Patient-ventilator interaction

One of the most frequent reasons for instituting mechanical ventilation is to decrease a patient's work of breathing. To achieve this goal, neuromuscu lar blockade was initially used to eliminate a patient's respiratory effort s, resulting in controlled ventilation. Neuromuscular blocking agents, howe ver, predispose to numerous hazards, and, today, some means of assisted ven tilation is usually employed. Surprisingly, we do not know the ideal level of respiratory muscle unloading during mechanical ventilation. To consider this issue, we need a means of measuring a patient's respiratory effort. In spiratory pressure-time product (PTP), which quantifies swings in intrathor acic pressure, is closely related to oxygen cost of breathing and has sever al advantages over measurements of mechanical work of breathing (1), In 17 patients failing a trial of weaning from mechanical ventilation, their aver age PTP was 388 cm H2O . s/min (2); the average value in healthy subjects i s 94 cm H2O . s/min. With carefully selected settings, a patient's inspirat ory PTP can be reduced into the normal range with assisted ventilation (3). For the most effective unloading of the inspiratory muscles, the ventilato r should cycle in synchrony with the activity of a patient's own respirator y rhythm. The interplay between these two pumps is complex, and problems ca n arise at several points in the respiratory cycle: the onset of ventilator triggering, the rest of inspiration after triggering, the switch from insp iration to expiration, and the end of expiration.