There is no universally optimal ventilation approach when patients are placed in Steep Trendelenburg position, typically defined as a 30-40 degree head-down tilt. This positioning causes upward displacement of the diaphragm due to increased intra-abdominal pressure. Consequently, abdominal contents exert pressure on the diaphragm, leading to heightened lung pressure, particularly in obese individuals. To mitigate risks, the ventilator can be configured to deliver a tidal volume of 6-8 mL/kg (based on ideal body weight) alongside a moderate positive end-expiratory pressure (PEEP) level of approximately 5-8 cmH2O. Lower tidal volumes can be employed either through volume control or pressure-controlled breaths. However, if the patient can only tolerate lower tidal volumes, significant hypercapnia and hypoxia may ensue.
Special attention is warranted when employing pressure control ventilation during abdominal insufflation, as a sudden reduction in abdominal pressure can result in excessive lung pressure. Vigilance is essential to prevent barotrauma.
Typically, the ventilator's inspiratory to expiratory phase ratio is set at 1:2. Increasing the inspiratory time (e.g., I:E ratios of 1:1 or 1:1.5) elevates mean airway pressure, thereby enhancing oxygenation at the expense of CO2 clearance. Conversely, ratios such as 1:3 or 1:4 facilitate prolonged exhalation, leading to decreased end-tidal CO2 levels (although ratios exceeding 1:4 may cause breath stacking). A recent clinical investigation focusing on laparoscopic bariatric surgery revealed that sequentially adjusting the I:E ratio from 1:2 to 1:1 to 2:1 while applying PEEP at 5 cmH2O significantly lowered peak airway pressure and elevated mean airway pressure and dynamic compliance without adverse hemodynamic effects.
| Setting | Effect | Caution |
|---|---|---|
| Tidal volume 6–8 mL/kg (ideal body weight) | Limits lung pressure when abdominal contents press upward on the diaphragm, particularly in obese patients. | If only lower tidal volumes are tolerated, significant hypercapnia and hypoxia may ensue. |
| PEEP 5–8 cmH2O | Moderate level paired with the lower tidal volume to mitigate risk. | — |
| I:E ratio 1:2 | The typical default setting. | — |
| I:E ratio 1:1 or 1:1.5 | Longer inspiratory time raises mean airway pressure and enhances oxygenation. | Comes at the expense of CO2 clearance. |
| I:E ratio 1:3 or 1:4 | Prolonged exhalation lowers end-tidal CO2. | Ratios exceeding 1:4 may cause breath stacking. |
| Pressure control during insufflation | One way to deliver lower tidal volumes, alongside volume control. | A sudden reduction in abdominal pressure can produce excessive lung pressure; stay vigilant for barotrauma. |
