Should a patient with Obstructive Sleep Apnea (OSA) or Obesity Hypoventilation Syndrome (OHS) who is undergoing a trial of Pressure Support Ventilation (PSV) and experiencing serially rising carbon dioxide (CO2) levels despite compensated pH be switched to Assist Control (AC) ventilation?

Should You Switch from PSV to Assist Control in OHS/OSA with Rising PCO2?

Yes, switch to assist control (pressure control) ventilation when a patient with OHS/OSA on PSV trial demonstrates serially rising PCO2 despite compensated pH, as this indicates inadequate ventilatory support that requires a controlled mode to guarantee minute ventilation and prevent respiratory muscle fatigue. 1

Why PSV is Failing in This Clinical Scenario

Pressure Support Ventilation (PSV) requires patient-initiated breaths and provides variable support based on patient demand 2. In your patient, the serially rising PCO2 despite compensated pH indicates:

• Progressive hypoventilation is occurring - The kidneys are compensating metabolically (maintaining pH) but the underlying ventilatory failure is worsening 1
• PSV cannot guarantee adequate minute ventilation when the patient's respiratory drive is insufficient or when work of breathing exceeds the support provided 2
• Sleep-disordered breathing likely pre-dates or complicates the acute presentation, making spontaneous triggering unreliable 1

Evidence Supporting the Switch to Controlled Ventilation

Guideline Recommendations for OHS

The BTS/ICS guidelines explicitly recommend pressure-controlled mechanical ventilation initially in patients with OHS 1. Key supporting evidence:

• High inspiratory pressures (IPAP >30) and expiratory pressures (EPAP >8) are commonly required in OHS patients 1
• Volume control or volume-assured modes may be more effective when high inflation pressures are required 1
• A switch from controlled to assisted ventilation should only be made once patient recovery allows 1

The Sleep Factor

Clinical research in stable sleep hypoventilation suggests that limiting the increase in hypercapnia during sleep is critical, and controlled ventilation mode may be more advantageous than assist mode 1. This is particularly relevant because:

• PSV can cause central apneas during sleep, especially when pressure support levels are high, leading to major sleep fragmentation 2
• Patient-ventilator desynchronization occurs frequently (55% of patients) in OHS patients on PSV, mostly during slow-wave and REM sleep 3
• If sleep-disordered breathing pre-dates AHRF or complicates an episode, the use of a controlled mode of NIV overnight is recommended 1

Practical Algorithm for Mode Selection

When to Use Pressure Control (Your Current Situation)

Patients requiring complete ventilatory control should be placed on pressure control ventilation (PCV) 2. This includes:

• Serially rising PCO2 on PSV (your patient) 1
• Heavily sedated or unable to trigger consistently 2
• Severe respiratory muscle weakness or fatigue 1
• Need for guaranteed minute ventilation regardless of patient effort 2

Initial Pressure Control Settings for OHS

Based on guideline recommendations 1, 4:

• Start with IPAP of 20-30 cm H2O (often >30 required in OHS) 1
• EPAP of 8-15 cm H2O (higher range often needed to recruit collapsed lung and treat concurrent OSA) 1
• Backup rate of 10-15 breaths/minute, set equal to or slightly less than spontaneous sleeping respiratory rate 4
• Inspiratory time of 30-40% of respiratory cycle 4
• Target tidal volume of 6-8 mL/kg predicted body weight 4

Critical Monitoring During Transition

Monitor these parameters to confirm adequate ventilatory support 1:

• Arterial blood gases within 1-2 hours - Look for pH normalization and PCO2 reduction 1
• Respiratory rate - Should decrease with adequate support 1
• Delivered tidal volumes - Adjust pressure settings to prevent volutrauma 2
• Patient-ventilator synchrony - Watch for fighting the ventilator 3

Common Pitfalls to Avoid

Don't Continue PSV When It's Failing

Continued use of NIV when the patient is deteriorating, rather than escalating to more controlled ventilation, increases mortality 1. Your patient's serially rising PCO2 is a clear deterioration signal.

Address Fluid Overload Aggressively

Fluid overload commonly contributes to ventilatory failure in OHS patients, and its degree is easily underestimated 1. Consider:

• Forced diuresis is often indicated - Fluid retention may exceed 20 liters 1
• BNP-directed fluid management should be considered if left ventricular dysfunction is present 1

High Pressures Are Expected and Necessary

Don't be afraid of high IPAP settings (>30 cm H2O) in OHS 1. These patients have:

• High impedance to inflation requiring high pressures 1
• Need for high PEEP to recruit collapsed lung units 1
• Concurrent severe OSA requiring high EPAP 1

When to Consider Switching Back to PSV

Only switch from controlled to assisted ventilation once patient recovery clearly allows 1. This means:

• pH normalized and PCO2 <6.5 kPa (approximately 49 mmHg) 1
• Precipitant cause of acute hypercapnic respiratory failure has been treated 1
• Fluid overload has been addressed 1
• Patient demonstrates consistent spontaneous respiratory effort 2

Long-term Considerations

Following an episode of acute hypercapnic respiratory failure, referral to a home ventilation service is recommended 1. Many OHS patients will require:

• Long-term domiciliary NIV or CPAP 1
• Continued pressure control mode if severe OSA (AHI >30) is not the predominant feature 1, 5
• Volume-targeted pressure support as an alternative once stable 4