Recommended Lung Volumes for Hypercarbia on BiPAP
For patients with hypercarbia on BiPAP, target tidal volumes of 6-8 mL/kg ideal body weight, with pressure support adjusted to achieve these volumes while maintaining pH >7.20 and reducing PCO2 toward the patient's baseline awake value. 1
Target Tidal Volume Parameters
The primary tidal volume goal is 6-8 mL/kg using ideal body weight for most patients with hypercapnic respiratory failure. 1 This range provides adequate alveolar ventilation while avoiding excessive lung stretch and barotrauma.
- For volume-targeted BiPAP specifically, the recommended tidal volume target is 8 mL/kg ideal body weight 1
- Slightly lower tidal volumes (closer to 6 mL/kg) with higher respiratory rates may be better tolerated in individual patients, particularly those with restrictive thoracic disease 1
- In patients with lung disease, higher minute ventilation is needed to deliver adequate alveolar ventilation due to increased physiological dead space (normally ~2 mL/kg) 1
Pressure Support Titration Strategy
Increase pressure support (IPAP-EPAP difference) by 1-2 cm H2O increments every 5 minutes if tidal volume remains below 6-8 mL/kg. 1
- Maximum IPAP should not exceed 25-30 cm H2O on most devices 1
- Minimum incremental change should be 1 cm H2O, as smaller changes are unlikely to be clinically meaningful 1
- Maximum incremental change should be 2 cm H2O to avoid over-titration 1
Blood Gas Targets for Hypercapnia
The acceptable goal for PCO2 is a value less than or equal to the patient's awake PCO2, not necessarily "normal" values. 1
- Increase pressure support if arterial PCO2 remains 10 mm Hg above goal at current settings for 10 minutes or more 1
- Target pH >7.20 as the primary goal rather than normalizing CO2, as this threshold represents acceptable permissive hypercapnia 1, 2
- In patients with chronic hypercapnia, the higher the pre-morbid PCO2 (inferred by elevated admission bicarbonate), the higher the target PCO2 should be 1
Additional Ventilatory Parameters
Pressure support may be increased if respiratory muscle rest has not been achieved after 10 minutes at current settings. 1
- Adequate respiratory muscle rest is indicated by resolution or improvement in tachypnea and excessive inspiratory effort 1
- If SpO2 remains below 90% for 5 minutes or more AND tidal volume is low (<6-8 mL/kg), increase pressure support 1
- For obstructive disease patterns, use lower respiratory rates (10-15 breaths/minute) with longer expiratory times (I:E ratio 1:2-1:4) to prevent air trapping 1, 2
Critical Monitoring Considerations
Check for excessive mask leak whenever increases in pressure support fail to raise tidal volume. 1
- The accuracy of BiPAP device estimates of tidal volume depends on flow signal accuracy, which is degraded by mouth leak or high mask leak 1
- Intervention for leak (mask refit or change) should be considered before further pressure increases, especially if prior increases in pressure support have been ineffective 1
- Discrepancy between device-estimated tidal volume and clinical scenario should prompt immediate leak assessment 1
Common Pitfalls to Avoid
- Do not rapidly normalize CO2 to "normal" levels if the patient has been chronically hypercapnic with metabolic compensation, as this can worsen cerebral perfusion 2
- Avoid setting PEEP greater than intrinsic PEEP (iPEEP) in obstructive disease, as this can be harmful and increase work of breathing 1
- Do not use sodium bicarbonate for respiratory acidosis, as it produces additional CO2 that must be eliminated through ventilation 2
- Recognize that small increases in alveolar ventilation produce relatively large decreases in PCO2 in hypercapnic patients due to the hyperbolic relationship between these variables 1