Postoperative Respiratory Care

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Chapter 28 Postoperative Respiratory Care

Thomas L. Higgins, MD, Jean-Pierre Yared, MD

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).⁴^–⁷ 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	Reduced LVEF Preexisting CHF Angina Current smoking Diabetes

Branca et al, 2001⁵ Mechanical ventilation > 72 hr

STS-predicted mortality estimate

Preoperative ventilation

Prior CABG

Female gender

Myocardial infarction within 30 days

Previous stroke

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

Pulmonary hypertension

Serum albumin ≤ 4.0 mg/dL

DO₂ ≤ 320 mL/mm/L² > 1 prior CABG

Thoracic aortic surgery ≥ 10 units of blood products

Total CPB time > 120 min

Canver and Chandra, 2003⁷ Mechanical Ventilation > 72 hr

CPB time

Sepsis and endocarditis

Gastrointestinal bleeding

Renal failure

Deep sternal wound infection

New cerebrovascular accident

Bleeding requiring reoperation

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.