More than a decade after low tidal volume ventilation for acute respiratory distress syndrome (ARDS) first gained widespread acceptance, it is still unknown how best to manage its effects on minute ventilation. High respiratory rate, or permissive hypercapnia? Limiting respiratory rate has been shown in preclinical models to reduce lung injury even at a constant arterial CO2 tension (; Vaporidi et al. 2008). Curley and colleagues (2013) go one step further, making the case for additional benefit from hypercapnia during low tidal volume ventilation.
As we find no great fault with the balanced case put forward by our Canadian colleagues, our rebuttal focuses on challenges in designing more definitive trials. First, how might the independent effects of respiratory rate, and pH each be considered? Tris-hydroxymethyl amino-methane (THAM) buffer might be used to identify the effects of hypercapnia independent of acidaemia. To isolate the effects of respiratory rate from , inspired CO2 could be administered during high respiratory rate to induce hypercapnia to a degree comparable to a low-rate strategy. Optimizing respiratory rate must also consider airflow dynamics since high rates may lead to auto-positive end-expiratory pressure (auto-PEEP).
Second, what is the minimum acceptable pH, and how should severe acidaemia be managed? A minimum pH approaching 7.15 was well tolerated haemodynamically in a heterogeneous ARDS population (Carvalho et al. 1997), while other studies specified a pH nadir between 7.05 (Brochard et al. 1998) and 7.30 (Brower et al. 2000) before encouraging intervention. THAM has shown promise as an effective buffer during fixed minute ventilation (Kallet et al. 2000), although further study is warranted before its widespread adoption as a rescue therapy.
Finally, how do we ensure the protection of patients at highest risk of harm from hypercapnia? Patients with intracranial hypertension may fare poorly from hypercapnia-induced cerebral vasodilatation, as may patients with pre-existent right ventricular compromise facing hypercapnic pulmonary vasoconstriction (Curley et al. 2010). Similarly, anti-inflammatory effects of hypercapnia may be deleterious in pulmonary or extra-pulmonary sepsis compared to other ARDS precipitants. Moreover, increased sedation or paralysis, with associated risk of iatrogenic injury, may be required during hypercapnia to maintain patient–ventilator synchrony and minimize large swings in transpulmonary pressures from spontaneous breathing efforts (Malhotra & Drazen, 2013).
Only with carefully designed studies will the role be defined for optimizing respiratory rate, and pH in individual patients with varying comorbidities and ARDS severity. The range of preclinical findings and heterogeneity of current clinical practice indicate a great need for further research in this area.