Management of Postoperative Mechanical Ventilation

Most relatively healthy patients who have not undergone extensive thoracic operations may have their tracheas safely extubated in the operating room or shortly after arrival in the recovery room (see Chapter 71 , Chapter 74 , and Chapter 75 ). However, patients with severe chronic pulmonary disease who undergo extensive thoracic operations require a period of postoperative mechanical respiratory support. Careful clinical assessment and judgment are necessary for postoperative tracheal extubation of patients who are intermediate in terms of their respiratory health and the extent of their thoracic procedures. Precise criteria for continued intubation and mechanical ventilatory support are presented in Chapter 42 , Chapter 71 , Chapter 74 , and Chapter 75 . Whenever doubt exists, we urge caution in removing mechanical ventilatory support too rapidly from these patients; it is safest to support patients until clinical and laboratory data clearly indicate that extubation is safe. The following briefly describes our current practice of mechanical ventilation, weaning, and extubation.

For most postoperative thoracotomy patients who require respiratory support, controlled mechanical ventilation with a tidal volume of 12 mL/kg, an intermittent mandatory ventilation (IMV) rate that produces a PaCO₂ of 40 mm Hg (which usually requires an initial rate of 8 to 12 breaths/min), and an inspired oxygen concentration of 60% to 100% is initiated after reintubation with a single-lumen tube. IMV used in this way is a full-support ventilation modality. Patients may breathe spontaneously, but at this initial ventilator stage, the spontaneous ventilation is not regarded as significantly contributing to minute ventilation. The spontaneous ventilation with IMV may increase cardiac output and

TABLE 49-17 -- Mechanical ventilation and weaning plan

Goal No. (Temporal Sequence to Be Followed)	Goal				Achieved Primarily by
(1) ↓FIO2				PaO2 > 60 mm Hg	PEEP titration
(2) ↓PEEP	↓ FIO2 < 0.5			PaO2 > 60 mm Hg	Respiratory care regimen
(3) ↓IMV rate	↓ FIO2 < 0.5	↓PEEP to <10 cm H2 O		PaO2 > 60 mm Hg	Patient with adequate breathing power
FIO2 , inspired oxygen concentration; IMV, intermittent mandatory ventilation; PaO2 , arterial oxygen tension; PEEP, positive end-expiratory pressure.

thereby decrease intravenous fluid requirements, as well as eliminate the need for paralysis and heavy sedation. IMV may also facilitate the transition to weaning. Arterial blood gas values and chest radiographs are obtained shortly after the institution of mechanical ventilation in the recovery room.

An FIO₂ greater than 0.5 is toxic to the lungs,^[430] and the first and compelling goal is to reduce FIO₂ to less than 0.5 while maintaining an acceptable PaO₂ ( Table 49-17 ). The decrease in FIO₂ below 50% is achieved by performing a dose (PEEP)-response (PaO₂ ) titration. PEEP is progressively added (range, 0 to 20 mm Hg) until PaO₂ is relatively normal for that patient (or at least >60 mm Hg) with an FIO₂ less than 0.5. PEEP should be raised in increments of 2.5 to 5.0 cm H₂ O. The patient should be allowed to stabilize with regard to respiratory mechanics (peak inspiratory pressure and compliance), hemodynamics (pulse rate and rhythm, systemic and central filling pressures, and cardiac and urine output), and gas exchange (arterial blood gases). Usually, each PEEP increment requires 0.5 to 1 hour to complete. Consequently, performance of a PEEP-FIO₂ /PaO₂ titration may take several hours. Of course, as the PEEP titration is being performed, other respiratory care modalities ( Table 49-18 ), such as suctioning, turning, and administration of antibiotics, are instituted. The advent of fiberoptic bronchoscopy has eliminated the need for the use of "super-PEEP" (PEEP >20 to 25 cm H₂ O) because regions of collapsed lung that are resistant to opening by PEEP are usually obvious on chest radiographs and can be suctioned and lavaged open under direct vision with a bronchoscope.

If a patient requires more than 10 cm H₂ O of PEEP to attain a PaO₂ greater than 60 mm Hg with an FIO₂ less than 0.5, the V̇/ mismatch is so great that it is illogical to make the patient spontaneously ventilate to any significant degree. In other words, lungs that have a PEEP requirement greater than 10 cm H₂ O are inefficient to the point that the work of breathing is likely to be excessive, and the patient will fatigue and fail.

