV) glycopyrolate (0.005–0.01 mg/kg), IV fentanyl (2 μg/kg) and IV propofol on the basis of loss of verbal response. Once the patient became unresponsive to verbal commands, succinylcholine was then administered in a dose of 1–1.5 mg/kg. This was immediately followed by mechanical ventilation with 100% oxygen using volume-controlled mode on anesthesia machine. Tidal volume was set according to 7 ml/kg ideal body weight and a respiratory rate of 14/min. PEEP of 10 cmH2O was added to ventilator settings in study group while the control group received zero PEEP. Mask was held using four hand techniques. After 1 min endotracheal intubation was done. PEEP was continued in study group after intubation for a minimum of 10 min. ABG analysis and hemodynamic parameters were recorded at following stages: preoperatively, just after inflation of cuff of endotracheal tube, 5 min postintubation, 10 min postintubation.

RESULTS

This study comprised total seventy patients. Eight patients in whom arterial line could not be placed preoperatively were excluded from the study. Two patients had to be excluded from the study as they were not able to tolerate a PEEP of 10 cm and showed marked hemodynamic instability. Thus, thirty patients each in study group and control group were included. The demographic profile was similar in both groups [Table 1]. All the patients were intubated in single attempt and time for placement and inflation of cuff did not require more than 1 min. Hence, the analysis of first sample was done at 1½–2 min of onset of PEEP and continued till 10 min thereafter.

Table 1

Patient characteristics

The results obtained were collected, tabulated, and analyzed by unpaired Student's t-test. Categorical variable was compared between groups by Student's t-test with two sample proportions. Paired t-test was employed for intra-group comparison of numerical variables. P < 0.05 was considered statistically significant for 95% confidence interval.

Systolic and diastolic BP was found to be comparable (P > 0.05) in both groups at all the times of measurement [Table 2]. Pulse rate was found to decrease significantly in the study group at 5 min of induction [Table 3]. Table 4 shows a comparison of ABGs between both groups. Significant improvement was seen in arterial oxygenation (PaO2) with a PEEP of 10 cmH2O just after induction. Thus, single minute of PEEP had improved oxygenation with the effect continuing after 5 and 10 min. Arterial carbon dioxide (PaCO2) did not show any significant difference just after induction but reduced significantly at 5 and 10 min.

Table 2

Comparison of hemodynamic parameters (systolic and diastolic blood pressure)

Table 3

Comparison of hemodynamic parameters (pulse rate beats/min)

Table 4

Comparison of blood gases (mmHg)

DISCUSSION

In this study, we found that application of PEEP during induction of anesthesia especially in morbidly obese patients even for a single minute significantly increases oxygenation.

It has been shown by Reinius et al.[6] that in morbidly obese patients, induction of anesthesia and paralysis reduces end expiratory lung volume, promotes atelectasis in dependent lung regions and causes a marked fall in arterial oxygenation. PEEP alone increases the normally aerated lung fraction. There is a reduction of poorly aerated lung tissue while atelectasis remains unchanged. Thus, there is no change in oxygenation. However, PEEP at the time of induction improves oxygenation by all three actions, namely, decreased atelectasis due to general anesthesia, increased normally aerated lung fraction and decreased hypoxic pulmonary vasoconstriction. Thus, oxygenation was significantly improved in our study [Table 4], thereby giving a longer period of nonhypoxic apnea. All the patients were intubated in single attempt. However, five patients were difficult to ventilate. Three patients belonged to control Group while two patients belonged to Study Group. It was found that even in these patients if PEEP was continued, arterial oxygenation either remained same as preoperative value or improved marginally. Oxygenation decreased when no PEEP was applied.

In our study, PaCO2 was comparable in two groups preoperatively [Table 4]. At the time of intubation, no significant difference was found between the two groups. However, after 5 and 10 min of ventilation with PEEP, PaCO2 significantly decreased. This shows that PEEP takes longer time to produce an effect on PaCO2. Reduction in PaCO2 occurs possibly due to increased fraction of normally ventilated lung and decreased ventilation-perfusion mismatch.

PEEP increases both peak inspiratory pressures and the mean airway pressures; it has the potential to decrease venous return and cardiac output. However, in our study, both groups were comparable regarding hemodynamics [Table 2]. No hemodynamic consequences were observed at high airway pressures. Although the pulse rate decreased significantly after 5 min, it did not alter the BP in the study group. Only two patients showed signs of severe hemodynamic instability and had to be excluded from the study, probably due to prolonged fasting and inappropriate preloading. Bohm et al.[8] have also shown that after optimization of preload, high positive airway pressures are hemodynamically well tolerated in morbidly obese patients with or without capnoperitoneum. Jellinek et al.[9] have demonstrated the absence of any hemodynamic compromise at high levels of PEEP if central venous pressures were kept higher than 10 mmHg. Therefore, PEEP can be used without any adverse effects on hemodynamics in patients who are adequately hydrated.

Only one similar study, which was done by Coussa et al.[10] could be found in which the effect of PEEP at the time of induction of general anesthesia was studied. They had shown that despite the use of 100% oxygen, application of PEEP throughout the induction period nearly completely prevents atelectasis formation in morbidly obese patients. However, they had studied the effect of PEEP given for 5 min along with continuous positive airway pressure (CPAP) and computed tomography scans were compared of patients receiving PEEP with those receiving no PEEP. Our study corroborates the above findings even without the application of CPAP.

A potential risk of mechanical ventilation by mask with PEEP is exposing sedated and paralyzed patient to stomach insufflations thereby, increasing the risk for regurgitation and aspiration. Gastric distension was found in some patients in our study, but it was not so significant so as to cause any major problem during the creation of pneumoperitoneum or thereafter. In our study, postoperative ABG analysis was not done which could have determined whether such a short duration of PEEP at the time of induction helps postoperatively or not.

With this study, we can conclude that PEEP of 10 cmH2O effectively improves oxygenation without altering the hemodynamic parameters even when applied for a single minute at the time of induction.

Financial support and sponsorship

Mohak Hi Tech Institute, Indore.

Conflicts of interest

There are no conflicts of interest.