Mechanical ventilation can be referred to as full or partial ventilatory support. This can be described by the equation of motion:
With full support, there is no respiratory muscle activity and thus Pmus is 0. With partial support, there is a contribution by the respiratory muscles, and thus, Pmus makes a contribution to the equation of motion.
With full support, the ventilator does all of the work needed for ventilation of the patient; the patient does not trigger the ventilator or breathe spontaneously. This can be achieved as the result of the patient's primary disease process (eg, neuromuscular disease), pharmacologic therapy (eg, paralysis), or a minute ventilation high enough to suppress the patient's respiratory drive (eg, hyperventilation). Full support is often preferred for patients who are severely ill to decrease the oxygen cost of breathing and achieve control of the patient's ventilatory pattern. Full support is provided by CMV.
With partial support, some of the work-of-breathing is provided by the ventilator and the remainder is provided by the patient. Partial ventilatory support is commonly used during weaning from mechanical ventilation. Partial support is also preferred by clinicians who believe that this maintains respiratory muscle tone, allows the patient to maintain some control of the ventilatory pattern, and improves patient comfort and synchrony. Total respiratory muscle suppression can quickly lead to atrophy and weakness. Partial ventilatory support can be achieved with CMV, SIMV, or PSV.