Hemodynamic and Oxygen Transport Characteristics of Common Ventilatory Modes
Assist control ventilation (ACV), synchronized intermittent mandatory ventilation (SIMV), and pressure support ventilation (PSV) are the three most conventional and commonly used modes of ventilatory support in patients requiring mechanical ventilation. The latter two modes are, in addition, applied very frequently to wean patients off ventilatory support. The PSV augments the patient’s spontaneous breaths with a preset positive pressure delivered by the ventilator; the patient controls the respiratory frequency, inspiratory flow, and inspiratory time. It reduces the ventilatory work done by the patient and improves the breathing pattern and patient’s comfort itat on canadian pharmacy levitra. As a result of these characteristics, PSV has been received with significant enthusiasm compared with the more standard ACV and SIMV modes. Indications and benefits of PSV, however, have not been clearly shown and many of the basic physiologic characteristics of PSV vs ACV and SIMV have not been studied.
These three modes of ventilatory support are applied with positive pressure and will increase the in-trathoracic pressure (ITP). Through a complex set of cardiopulmonary interactions, the increase in ITP can affect cardiac output (CO) in patients on ventilatory support. This, in turn, may alter oxygen transport (TO2) to tissues. Mechanical ventilation can also affect the ventilatory pump (principally the diaphragm), the work of breathing (WOB), and the level of oxygen consumption (V02) by the pump depending on the degree of the unloading of the pump and the level of ventilatory support. The ACV and PSV can be applied in such a way as to almost totally unload the ventilatory pump, decrease the WOB, and reduce the V02. Since the SIMV mode allows spontaneous breathing, it may not have similar effects as the other two ventilatory modes.
The ultimate goal of ventilation, spontaneous or assisted, is the transport of adequate amounts of oxygen to tissues for the preservation and maintenance of aerobic cellular metabolism. Different modes of positive-pressure mechanical ventilation can affect oxygen transport and consumption differently through changes in various factors that control tissue oxygenation. These include (1) changes in CO as the result of the effects of ITP swings on heart-lung interaction, (2) adequacy of alveolar ventilation and the resultant levels of Sa02 and РаОг, and (3) level of WOB and the extent of V02 by respiratory muscles.
Information regarding the hemodynamic and oxygen transport parameters with ACV, SIMV, and PSV modes and comparison among these are scanty and inconclusive. The preference for a specific ventilatory mode for a particular patient at the present time, therefore, is somewhat arbitrary and based on trial and error.
The aim of our study was to compare the hemodynamic and oxygen transport parameters on ACV, SIMV, and PSV modes under various pathophysiologic conditions in patients requiring assisted ventilation and hemodynamic monitoring.