NMS. Surgery

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Chapter 1 ♦ Principles of Surgical Physiology

facilitates both increased fraction of inspired oxygen ( Fio 2 ) and airway pressure provision to reduce intrapulmonary shunt and improve oxygenation. d. Impending airway obstruction: Indication for intubation to prevent complete obstruction and loss of the airway. (1) Anatomic (e.g., from trauma, tumors, edema, or vocal cord abnormalities) and functional changes (e.g., from depressed neurologic status, drugs, anesthesia, or stroke). (2) If there is a concern, then intubate early under controlled conditions. B. Ventilator support: Most common reason to be admitted to an ICU. Ventilator support should address ventilation and oxygenation . 1. Ventilation: Determines CO 2 elimination and depends on alveolar minute ventilation. a. Alveolar minute ventilation: Total minute ventilation − dead space ventilation. b. Total minute ventilation: Product of RR × tidal volume, expressed as L/min. 2. Oxygenation (or Po 2 ): Determined by the partial pressure of alveolar oxygen and the intrapulmonary shunt. Increasing Fio 2 will increase alveolar oxygen, whereas increasing mean airway pressure (e.g., by increasing positive end-expiratory pressure [PEEP] ) will decrease shunt and increase Po 2 . 3. Mode of ventilation: Determines how the ventilator will provide a mechanical breath to the patient; based on pressure or volume. a. Synchronized intermittent mandatory ventilation (SIMV): Provides a preset rate and tidal volume. (1) Spontaneous breathing by the patient provides additional minute ventilation (tidal volume × rate) dependent on the patient’s work of breathing capability. (2) The work of breathing is shared between the machine’s minute ventilation and the patient’s spontaneous minute ventilation. (3) Usual mode of ventilation when weaning a patient: By turning down the ventilator rate, the patient assumes more work of breathing until the ventilator is no longer needed. (4) Pressure support (PS) can be added to facilitate spontaneous breaths. b. Assist or volume control (AC/VC): Provides a preset rate and tidal volume, and all additional patient breaths are fully assisted by the ventilator to the preset tidal volume. (1) Minute ventilation becomes the result of the preset tidal volume × (the preset ventilator’s + patient’s rates). (2) This allows the patient to receive full ventilatory support without expending extra energy on the work of breathing.