Does Pulmonary Artery Systolic Pressure as Estimated by Transthoracic Echocardiography Alter the Effect of Positive End-Expiratory Pressure on Arterial Blood Gases and Hemodynamics in Morbidly Obese Patients?

BACKGROUND: Positive end-expiratory pressure (PEEP) at the time of induction increases oxygenation by preventing lung atelectasis. However, PEEP may not prove beneficial in all cases. Factors affecting the action of PEEP have not been elucidated well and remain controversial. Pulmonary vasculature has direct bearing on the action of PEEP as has been proven in the previous studies. Thus, this prospective study was planned to evaluate the action of PEEP on the basis of pulmonary artery systolic pressure (PASP) which is noninvasive and easily measured by transthoracic echocardiography.
MATERIALS AND METHODS: Seventy morbidly obese patients, the American Society of Anesthesiologists Grade II, or III, aged 20-65 years with body mass index >40 kg/m2, scheduled for elective laparoscopic bariatric surgery were included. Patients who denied consent, those undergoing emergency and/or open surgery and those requiring >2 attempts for intubation were excluded from the study. Ten patients had to be excluded. Thus, a total of sixty patients participated in the study. Thirty patients received no PEEP at the time of induction while other thirty patients were given a PEEP of 10 cm of H2O. Serial ABG samples were taken preoperatively, at the time of intubation, 5 min after intubation, and 10 min after intubation. Patients were then divided into four groups on the basis of PASP value of ≤30 mm Hg with and without PEEP or >30 mm Hg with and without PEEP. PRIMARY OUTCOME: The primary outcome was the effect of PEEP of 10 cm of H2 O on ABG and hemodynamics in morbidly obese patients. SECONDARY OUTCOME: The secondary outcome was the effect of PASP on the action of PEEP in morbidly obese patients undergoing laparoscopic surgery.
RESULTS: Patients having PASP of >30 mm Hg had significant improvement in oxygenation on PEEP application (270.11 ± 119.26 mm Hg) as compared to those without PEEP (157.57 ± 109.29 mm Hg) just after intubation. The increase in oxygenation remained significant at all time intervals. Patients with PASP ≤30 mm Hg did not show significant improvement in oxygenation with PEEP application (177.09 ± 85.85 mm Hg as compared to 226.27 ± 92.42 mm Hg without PEEP). Hemodynamic parameters did not show statistically significant alterations.
CONCLUSION: Morbidly obese patients who have PASP >30 mm Hg benefit most from the PEEP. Thus, PASP which is an easily measurable noninvasive parameter can be used as a criterion for selecting patients who benefit from PEEP application.

INTRODUCTION

Pulmonary gas exchange and respiratory mechanics are altered due to atelectasis during general anesthesia and paralysis, more so in obese population.[12] Application of positive end-expiratory pressure (PEEP) once atelectasis has set in may prove futile.[3] Application of PEEP during the induction of general anesthesia prevents atelectasis formation and provides more time for intubation.[4] Effects of PEEP on arterial oxygenation and hemodynamics are well documented. However, factors that influence the action of PEEP are still not known.

Dai et al. have concluded that echocardiographic measurements from an epidemiological study compare favorably with those taken in a clinical setting with experienced technical support. Thus, these measurements can be applied meaningfully to clinical observation.[5] Pulmonary artery systolic pressure (PASP) values obtained noninvasively show reliable results and good correlation to invasive measurements.[6]

The previous studies have shown that PEEP affects PA pressure. Thus, PASP should affect the action of PEEP. This study demonstrates the effect of PASP as estimated by transthoracic echocardiography (TTE) on PEEP of 10 cm H2O when applied at the time of induction in morbidly obese patients undergoing laparoscopic surgery.

MATERIALS AND METHODS

After Ethics Committee Approval, this prospective study was conducted in the Department of Anaesthesiology and Critical Care, Sri Aurobindo Institute of Medical Sciences and P.G. Institute and Mohak Hospitals, Indore, over a period of 1 year. Seventy morbidly obese patients, the American Society of Anesthesiologists Grade I, II, or III, aged 20–65 years with body mass index (BMI) >40 kg/m2, scheduled for elective laparoscopic bariatric surgery were selected, and a written informed consent was obtained. All the recruited patients underwent two-dimensional transthoracic echocardiography, and PASP was recorded. Echocardiography was performed by same cardiologist as this measurement is operator dependent. Patients who denied consent, those undergoing emergency and/or open surgery and those requiring >2 attempts for intubation were excluded from the study.

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 the induction of anesthesia in all the patients. No premedication was given. All the patients were induced with intravenous (i.v.) glycopyrrolate (0.005–0.01 mg/kg), i.v. fentanyl (2 μg/kg), and i.v. 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 cm of H2O was added to ventilator settings in study group while the control group received zero PEEP. Mask was held using four-hand technique. 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 post intubation.

Patients were then again divided into four groups on the basis of PASP:

Group 1: Patients with PASP ≤30 mm Hg receiving no PEEP (n = 11)

Group 2: PASP ≤30 mm Hg receiving PEEP of 10 cm H2O (n = 11)

Group 3: PASP >30 mm Hg receiving no PEEP (n = 19)

Group 4: PASP >30 mm Hg receiving PEEP of 10 cm H2O (n = 19).

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 H2O and showed marked hemodynamic instability. Thus, 60 patients completed the study and were divided into four groups. Demographic distribution of all four groups is shown in Table 1.

Table 1

Demographic profile

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 intragroup comparison of numerical variables. P < 0.05 was considered statistically significant for 95% confidence interval. Group 1 was compared with Group 2 while Group 3 was compared with Group 4.