Thus, the second goal is to reduce PEEP to less than 10 cm H₂ O; of course, it is assumed that what was achieved before (FIO₂ <0.5, acceptable PaO₂ ) is retained. This second goal is accomplished primarily by instituting an intensive and aggressive respiratory care regimen. Such a regimen has four components ( Table 49-18 ), and all four proceed in parallel. Although no hard data prove that any one of these respiratory care treatments is beneficial

1905

Use of the respiratory care regimen outlined in Table 49-18 usually allows the physician, within a few days, to decrease PEEP to less than 10 cm H₂ O with continued maintenance of an adequate PaO₂ along with a low FIO₂ . Once the PEEP requirement is less than 10 cm H₂ O, no special attempt is made to reduce it further. Maintenance of some PEEP is desirable in terms of maintaining normal FRC and lung compliance and providing a subjective sensation of good lung expansion. Achievement of an FIO₂ less than 0.5 and a PEEP level less than 10 cm H₂ O with an acceptable PaO₂ means that the blood and gas within the lungs are matched well enough that the lungs

TABLE 49-19 -- Bedside physiologic parameters conventionally and commonly used to predict success in weaning from ventilatory support and weakness of each test

Test	Problem with Test
(1) Tidal volume > 5 mL/kg	(1) Ignores determination of inspiratory reserve
(2) Vital capacity > 10–15 mL/kg	(2) Requires cooperation
(3) Respiratory rate < 25–35 breaths/min	(3) Multifactorial
(4) Minute ventilation > 10 L/min	(4) Multifactorial
(5) Maximum voluntary ventilation >2 × minute ventilation	(5) Requires cooperation
(6) Maximum inspiratory pressure more negative than 20–25 cm H2 O	(6) Ignores respiratory mechanics
(7) Respiratory rate-tidal volume ratio	(7) Combination of above, but the ratio defines rapid shallow breathing

perform their basic gas exchange function in a reasonably efficient manner. It now makes sense to have such patients do some of the work of breathing on their own, and they are now candidate for weaning from mechanical ventilation.

The third goal is to retain what was achieved before (FIO₂ less than 0.5, PEEP less than 10 cm H₂ O, and acceptable PaO₂ ) and reduce the IMV rate to less than 1 breath/min. This weaning process is accomplished by a progressive decrease in the IMV rate to allow a gradual transition from 100% to 0% ventilator dependence. The rapidity at which the IMV rate can be reduced is dictated by and directly proportional to the vital capacity and peak inspiratory force. Other simple indices of readiness for decreasing the IMV rate are shown in Table 49-19 . When the decrease in IMV rate is begun, the head of the bed should be elevated as much as possible to reduce the pressure of the abdominal contents on the diaphragm and also to allow the patient to reestablish contact with the environment. During the transition, IMV provides a progressively decreasing amount of partial ventilatory support.^[433]

As the IMV rate is decreased, the patient is monitored by the spontaneous respiratory rate, vital capacity, peak inspiratory force, and PaCO₂ . In addition, the patient should be asked to indicate whether a sensation of dyspnea is perceived. When the rate of IMV withdrawal is appropriate and is being tolerated, the patient's spontaneous respiratory rate remains constant or only slightly increases, vital capacity and peak inspiratory force remain constant or improve, arterial blood gas concentrations do not deteriorate, and the patient remains reasonably comfortable. Conversely, when the IMV withdrawal rate is too rapid and is not being tolerated, the spontaneous respiratory rate increases greatly (which is usually the first indication of a problem with tolerance), vital capacity and peak inspiratory force decrease, and arterial blood gas values deteriorate; the patient becomes hypertensive and tachycardiac, may have arrhythmias, and communicates a sensation of air hunger.

The patient is ready for extubation when PaO₂ is adequate, FIO₂ is less than 0.5 (goal 1), the PEEP level is less than 10 cm H₂ O (goal 2), vital capacity is larger than 15 mL/kg, peak inspiratory force is greater than -25 cm H₂ O, IMV is 1 breath/min or less, the spontaneous respiratory

1906

The logic of the preceding approach is that the patient's lungs are first required to have the ventilation and perfusion well enough matched, as indicated by an adequate or reasonable PaO₂ , an FIO₂ less than 0.5, and a PEEP level less than 10 cm H₂ O, so that the lungs function as an efficient gas exchange organ. Next, the patient's lungs are required to sustain this efficient gas exchange without respirator aid as indicated by an adequate or reasonable vital capacity, peak inspiratory force, spontaneous respiratory rate, respiratory rate-to-tidal volume ratio (<105 breaths/min/L, which defines the absence of rapid shallow breathing ^[434] ), and PaCO₂ on a low IMV rate. Finally, the process requires the absence of any new complicating factor, as indicated by a lack of new chest radiographic findings, resolution of previous pathologic chest radiographic findings, and the absence of any significant disorder or complication in any of the other major organ systems.

During the postextubation period, high alveolar inflating pressure—the ideal respiratory care maneuver—is sustained for a relatively long time and can be achieved only with a large inhaled volume. Such deep-breathing exercises with an emphasis on achieving sustained inspiration to total lung capacity can be accomplished by using the incentive spirometer.

In addition, a variety of other techniques can help minimize postoperative pulmonary complications. Both upright position in bed and early ambulation increase FRC and help restore a favorable FRC-closing volume relationship. Percussion and postural drainage aid in the mobilization of secretions in patients with chronic bronchitis. Continuing the administration of bronchodilating drugs or steroids that were given preoperatively is important in continuing to keep reactive airways quiescent.