Mechanical ventilation utilizes positive-pressure devices to improve oxygen (O2) and carbon dioxide (CO2) exchange. There are two main goals of mechanical ventilation: (1) maintain appropriate levels of arterial O2 and CO2; and (2) reduce the patient’s work of breathing. Mechanical ventilation is a supportive intervention that does not treat the underlying disease process.
Positive-pressure ventilation can be administered with an endotracheal tube (ETT) or noninvasively with a mask. Noninvasive management can be used for patients who have a nonobstructed airway, a preserved respiratory drive, and protective airway mechanisms intact. Invasive airway management is required if there is acute airway obstruction, inability to handle secretions, loss of protective airway reflexes, or respiratory failure that is refractory to noninvasive positive- pressure ventilation with persistent hypoxemia and hypercapnia.
Mechanical ventilation can be used to ensure a controlled airway for patients who require sedation, such as during surgical procedures, or to tolerate resuscitation and life support. Other goals include oxygenation, minute ventilation (MV) and pH control, and work of breathing reduction.
Oxygenation is improved by titrating fraction of inspired oxygen (Fio2), and improving mean airway pressures by adjusting tidal volume (VT) and positive end-expiratory pressure (PEEP). Control of MV allows for regulation of CO2 and pH. Depending on the mode of mechanical ventilation selected, an MV can be guaranteed regardless of effort, which is useful for treatment of hypercapnic respiratory failure as well as for maintaining physiologic pH. Mechanical ventilation decreases work of breathing by ensuring adequate VT, optimizing inspiratory and expiratory times during respiration to prevent air trapping, and preventing airway collapse.
During mechanical ventilation, VT, PEEP, and Fio2 control oxygenation. VT and PEEP work together by increasing alveolar volume and mean airway pressures. In patients with obstructive airway disease, larger VT with slower respiratory rate (RR) prevents air trapping. With noncompliant lungs, smaller VT and faster RR avoid volutrauma and barotrauma. Decreasing Fio2 minimizes toxicity while also maintaining adequate O2 saturation (Spo2).
Positive end-expiratory pressure improves oxygenation by maintaining airway pressures more than 0 cm H2O during exhalation, preventing alveoli collapse, and improving recruitment of atelectatic areas. PEEP increases functional residual capacity (FRC), which is the volume remaining in the lung after normal exhalation. Closing capacity (CC) is the volume in the lungs at which small airways that do not have cartilaginous support begin to close. If CC exceeds FRC, atelectasis occurs. PEEP increases FRC, preventing atelectasis. Assessment and optimization of volume status prior to increasing PEEP levels avoid reduction in right heart blood return.
Minute ventilation adjustments alter either ...