in: Non- Invasive Ventilation: A Practical Handbook for Understanding the Causes of Treatment Success and Failure,, Antonio Esquenas, Editor, NOVA Science Publishers Inc. , New-York, pp.258-276, 2019
The main mechanisms for hypercapnia and subsequent respiratory acidosis are hypoventilation as well as ventilation-perfusion mismatch. Acute respiratory acidosis is generally defined as pH <7.35 and hypercapnia (PaCO2 >45 mmHg). Patients with chronic respiratory failure usually preserve the pH levels within normal ranges despite the increased levels of pCO2, unless there is an acute reason for decompensation. Acute hypercapnic respiratory failure can develop due to exacerbation in patients with chronic obstructive pulmonary disease (COPD), chest wall deformities, and neuromuscular diseases (NMD). Other causes of AHRF include status asthmaticus and drug intoxication. Non-invasive ventilation (NIV) is the first choice of ventilatory assistance in AHRF. NIV can reverse respiratory acidosis, and there’s growing research evidence suggesting that NIV reduces intubation rates and mortality. However, NIV failure, resulting in higher in-hospital mortality, length of hospital and intensive care unit (ICU) stay, and charges for hospitalization, has been observed especially in cases with low levels of baseline pH and on-going persistence in low pH despite optimal NIV treatment. Initiation of NIV is recommended in patients with pH<7.35. Moreover, it can be applied to acutely unwell hypercapnic patients with NMD or chest wall deformities, or to hypercapnic, obese, somnolent, and hospitalized patients in the absence of respiratory acidosis. Even severe acidosis can be successfully managed with NIV in an appropriate environment with experienced staff. It is crucial to monitor pH and PaCO2 levels closely, especially during the first hours of the NIV treatment. In cases with persisting or worsening conditions, patients should be intubated without delay to prevent adverse outcomes.