In this study, researchers tested whether higher temperature and flow decrease patient comfort. In more severe patients, instead, they hypothesized that higher flow might be associated with improved comfort. The high-flow nasal cannula (HFNC) delivers up to 60 l/min of humidified air/oxygen blend at a temperature close to that of the human body.

Several recent randomized clinical trials in patients with or at risk for AHRF described decreased need for invasive mechanical ventilation and improved survival by early application of HFNC compared to standard oxygen or non-invasive positive pressure ventilation (NIPPV). The HFNC is associated with several beneficial physiologic effects, potentially promoting spontaneous breathing, avoiding patient exhaustion, and decreasing the risk of patient self-inflicted lung injury (P-SILI).

Non-intubated AHRF patients admitted to the intensive care unit (ICU), Inclusion criteria were: new/worsening respiratory symptoms (e.g., dyspnea) following a known clinical insult (e.g., pneumonia) within 1 week; PaO2/FiO2 ≤ 300 despite additional oxygen as per clinical decision; and evidence of pulmonary infiltrates on chest X-ray or CT scan.

Patients were nonsedated and kept in a semirecumbent position in a calm environment. Each patient underwent, in random order, four 20-min steps:

1. Gas flow 30 l/min and temperature 31 °C (HF30-T31)

2. Gas flow 60 l/min and temperature 31 °C (HF60-T31)

3. Gas flow 30 l/min and temperature 37 °C (HF30-T37)

4. Gas flow 60 l/min and temperature 37 °C (HF60-T37).

Optimum HFNC settings for patient comfort

In the whole population, on average, comfort during HFNC was elevated in all study phases (median 3 (IQR 2–4), mean 3.2 ± 1.4); at the patient level, fair to high comfort (i.e., ≥ 3) was always achieved in at least one phase.

The main findings of our study can be summarized as follows: in AHRF patients undergoing HFNC, comfort was higher at lower temperature, regardless of flow rate; in the subgroup of more severe patients, both lower temperature and higher flow were associated with improved comfort; and HFNC settings associated with best and worst comfort presented large variability at the individual patient level.

Set temperature during use of the HFNC seems to significantly impact the comfort of AHRF patients. Higher flow does not decrease comfort: on the contrary, in the subgroup of more severely hypoxemic patients, higher flow improves comfort. At the individual patient level, high variability exists in the settings associated with highest and lowest comfort, and the HFNC might need personalized titration.

To this end, patient comfort might already represent a smart and crafty indicator to guide settings of non-invasive respiratory support by HFNC.