Models of Noninvasive Mechanical Ventilation in Pandemic Conditions

Abstract

SARS-CoV-2 pneumonia can be associated with hypoxaemia, which can be very severe and lead to death. Treatment of hypoxaemia with oxygen therapy and respiratory support is therefore key to prevent mortality. In most cases, hypoxaemia can be adequately controlled with conventional oxygen therapy (COT) in a conventional hospitalisation area. However, in a minority of cases, hypoxaemia worsens and escalation to noninvasive respiratory support (NIRS) devices is necessary: high-flow nasal cannula (HFNC), continuous positive airway pressure (CPAP), bilevel positive airway pressure (BPAP); under close monitoring in an intermediate respiratory care unit (IRCU). Early initiation of NIRS is of vital importance in order to halt the progression of hypoxaemia and, consequently, to avoid escalation to orotracheal intubation-invasive mechanical ventilation (OTI-IMV), with the complications that this entails (bacterial pneumonia associated with IMV, nosocomial infections, polyneuromyopathy etc.). The main criteria to proceed with escalation from COT to NIRS is the need for an FiO₂ ≥ 0.40 under COT to maintain an SpO₂ ≥ 94%; other criteria are a PaO₂/FiO₂ ratio ≤ 200, a tachypnoea >25 breaths/min and the use of accessory respiratory musculature. HFNC or CPAP can be used alone or in alternating combination, with the combination being preferred in more severe cases (SpO₂ < 92% despite HFNC, PaO₂/FiO₂ ratio < 100, tachypnoea > 25 breaths/min and/or use of accessory respiratory musculature despite HFNC) and in subjects with obesity (BMI > 30 kg/m²) or with sleep apnoea. Prone decubitus sessions are recommended, especially under HFNC, to increase oxygenation and thus avoid OTI-IMV. The most important parameters that allow us to assess the patient’s response to NIRS are the PaO₂/FiO₂ ratio (or, failing that, the SpO₂/FiO₂ ratio), the respiratory rate (RR), the use or non-use of accessory respiratory muscles, and the dyspnoea. Others are the alveoloarterial oxygen gradient, the HACOR index, the ROX index and the oesophageal pressure. It is reasonable to allow a window of opportunity of 48–72 h for NIRS from its initiation before considering escalation to OTI-IMV, without entailing to a significant delay in OTI-IMV leading to a significant impact on mortality in the case of NIRS failure; the benefit of avoiding OTI-IMV (which is avoided in 74.4% of cases) is considered much greater in the event of successful NIRS. After 48–72 h, if the patient remains the same or even worsens (SpO₂ < 92%, PaO₂/FiO₂ ratio ≤ 100, decreasing curve in SpO₂ even though SpO₂ is ≥92%, tachypnoea >25 breaths/min, use of accessory respiratory muscles), despite optimised NIRS (with high FiO₂ and high flows or pressures), OTI-IMV will be performed, since after this time the probabilities of successful NIRS are low and, in contrast, the delay in OTI-IMV becomes significant and, consequently, will have a significant impact on mortality. Finally, in case of successful NIRS, de-escalation from NIRS to COT will generally be performed when NIRS can be lowered to an FiO₂ ≤ 0.40 so that the patient maintains an SpO₂ ≥ 92%, a RR ≤ 25 breaths/min and no accessory respiratory muscle use for at least 30 min.