Groups 1 and 2 did not show any significant difference in PaO2 and PaCO2 [Table 2]. They were comparable hemodynamically as well, indicating that the addition of PEEP has no effect in patients having PASP ≤30 [Tables 3 and 4]. Groups 3 and 4 were comparable hemodynamically [Tables 4 and 5], but PaO2 showed a significant increase in Group 4 [Table 6].

Table 2

Comparison of arterial blood gases in patients having pulmonary artery systolic pressure ≤30 mm Hg

Table 3

Hemodynamic parameters (systolic and diastolic blood pressure) when pulmonary artery systolic pressure ≤30 mm Hg

Table 4

Hemodynamic parameter (pulse rate) according to pulmonary artery systolic pressure

Table 5

Hemodynamic parameters (systolic and diastolic blood pressure) when pulmonary artery systolic pressure >30 mm Hg

Table 6

Comparison of arterial blood gases in patients having pulmonary artery systolic pressure >30 mm Hg

DISCUSSION

This is the only study which evaluates the effect of PASP on PEEP in obese patients. We found that PEEP has no effect in morbidly obese patients having PASP in the normal range. Patients having higher PASP (>30 mm Hg) benefit more with the application of PEEP. Since patients with high BMI generally have higher PASP, the effect of PEEP is better seen in obese individuals.

It has been observed that PEEP is beneficial in some patients only. PEEP alone does not decrease atelectasis.[37] It simply increases the normally aerated lung fraction.[3] However, when PEEP is applied at the time of induction of general anesthesia, it mainly acts by recruiting atelectatic areas.[4] In our study, this effect was predominantly seen in patients having PASP ≥30 mm Hg. The effects of PEEP on the pulmonary microcirculation are expected to differ depending on whether PEEP improves lung mechanics by recruiting atelectatic areas or deteriorates lung mechanics by over distending some areas.[3] Recruitment can be evaluated at the bedside by the oxygenation response while overdistension is likely if PaCO2 increases and compliance of the respiratory system decreases. Oxygenation has been found to correlate with recruitment in several experimental[78] and clinical studies.[910] An increase in PaCO2 indicates overstretching of the alveolar units associated with an increase in dead space.[11] This is corroborated in our study. We found that when PASP is <30 mm Hg, oxygenation actually decreased although insignificantly, after single minute of PEEP while PaCO2 rises significantly [Table 2]. Thus, positive alveolar pressure produced by PEEP sufficiently compresses the microvasculature due to overdistension of the alveoli when PASP is less. Resultant ventilation-perfusion mismatch offsets the effect of PEEP. There is no increase in oxygenation while PaCO2 increases significantly. When PASP is more, the pressure in the microvasculature is able to withstand the positive alveolar pressure. Hence, decreased ventilation-perfusion mismatch is seen and oxygenation improves significantly, and the corresponding PaCO2 remains the same initially and then gradually decreases [Table 6].

Gattinoni et al. in their study in patients of adult respiratory distress syndrome (ARDS) found that the effects of PEEP on pulmonary arterial pressure appear to be related to anatomic recruitment.[12] When application of PEEP determined effective alveolar recruitment, mean PA pressure decreased while cardiac output is not severely affected. This is due to the fact that alveolar recruitment is paralleled by a simultaneous recruitment of pulmonary vessels with a subsequent increase in the pulmonary vascular volume. A similar effect is produced when PASP is more as larger number of pulmonary vessels remain open during PEEP application. This results in better gas exchange at alveolar level.

Fougères et al. found that in ARDS patients a PEEP increase with limited tidal volume and plateau pressure reduced cardiac output by increasing the right ventricular afterload.[13] Passive leg raising restored cardiac output by increasing venous return thereby reducing the transpulmonary pressure difference and the pulmonary vascular resistance. This suggests that some pulmonary microvessels were collapsed by PEEP elevation and were recruited by increasing the central blood volume. Thus, adequate preloading ensures not only hemodynamic stability but also better alveolar gas exchange during PEEP application. The hemodynamic variables were comparable in Groups 1 and 2 and also Groups 3 and 4 in this study [Tables 3–5].

In our study, BMI and age of all the four groups were comparable [Table 1]. McQuillan et al. in their study have demonstrated an independent association of age, male sex, and BMI with PASP among echocardiographic normals.[14] Age was the strongest predictor of PASP, with an average increase in PASP of 0.8 mm Hg per decade. Interestingly, in our study Group, 4 patients (PASP ≥30 mm Hg with PEEP of 10 cm H2O) had the highest mean age and they benefitted the most with PEEP application [Tables 1 and 6].

The main limitation of our study is that it fails to demonstrate the effect of PEEP in patients with pulmonary hypertension as cardiac catheterization was not done. Doppler TTE tends to systematically overestimate PA pressures and hence cannot predict pulmonary hypertension. Effect of PEEP was evaluated only for a short duration starting from the time of induction resulting in minimum atelectasis. Hence, whether higher PASP enhances the effect of PEEP once atelectasis sets in cannot be commented upon. Also its effectiveness when applied for longer duration needs to be tested. Also whether, there is an upper limit of PASP after which the beneficial effect of PEEP ceases was not determined. Effects of associated respiratory comorbidities have not been taken into account. Most of the studies done till date have evaluated the effect of PEEP in ARDS patients who normally have elevated PASP. This subset also benefits from PEEP. Larger studies are required to see the comprehensive effect of PEEP in obese patients.

CONCLUSION

With this study, we can conclude that those morbidly obese patients who have PASP >30 mm Hg benefit most from the PEEP. Thus, PASP should be one of the criteria used for patient selection at the time of PEEP application.

Financial support and sponsorship

This study was financially supported by Sri Aurbindo Institute of Medical Sciences and Post graduate Institute, Indore.

Conflicts of interest

There are no conflicts of interest.