NMS. Surgery

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Chapter 1 ♦ Principles of Surgical Physiology

7. Weaning and Extubating: Ventilator weaning is the progressive transfer of the work of breathing from the ventilator to the stable patient with the ultimate goal of extubation. a. Muscle atrophy: Ventilated patients are usually in a catabolic state, thus breaking down muscle protein for fuel. Respiratory muscles rapidly atrophy. b. Stable patient: Adjust the amount of ventilatory support to allow the patient to do some work of breathing to keep the respiratory muscles intact. c. Spontaneous breathing trial (SBT): This has been shown to shorten mechanical ventilation duration. An awake patient who controls their airway, passes the SBT with adequate parameters (described in the following section), and has acceptable acid–base balance may be ready for extubation. The SBT can be a low amount of CPAP and pressure support (5 mm Hg for each) or more classically be a flow-by oxygen trial off the ventilator. (1) Rapid shallow breathing index (RR bpm ÷ tidal volume in L) less than 100. (2) Vital capacity: Greater than 15–20 mm Hg. (3) Negative inspiratory force greater than 20 cm H 2 O (4) Arterial blood gases: The patient must have acceptable blood gases as well as spirometry values. (a) Oxygen saturation should be higher than 90%. (b) PCO 2 should be 35–45 mm Hg with pH of 7.35–7.45. C. Hemodynamic and other invasive monitoring: ICU monitoring devices include arterial lines for blood pressure; the pulmonary artery catheters; and intracranial catheters for intracranial pressure (ICP) monitoring. However, pulmonary artery catheters are now used much less. Cardiac performance is monitored with noninvasive technologies such as arterial line tracings and echocardiography. 1. Arterial catheter: Usually placed in radial arteries, other sites are femoral, axillary, and brachial arteries. a. The arterial line provides continuous blood pressure monitoring and is a simple, nonpainful source for blood sampling. b. Three pressure measurements obtained: systolic, diastolic, and mean. (1) Systolic: Highest cardiac cycle pressure recorded. (2) Diastolic: Lowest cardiac cycle pressure recorded. (3) Mean: Measured by integrating the area under the curve of the cardiac pressure wave. The mean pressure can be indirectly determined as (BPsystolic + 2 × BPdiastolic)/3 and represents the perfusion pressure. 2. Pulmonary artery (Swan-Ganz) catheter: Flow-directed catheter inserted into a central vein with an inflatable balloon on its tip to float through the heart and into the pulmonary artery (Fig. 1-3). a. Pulmonary capillary wedge pressure (PCWP)/occlusion pressure (PCOP): When the catheter is in position in a distal pulmonary