Physiological Effects of Continuous Negative External Pressure for Acute Hypoxemic Respiratory Failure

This study is being conducted to evaluate if wearing a non-invasive breathing support device over the chest/abdomen improves markers of breathing in patients with lung injury requiring high-flow oxygen. The breathing support device consists of a plastic shell that sits over the chest and abdomen and connects to a vacuum that helps the chest expand with breathing. This breathing support is known as continuous negative external pressure (CNEP). Study findings will help determine if this breathing support device might be useful for patients with acute hypoxemic respiratory failure (AHRF).

This study is a prospective randomized cross-over trial. Eligible, consenting participants will undergo 4 strategies of respiratory support: high-flow nasal cannula (HFNC) only, HFNC + continuous negative external pressure (CNEP) of 10 cmH2O, HFNC + CNEP of 20 cmH2O, HFNC + CNEP of 30 cmH2O. Throughout the study period, HFNC will be managed at a constant flow rate with fraction of inspired oxygen (FiO2) titrated to achieve goal oxygen saturation (SpO2) of 90-97%, measured via continuous pulse-oximetry. Each of the 4 strategies will be performed for 45 minutes per strategy, interspersed with a 15-minute washout period of HFNC only. Participants will be randomized to the sequence of strategies for respiratory support.

A summary of the sequence of trial procedures is as follows:

1. Perform baseline measures.
2. Wait 5-10 minutes for recovery.
3. Initiate first random treatment assignment for up to 45 minutes.
4. HFNC-only washout for 15 minutes
5. Initiate second random treatment assignment for up to 45 minutes.
6. HFNC-only washout for 15 minutes
7. Initiate third random treatment assignment for up to 45 minutes.
8. HFNC-only washout for 15 minutes
9. Initiate fourth random treatment assignment for up to 45 minutes.
10. Return to usual care (HFNC only)

If a participant does not tolerate a given level of CNEP, the patient will be returned to HFNC for 5 minutes and then given the option to attempt that CNEP level again. If the patient declines reattempt or does not tolerate that CNEP level on reattempt, the HFNC-only washout will be instituted for 15 minutes before proceeding to the next protocol step.

Eligibility Criteria:
Inclusion Criteria:

* Age 18 years or older
* Acute hypoxemic respiratory failure
* Non-cardiogenic airspace opacities on chest radiograph or computed tomography (CT) scan
* High-flow nasal cannula (flow ≥ 30 LPM) initiated within last 96 hours (4 days)
* FiO2 ≥ 40%
* SpO2 ≥ 92%

Exclusion Criteria:

* Do-not-intubate order
* Anatomical factor predisposing to poor fit of cuirass (e.g. severe kyphosis or scoliosis)
* Use of cuirass precluded, e.g. due to:

  * Clinically prescribed prone positioning
  * Tense ascites
  * Severe abdominal pain
  * Abdominal wound or surgery
  * Pregnancy
  * Agitated delirium
* Prior intubation during hospital stay
* Cardiogenic pulmonary edema
* Exacerbation of asthma or COPD
* Chronic lung disease, including:

  * Interstitial lung disease
  * Cystic fibrosis
  * Lung mass, lung cancer, or metastasis to the lung
  * Lung transplant recipient
  * Any disease that requires home oxygen
* Glasgow coma score \< 15
* Chest tube, pneumothorax, or pneumomediastinum
* Hemodynamic instability (mean arterial pressure \< 55 mmHg or norepinephrine-equivalent vasopressor requirement \> 0.1mcg/kg/min)
* Implantable electrical device (e.g. pacemaker, defibrillator, neurostimulator)
* Unreliable pulse-oximetry tracing
* Imminent intubation
* Anticipated lack of patient availability to complete study procedures (e.g. due to planned clinical procedure such as CT scan or dialysis during potential time of study)
* Attending physician refusal