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Distribution of ventilation and oxygenation in surgical obese patients ventilated with high versus low positive end-expiratory pressure

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AUTHORS: Ellenberger, Christoph et al 

European Journal of Anaesthesiology 39(11):p 875-884, November 2022.

BACKGROUND

Intra-operative ventilation using low/physiological tidal volume and positive end-expiratory pressure (PEEP) with periodic alveolar recruitment manoeuvres (ARMs) is recommended in obese surgery patients.

OBJECTIVES

To investigate the effects of PEEP levels and ARMs on ventilation distribution, oxygenation, haemodynamic parameters and cerebral oximetry.

PATIENTS

One hundred and sixty-two patients with a BMI at least 35 kg per square metre undergoing elective open or laparoscopic surgery lasting at least 120 min.

INTERVENTION

Patients were randomised to PEEP of 4 cmH2O (n = 79) or PEEP of 12 cmH2O with hourly ARMs (n = 83).

MAIN OUTCOME MEASURES

The primary endpoint was the fraction of ventilation in the dependent lung as measured by electrical impedance tomography. Secondary endpoints were the oxygen saturation index (SaO2/FIO2 ratio), respiratory and haemodynamic parameters, and cerebral tissue oximetry.

RESULTS

Compared with low PEEP, high PEEP was associated with smaller intra-operative decreases in dependent lung ventilation [-11.2%; 95% confidence interval (CI) -8.7 to -13.7 vs. -13.9%; 95% CI -11.7 to -16.5; P = 0.029], oxygen saturation index (-49.6%; 95% CI -48.0 to -51.3 vs. -51.3%; 95% CI -49.6 to -53.1; P < 0.001) and a lower driving pressure (-6.3 cmH2O; 95% CI -5.7 to -7.0). Haemodynamic parameters did not differ between the groups, except at the end of ARMs when arterial pressure and cardiac index decreased on average by -13.7 mmHg (95% CI -12.5 to -14.9) and by -0.54 l min-1 m-2 (95% CI -0.49 to -0.59) along with increased cerebral tissue oximetry (3.0 and 3.2% on left and right front brain, respectively).

CONCLUSION

In obese patients undergoing abdominal surgery, intra-operative PEEP of 12 cmH2O with periodic ARMs, compared with intra-operative PEEP of 4 cmH2O without ARMs, slightly redistributed ventilation to dependent lung zones with minor improvements in peripheral and cerebral oxygenation.

KEY POINTS

  • In obese patients, small airway and alveolar collapse tends to develop during anaesthesia and mechanical ventilation.
  • Although the use of low/physiological tidal volume with PEEP and ARM has been largely advocated to prevent lung injury and atelectasis, the level of PEEP and the impact of ARM on respiratory, haemodynamic and oxygenation parameters remain controversial.
  • Compared with a PEEP level of 4 cmH20, setting the PEEP at 12 cmH20 with hourly ARMs resulted in a small gain in dependent lung ventilation along with lower driving pressure and minor improvements in systemic oxygenation.
  • At the end of each ARM, cerebral tissue oxygenation increased, whereas cardiac index and blood pressure transiently decreased.

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