Chapter 76 Mechanical Ventilation
I am frequently asked to render pulmonary critical care consultation in the following settings: when the patient requires (1) urgent intubation and mechanical ventilation, (2) adjustment of mechanical ventilation already instituted, (3) weaning from mechanical ventilation, (4) intervention for an acute problem that has developed related to mechanical ventilation, or (5) the development of ARDS. Accordingly, we have set forth consideration of the five patients above to consider how to speak intelligently and how to proceed in each of these contexts.
An 80-year-old male admitted for an elective laparoscopic cholecystectomy is found to be breathing at a rate of 35 breaths/min; he is diaphoretic with nasal alar flaring, he holds onto the side rails of his stretcher; he has rales up to his midchest on auscultation. You find out that he has not taken his furosemide (“water pill”) during the past week.
You decide to place him on 100% O2 by nonrebreather mask while you prepare for urgent intubation to reduce his work of breathing. Once he is intubated, you move him to the ICU to complete the evaluation, which will include CXR, EKG, and cardiac enzyme evaluation to assess for possible acute myocardial infarction as a cause of the pulmonary congestion. In addition, you administer furosemide empirically to reduce the pulmonary congestion.
If these three criteria cannot be met, tracheal intubation and mechanical ventilatory support may be required. In the setting of respiratory distress, careful judgment is needed to determine if and when to initiate mechanical ventilation.
Blood gases, while able to provide objective evidence of distress, are the last parameters to deteriorate in the evolution of pulmonary dysfunction. The decision to intubate and initiate mechanical ventilation should be made on clinical appreciation of respiratory distress. See Sabiston 24; Becker 11, 12.
A 25-year-old male presents with a gunshot wound to his abdomen. He is intubated and taken to the OR, where his abdominal injuries are addressed definitively by a single resection of his small bowel. He is brought to the ICU on a ventilator at the following settings: assist-control (A/C) mode, RR 12, tidal volume (Vt) 500 mL, fraction of inspired O2 (FIO2) 100%, positive end-expiratory pressure (PEEP) 5. He is sedated and paralyzed. Your baseline ABG returns: pH 7.24, PaCO2 55 mmHg, PaO2 240 mmHg, HCO3 16 mEq/L.
(PaO2 : FIO2 ratio 240; see discussion of ARDS under Case 5.)
Editor’s note: Understanding mechanical ventilation first requires familiarization with the vernacular used in the respiratory care field. A glossary of the different types of ventilatory approaches follows. Note that there are two basic modes of ventilation: volume limited (gas flow continues until a predetermined inspired volume is achieved) and pressure limited (gas flow continues until a predetermined airway pressure is reached). Following a description of the ventilation modes, primary and secondary ventilatory controls are outlined.
Volume-limited ventilation involves the delivery of gas until a preset tidal volume (VT) is reached; airway pressures can vary depending on compliance or stiffness of the lungs. Expiration is passive. The following settings vary by how the ventilator responds to spontaneous inspiratory effort.