Why should shock patients receive high Positive End-Expiratory Pressure (PEEP) in mechanical ventilation?

Use of High PEEP in Shock Patients on Mechanical Ventilation

Shock patients should not routinely receive high PEEP during mechanical ventilation as positive intrathoracic pressure can reduce venous return and worsen right ventricular failure, potentially exacerbating hemodynamic instability. 1

Hemodynamic Effects of PEEP in Shock

• Positive intrathoracic pressure induced by mechanical ventilation, particularly high PEEP, can reduce venous return and worsen right ventricular (RV) failure in patients with shock 1
• In patients with septic shock, PEEP should be applied with caution to avoid circulatory depression and further hemodynamic compromise 1
• Mechanical ventilation with high PEEP can cause adverse hemodynamic effects that are more prominent in patients with shock due to their already compromised cardiovascular status 1

PEEP Recommendations Based on Shock Type

Septic Shock with ARDS:

• For sepsis-induced moderate to severe ARDS, higher PEEP is suggested over lower PEEP (weak recommendation, moderate quality evidence) 1
• However, this recommendation must be balanced against the potential for hemodynamic deterioration in shock states 1
• PEEP titration should be done cautiously while monitoring for signs of decreased cardiac output 1

Cardiogenic Shock (including PE-induced):

• In pulmonary embolism with shock, PEEP should be applied with extreme caution as it may worsen RV failure 1
• When mechanical ventilation is required in PE with shock, positive end-expiratory pressure should be applied with caution due to its potential to reduce venous return 1
• Low tidal volumes (approximately 6 ml/kg lean body weight) should be used to keep end-inspiratory plateau pressure below 30 cm H₂O 1

Balancing PEEP Benefits and Risks

• Higher PEEP can improve oxygenation and reduce ventilator-induced lung injury by preventing alveolar collapse 2
• However, PEEP may also cause circulatory depression and contribute to lung injury through alveolar overdistention 2, 3
• The overall effect of PEEP depends on the balance between alveolar recruitment and overdistention when PEEP is applied 2

Monitoring During PEEP Titration

• Careful cardiovascular monitoring is essential when applying PEEP in shock patients 4, 5
• Serial determinations of cardiac output and blood pressure are necessary to assess the hemodynamic response to PEEP 4
• In some patients, increasing PEEP may paradoxically worsen oxygenation by decreasing cardiac output and increasing intrapulmonary shunt 5

Practical Approach to PEEP in Shock Patients

1. Start with lower PEEP levels (5-8 cm H₂O) in shock patients 1, 6
2. If the patient has ARDS, cautiously increase PEEP while monitoring hemodynamic parameters 1
3. If hemodynamic deterioration occurs (>20% drop in blood pressure), reduce PEEP immediately 5
4. Consider fluid resuscitation before applying higher PEEP, but avoid aggressive volume loading which can worsen RV function 6
5. If vasopressors are required, norepinephrine is preferred as it can improve RV function and coronary perfusion 1

Special Considerations

• In pediatric sepsis with PARDS (Pediatric ARDS), higher PEEP is suggested but with careful monitoring for adverse hemodynamic effects 1
• For patients with pulmonary embolism and shock, avoid aggressive volume expansion as it may worsen RV function through mechanical overstretch 6
• Recruitment maneuvers, while potentially beneficial for oxygenation, should be used with extreme caution in shock patients due to their potential to cause profound hemodynamic instability 1

Remember that the primary goal in shock patients is to stabilize hemodynamics while providing adequate oxygenation. PEEP should be carefully titrated to optimize these sometimes competing objectives.