Mechanical Ventilatory Support

The knowledge of the pathophysiology of acute respiratory failure and the changes in lung physiology during positive-pressure ventilation will aid in the selection of an appropriate ventilatory modality and in the selection of the initial settings. 20 Ventilators are pressure- or volume-cycled. Volume-cycled ventilators are used routinely in EDs. Other decisions regarding mechanical ventilatory support in the ED include the rate, mode, F p2, minute ventilation, and use of positive end-expiratory pressure (PEEP) or continuous positive airway pressure (CPAP).

There are three common ventilator modes or methods for providing the tidal volume: controlled mechanical ventilation (CMV), assist-control (A/C), and intermittent mandatory ventilation (IMV). Use the control mode for apneic patients. The A/C mode allows the patient to trigger a cycle by inhaling and lowering the air pressure, which can be adjusted by the ventilator's trigger "sensitivity" (1 to 3 cmH 20). The ventilator will provide an untriggered "controlled" breath unless one is triggered during the selected time cycle. Finally, a predetermined number of ventilator-generated tidal volumes can be assured either unsynchronized (IMV) or more commonly synchronized (to patient effort) intermittent mandatory ventilation (SIMV). In the ED, the A/C or SIMV is the preferred initial mode except with an apneic patient.

The initial Fio2 should be guided by the oximetry. Set the tidal volume at 10 to 15 mL/kg ideal body weight and adjust the rate accordingly. Allow sufficient time for expiration. Maintain the peak airway pressure (PAP) below 35 to 45 cmH20 to prevent barotrauma. The PAP appears to be related to barotrauma more than the level of CPAP. The tidal volume can be increased up to 15 mL/kg to adjust the PaCo2 unless it elevates the PAP excessively.

To reclaim lung volumes, PEEP or CPAP should be considered if the decreased pulmonary compliance prevents delivery of an adequate tidal volume or if hypoxemia persists despite 100% Fio2. Even low levels (3 to 5 cmH20) of PEEP/CPAP usually render ventilator "sighs" (1.5 * tidal volume) unnecessary. If hypotension develops, adjust the respiratory rate and PEEP to lower the mean airway pressure.

Some alert patients with mild respiratory insufficiency who do not meet intubation criteria can be temporarily managed with continuous positive airway pressure (CPAP) through a snug-fitting face mask. This reduces the functional residual capacity and the work of breathing. Mask CPAP may thus delay or reduce the need for intubation. In patients with severe maxillofacial trauma and potential basilar skull fractures, pneumocephalus is a hazard. Noninvasive positive-pressure ventilation delivered through a face mask has been compared to conventional mechanical ventilation and is an effective option for many types of acute respiratory failure. 21

Face-mask CPAP may also prove to be a valuable initial adjunct in some patients with pulmonary edema. The pressure can be maintained below 18 cmH 20 if it is intermittently released. This decreases the potential for gastric insufflation and subsequent aspiration.

Patients who require prolonged CPAP to maintain oxygenation and adequate alveolar ventilation may subsequently benefit from airway pressure release ventilation (APRV). APRV is basically a CPAP system for spontaneously breathing patients who cannot maintain adequate alveolar ventilation on their own and who also require an increased functional residual capacity. This technique attempts to ventilate the lungs while avoiding higher PAPs by releasing the level of CPAP to a lower pressure. These ventilators incorporate a second relief valve that allows for the cyclical release of CPAP to the desired lower level. Inverse ratio ventilation (IRV), in contrast, is the technique to completely control ventilation and the PAP following NMB. In other words, APRV and IMV allow unrestricted spontaneous ventilation, while IRV and CMV do not.

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