Respiratory drive, the intensity of the respiratory center's output, determines the effort exerted in each breath. The increasing awareness of the adverse effects of both strong and weak respiratory efforts during mechanical ventilation on patient outcome brings attention to the respiratory drive of the critically ill patient. Critical illness can affect patients' respiratory drive through multiple pathways, mainly operating through three feedback systems: cortical, metabolic, and chemical. The chemical feedback system, defined as the response of the respiratory center's output to changes in arterial blood gases and pH, is one of the most important determinants of respiratory drive. The purpose of this state-of-the-art review is to describe the determinants of respiratory drive in critically ill patients, review the tools available to assess respiratory drive at the bedside, and discuss the implications of altered respiratory drive during mechanical ventilation. An analysis that relates arterial carbon dioxide levels with brain's response to this stimulus will be presented, contrasting the brain's responses to the patient's ability to generate effective alveolar ventilation, both during unassisted breathing and with different modes of ventilatory assist. This analysis may facilitate comprehension of the pathophysiology of respiratory drive in critically ill patients. As we aim to avoid both over- and under-assistance with mechanical ventilation, considering the patients' respiratory drive at the bedside may improve clinical assessment and management of the patient and the ventilator.
Keywords: assisted ventilation; chemical feedback; effort; respiratory center.