Why is my patient with respiratory acidosis breathing over their ventilator (ventilator-associated respiratory distress)?

Managing Patient Breathing Over the Ventilator in Respiratory Acidosis

Patient-ventilator asynchrony is likely causing your patient with respiratory acidosis (RAAS -5) to breathe over the ventilator, requiring immediate adjustment of ventilator settings to match the patient's respiratory demands.

Understanding the Problem

When a patient breathes over the ventilator (patient-ventilator asynchrony), it indicates a mismatch between the patient's respiratory demands and the ventilator's delivered support. This commonly occurs in respiratory acidosis when:

1. Flow delivery is inadequate: The ventilator's inspiratory flow doesn't meet the patient's demand 1
2. Intrinsic PEEP (auto-PEEP): Air trapping creates an inspiratory threshold load 1
3. Trigger sensitivity issues: Delayed or ineffective triggering 1
4. Inappropriate termination criteria: Flow cycling occurs too early or too late 2

Immediate Assessment and Intervention

Step 1: Examine Flow-Volume Waveforms

• Look for evidence of patient-ventilator asynchrony on pressure/flow waveforms 1
• Check for ineffective triggering efforts (patient effort without ventilator response)
• Assess for flow starvation (concave pressure curve during inspiration)

Step 2: Adjust Ventilator Settings

1. Flow Settings:

   • Increase inspiratory flow rate if flow starvation is evident
   • Consider switching to pressure control ventilation (PCV) which provides a decelerating flow pattern with higher initial flow rates 3
   • PCV significantly reduces patient work of breathing compared to volume control in acute lung injury 3

2. Trigger Sensitivity:

   • Optimize trigger sensitivity to detect patient effort without auto-triggering
   • Flow triggering is more sensitive than pressure triggering 1

3. Address Auto-PEEP:

   • If intrinsic PEEP is present (common in obstructive disease):
     • Increase expiratory time by reducing respiratory rate
     • Reduce tidal volume to 6-8 mL/kg predicted body weight 4
     • Consider adding external PEEP to offset intrinsic PEEP (typically 80% of measured auto-PEEP) 1

4. Termination Criteria:

   • Adjust flow termination criteria to 5-20% of peak inspiratory flow 2
   • Too low (1%) causes delayed termination; too high (>35%) causes premature termination 2

Step 3: Manage Acid-Base Status

• Target pH 7.2-7.4 (permissive hypercapnia is acceptable) 1
• Avoid rapid correction of chronic hypercapnia 1
• The higher the pre-morbid pCO2 (suggested by high bicarbonate), the higher the target pCO2 should be 1

Special Considerations Based on Underlying Pathology

For Obstructive Disease (COPD/Asthma):

• Keep PEEP low (3-5 cmH₂O) to avoid worsening air trapping 4
• Prolong expiratory time by reducing respiratory rate and I:E ratio (1:2-1:4) 1
• Target I:E ratio of 1:2-1:4 1

For Restrictive Disease (Pulmonary Fibrosis, ARDS):

• Higher PEEP (>10 cmH₂O) may be beneficial 4
• Lower tidal volumes (4-6 mL/kg PBW) 4
• Target I:E ratio closer to 1:1 1

For Neuromuscular Disease:

• Lower inflation pressures (10-15 cmH₂O) are usually sufficient 1
• Patient may need more ventilatory support if triggering is inadequate 1

Consider Sedation if Necessary

• If adjustments fail to resolve asynchrony, consider:
  • Short-acting sedation to reduce respiratory drive temporarily 1
  • In severe ARDS, neuromuscular blockade may be needed for the first 48 hours 1
  • For symptom control in agitated patients, intravenous morphine 2.5-5 mg may improve tolerance 1

Pitfalls to Avoid

1. Excessive sedation: Can prolong ventilator dependence and ICU stay 1
2. Ignoring patient comfort: Discomfort increases work of breathing and worsens outcomes 1
3. Over-correction of hypercapnia: Rapid correction in chronic hypercapnia can cause alkalosis and adverse effects 1
4. Overlooking the underlying cause: Address any reversible causes of increased respiratory drive (pain, anxiety, fever, metabolic acidosis) 1
5. Setting PEEP higher than intrinsic PEEP: Can worsen hyperinflation 1

By systematically addressing these factors, you can improve patient-ventilator synchrony, reduce work of breathing, and optimize management of respiratory acidosis in your ventilated patient.