BPAP vs AVAPS for Hypercapnic Respiratory Failure in the ED

Background: Non-invasive positive pressure ventilation (NIPPV) is an effective treatment modality for patients with both hypoxemic or hypercapnic respiratory failure and has been shown to decrease the need for intubation [2].  Continuous positive airway pressure (CPAP) provides a constant positive pressure support throughout the breathing cycle, while bilevel positive airway pressure (BPAP) provides different levels of positive airway pressure during inspiration and expiration. The two pressure levels (inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP)) are useful in increasing ventilation and improving CO2 elimination. A newer mode of NIPPV, average volume-assured pressure support (AVAPS) aims to achieve a target tidal volume with variable pressure support (IPAP min and IPAP max) during inspiration.  Advocates feel that AVAPS is a more comfortable pressure support mode, which varies according to the patient’s needs (This is like PRVC for mechanical ventilation).  There are very limited studies however in the literature on the AVAPS mode of NIPPV.

Paper: Goren NZ et al. Comparison of BPAP S/T and Average Volume-Assured Pressure Support Modes for Hypercapnic Respiratory Failure in the Emergency Department: A Randomized Controlled Trial. Balkan Med J 2021. PMID: 34462252

Clinical Question: Does AVAPS NIPPV in patients with hypercapnic respiratory failure improve PaCO2 and pH values better than traditional BiPAP NIPPV?

What They Did:

• Single-center, single blind, prospective, randomized clinical trial
• Patients with hypercapnic respiratory failure presenting to ED
• Patients randomized to:
  • Average volume-assured pressure support (AVAPS)
    • Target TV: 6 – 8mL/kg
    • EPAP: 6cm H20
    • IPAPmin: 12cmH20
    • IPAPmax: 26cmH20
  • Bilevel positive airway pressure spontaneous/timed (BPAP S/T)
    • EPAP 6cmH20
    • IPAP 12cmH20
    • IPAP increased by 2cm H2O according to patients’ blood gas and clinical findings, under the physician’s decision on patients’ needs

Outcomes:

• Primary: Improvement in PaCO2 and pH values
• Secondary:
  • Change in mode (Transition to the second mode) due to treatment failure or patient’s non-compliance
  • Need for intubation
  • ED outcomes 

Inclusion:

• Shortness of breath
• PaCO2 >45mmHg on ABG
• Indications for NIPPV in the ED
  • SpO2 <90% (on room air)
  • SpO2 <93% (Despite 6LPM)
  • Use of accessory respiratory muscles
  • Inability to speak in whole sentences due to respiratory distress
  • RR > 24
  • Change in mental status

Exclusion:

• Urgent need for intubation
• Patient’s inability to maintain airway continuity
• Presence of pneumothorax
• Trauma
• Cardiac arrhythmias or suspected MI
• Patients with tracheostomy

Results:

• 148 patients presented to the ED with hypercapnic respiratory failure during the trial period
  • 8 patients refused to participate in the study and 60 patients were excluded
  • 80 patients with hypercapnic respiratory failure requiring NIPPV
    • BPAP S/T: 33pts
    • AVAPS: 47pts
    • COPD most common comorbid disease: 63pts (78.8%)
    • GCS Range: 9 to 15
  • pH Change in 1^st Hour
    • BPAP S/T: 0.03
    • AVAPS: 0.07
    • P = 0.015
  • PaCO2 Excretion in 1^st Hour
    • BPAP S/T: 4.75
    • AVAPS: 10.20
    • P = 0.033
  • Secondary Outcomes

Strengths:

• Randomized clinical trial
• Age, comorbid diseases, vital signs, and baseline blood gas parameters similar between groups
• Important study which compares BPAP S/T vs AVAPS NIPPV

Limitations:

• Study was underpowered
• Unable to collect information on patients’ level of comfort with the different NIPPV modes
• 13 patients were transferred to ICUs of other hospitals which limited the ability to obtain further clinical information on these patients
• Study too small to make any conclusions on safety
• As clinicians knew what type of NIPPV each patient was receiving this could impact treatment decisions (i.e. need for intubation)
• Unclear what other treatments were given to patients
• Randomization scheme is weak (envelopes) and this could be why the split of patients (33 vs 47) was odd
• Primary outcome non-patient centered and may have influenced clinical decisions on treatment and intubation
• Baseline demographic information is rather limited

Discussion:

• Study needed a minimum sample size of 94 patients achieve a power of 95% which they did not achieve (Only recruited 80 patients)
• Of note patients with hypercapnic respiratory failure had comparatively similar improvements in blood gas parameters, but with faster rates in the AVAPS mode than in the BPAP S/T. Authors feel this is due to patients’ comfort and compliance, but this was not evaluated in this trial

Author Conclusion: “In this study, improvements in blood gas parameters in the AVAPS groups were faster compared to the S/T group; However, we did not find any significant difference between the groups in terms of clinical parameters.  The AVAPS mode is as effective and safe as BPAP S/T in treating patients with hypercapnic respiratory failure in the ED.” 

Clinical Take Home Point: In patients presenting to the ED with hypercapnic respiratory failure, AVAPS did lead to a faster improvement in pH and PaCO2 levels compared to BPAP S/T. Unfortunately, this RCT is too small with poor methodology which makes it difficult to draw any absolute conclusions.  AVAPS may be useful, but we still need a well conducted RCT to find out if it is (i.e which patients and which conditions).

References:

1. Goren NZ et al. Comparison of BPAP S/T and Average Volume-Assured Pressure Support Modes for Hypercapnic Respiratory Failure in the Emergency Department: A Randomized Controlled Trial. Balkan Med J 2021. PMID: 34462252
2. Abubacker AP et al. Non-Invasive Positive Pressure Ventilation for Acute Cardiogenic Pulmonary Edema and Chronic Obstructive Pulmonary Disease in Prehospital and Emergency Settings. Cureus 2021. PMID: 34277241

For More Thoughts on This Topic Checkout:

• EMCrit 341: AVAPS (Average Volume Assured Pressure Support) NIPPV with Alex Bracey

Post Peer Reviewed By: Anand Swaminathan, MD (Twitter/X: @EMSwami)