MECHANICAL VENTILATION FOR HEART FAILURE: LET THEM REST OR REST IN PEACE?

摘要

Let them rest or rest in peace? Quite frankly, this little question reminds me of two key considerations for mechanical ventilation in dogs and cats with acute, severe medically-refractory decompensated congestive heart failure. The first consideration (let them rest) or better said, indication for mechanical ventilation is to reverse acute heart failure-induced respiratory muscle fatigue and to decrease the work of breathing. Respiratory muscle fatigue may be diagnosed by a decrease in respiratory rate, associated with hypercapnea and declining level of consciousness. Invasive mechanical ventilation should only be used after acute respiratory failure does not respond to diuretics, vasodilators, oxygen therapy and in humans, non invasive ventilatorysupport.(see below) General goals of mechanical ventilation in these patients is to support the pulmonary system to maintain an adequate level of alveolar ventilation and to restore normal acid-base balance and oxygenation to the organs and tissues. Additionally, a general goal of mechanical ventilation is to reduce the work of breathing (WOB) until the cause of respiratory failure can be reversed. Normally in a heart failure patient, the ventilatory therapy buys you more time, that is, time to let aggressive medical therapy to take effect or time for the left atrium to stretch if an acute mitral valve chordae tendinae has ruptured. Normally, WOB accounts for 1 - 4 % of the total oxygen consumption at rest. Oxygen consumption by the respiratory muscles increases during respiratory failure of any cause. Oxygen consumption by the respiratory muscle increases to as much as 35% - 40% in some patients. This increased WOB can lead to respiratory failure secondary to respiratory muscle fatigue causing significant tissue hypoxemia and acid base abnormalities. By taking over breathing, one can divert this cardiac output away from the muscles of respiration to more vital organs and tissues.