Postoperative Respiratory Care

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Chapter 28 Postoperative Respiratory Care

Patients undergoing cardiac surgery experience physiologic stresses from anesthesia, thoracotomy, surgical manipulation, and cardiopulmonary bypass (CPB). Each of these interventions can create transient deleterious effects on pulmonary function even with normal lungs; the effects may be exaggerated in the presence of preexisting pulmonary pathologic processes. Important pulmonary changes after cardiac surgery include diminished functional residual capacity (FRC) after general anesthesia and muscle relaxants, transient 50% to 75% reduction in vital capacity (VC) after median sternotomy and intrathoracic manipulation, atelectasis, and increased intravascular lung water. Acute FRC reduction results in arterial hypoxemia due to mismatch between ventilation and perfusion and in diminished lung compliance with increased work of breathing. This additional work of breathing, which increases oxygen consumption by up to 20% in spontaneously breathing patients, also increases myocardial work at a time when myocardial reserves may be limited. Changes in spirometric measurements and respiratory muscle strength can last up to 8 weeks postoperatively.1

Thus, a sizeable proportion of cardiac surgical patients can be expected to have respiratory complications. Acute lung injury, sometimes progressing to acute respiratory distress syndrome (ARDS), can occur in up to 12% of postoperative cardiac patients.

RISK FACTORS FOR RESPIRATORY INSUFFICIENCY

The lung is especially vulnerable because disturbances may affect it directly (atelectasis, effusions, pneumonia) or indirectly (via fluid overload in heart failure, as the result of mediator release during CPB, shock states, or infection, or via changes in respiratory pump function as with phrenic nerve injury). Postoperative status will be determined in part by the patient’s preoperative pulmonary reserve, as well as by the level of stress imposed by the procedure. Thus, a patient with reduced VC due to restrictive lung disease, undergoing minimally invasive surgery, may have fewer postoperative pulmonary issues than a relatively healthy patient undergoing simultaneous coronary artery bypass grafting and valve replacement with its accompanying longer operative/anesthetic and CPB times. Respiratory muscle weakness contributes to postoperative pulmonary dysfunction, and prophylactic inspiratory muscle training has been shown to improve respiratory muscle function, pulmonary function tests, and gas exchange. Training reduces the percentage of patients requiring more than 24 hours of postoperative ventilation support from 26% to 5%.

Assessing Risk Based on Preoperative Status

The Society of Thoracic Surgeons (STS) National Adult Cardiac Surgery Database is widely used and offers, in addition to a mortality model, a model customized to predict prolonged ventilation.2 Chronic obstructive pulmonary disease (COPD) might be expected to be a major risk for postoperative morbidity and mortality. However, hospital mortality with mild-to-moderate COPD is not especially high; it is the minority of patients with severe COPD, especially those older than the age of 75 years or receiving corticosteroids, who are at highest risk. Patients with preexisting COPD have higher rates of pulmonary complications (12%), atrial fibrillation (27%), and death (7%).3 Obesity, defined by increased body mass index, does not appear to increase the risk of postoperative respiratory failure.

Studies have used multivariate regression techniques to elucidate factors specifically associated with postoperative respiratory failure (Table 28-1).47 They differ in their endpoints for outcome and in their choice of preoperative versus operative versus postoperative variables. The STS model was found to be the single best predictor of mechanical ventilation support for longer than 72 hours but also identified mitral valvular disease, age, vasopressor and inotrope use, renal failure, operative urgency, type of operation, preoperative ventilation, prior cardiac surgery, female gender, myocardial infarction within 30 days, and previous stroke as contributors.5 None of these models, general or specific for respiratory complications, is sufficiently sensitive or specific to prohibit consideration of surgery for an individual patient, but all provide the clinician with early warning for patients at high risk.

Table 28-1 Factors Predicting Postoperative Respiratory Outcome

Study Endpoint Risk Factors
Spivack et al, 19964 Mechanical ventilation > 48 hr

Branca et al, 20015 Mechanical ventilation > 72 hr

Rady et al, 19996 Extubation failure (reintubation after initial extubation)

Canver and Chandra, 20037 Mechanical Ventilation > 72 hr

LVEF = left ventricular ejection fraction; CHF = congestive heart failure; STS = Society of Thoracic Surgeons; CABG = coronary artery bypass grafting; COPD = chronic obstructive pulmonary disease; BUN = blood urea nitrogen; DO2 = systemic oxygen delivery; CPB = cardiopulmonary bypass; Hct = hematocrit.

Postoperative Events

The expected postoperative course is a short period of ventilation support while the patient is warmed, allowed to awaken, and observed for bleeding or hemodynamic instability. Preoperative risks, issues with difficult intubation, and operating room events should be communicated from the operating room team to the ICU team at the time of ICU admission. Box 28-1 outlines criteria to be met before routine extubation.

Before extubation, a quick neurologic examination should be performed to rule out new cerebrovascular events, presence of excess opioids, or residual neuromuscular blocking agents. Knowledge that the work of breathing can consume up to 20% of CO should preclude extubation in the hemodynamically unstable patient. Although patients may be successfully extubated while on IABP, the need to lie flat after balloon and sheath removal may dictate continued temporary ventilator support.

Postoperative care of low-risk cardiac surgical patients has come to resemble a recovery room model, but high-risk patients benefit from postoperative involvement of anesthesiologists, cardiologists, and critical care specialists. The presence of full-time ICU staff physicians improves outcome and is now recommended by the Leapfrog Group as a patient safety standard.8

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