Despite early intervention and appropriate respi-ratory care, patients with critical illness will often require mechanical ventilation. Mechanical venti-lation can replace or supplement normal sponta-neous ventilation. In most instances, the problem is primarily that of impaired CO2 elimination (ventilatory failure). In other instances, mechani-cal ventilation may be used as an adjunct (usu-ally to positive-pressure therapy; see) in the treatment of hypoxemia. The decision to initiate mechanical ventilation is made on clinical grounds, but certain parameters have been suggested as guidelines (Table 57â€“4).
Of the two available techniques, positive-pressure ventilation and negative-pressure ventila-tion, the former has much wider applications and is
almost universally used. Although negative-pressure ventilation does not require tracheal intubation, it cannot overcome substantial increases in airway resistance or decreases in pulmonary compliance, and it also limits access to the patient.
During positive-pressure ventilation, lung inflation is achieved by periodically applying posi-tive pressure to the upper airway through a tight-fitting mask (noninvasive mechanical ventilation) or through a tracheal or tracheostomy tube. Increased airway resistance and decreased lung compli-ance can be overcome by manipulating inspiratory gas flow and pressure. The major disadvantages of positive-pressure ventilation are altered ventila-tion-to-perfusion relationships, potentially adverse circulatory effects, and risk of pulmonary baro-trauma and volutrauma. Positive-pressure ventila-tion increases physiological dead space because gas flow is preferentially directed to the more compli-ant, nondependent areas of the lungs, whereas blood flow (influenced by gravity) favors dependent areas. Reductions in cardiac output are primarily due to impaired venous return to the heart from increased intrathoracic pressure. Barotrauma is closely related to repetitive high peak inflation pressures and under-lying lung disease, whereas volutrauma is related to the repetitive collapse and reexpansion of alveoli.