Many patients require mechanical ventilation in the operating room or inten-sive care unit. While an intubated patient can breathe spontaneously through an endotracheal tube (which imposes significant resistance), many operative proce-dures require muscle relaxation, making mechanical ventilation mandatory. Most anesthesia machines are equipped with ventilators capable of providing volume-or pressure-controlled ventilation. For volume-controlled ventilation, the oper-ator sets a tidal volume, respiratory rate, and inspiratory to expiratory time ratio (I : E ratio), and the ventilator does its best to comply. If compliance deteriorates, the machine will generate additional pressure (up to a set limit) in an attempt to deliver the desired tidal volume. In pressure-controlled mode, as the name sug-gests, the selected pressure will be maintained for a set time, which might mean variable tidal volumes, depending on the patient’s pulmonary compliance and resistance.
In general, ventilators used in the ICU offer more options than anesthesia machine ventilators. For example, they might offer SIMV (synchronized inter-mittent mandatory ventilation) in which the mechanical breath is synchronized with the patient’s inspiratory effort, and the patient can breathe spontaneously between mechanical breaths. SIMV is often combined with pressure support ven-tilation (PSV), in which spontaneous respiratory efforts are met with a set level of positive pressure, assisting with inhalation and designed to overcome the resis-tance imposed by the endotracheal tube and ventilator.
Another ventilator mode that requires explanation is continuous positive air-way pressure (CPAP) and positive end-expiratory pressure (PEEP). Ordinarily, when we exhale, some gas remains in the lungs (the FRC – see Fig. 10.6). Supine positioning and anesthesia reduce the FRC, potentially resulting in hypoxemia. Normal FRC can be restored with the addition of end-expiratory pressure, PEEP. It becomes particularly useful if increased intra-abdominal pressure or extravas-cular fluid (pulmonary edema, atelectasis, aspiration of gastric contents or res-piratory distress syndrome (ARDS)) decreased FRC or caused collapse of alveoli. Two major factors limit the amount of PEEP that we can apply: (i) the increase in intrathoracic pressure will impede venous return; and (ii) the inspired tidal vol-ume is administered on top of this baseline positive pressure, causing increased peak inflation pressure and possibly barotrauma.