PULMONARY FUNCTION TESTING IN SURGICAL PATIENTS

Significant postoperative changes in pulmonary function in many surgical patients include phenomena such as reduced lung volumes, rapid and shallow breathing, and impaired gas exchange. These alterations in pulmonary function may occur as a result of the anesthetic, the surgical procedure, the associated body position, or the medications administered immediately after surgery. These changes, which occur in normal patients, may be more severe in patients undergoing surgery who have compromised pulmonary function and therefore may produce significant postoperative pulmonary complications. Such complications usually include bronchospasm, bronchitis with purulent sputum, disabling cough pneumonia, and respiratory failure, as indicated by altered blood gas values. Preexisting lung dysfunction is the major factor associated with postoperative pulmonary problems. Also important are incisional site and size, as well as possible surgical trauma to lung tissue. Both considerations render the patient who is to undergo thoracic surgery a prime candidate for evaluation.

Prevailing opinion suggests that preoperative pulmonary function testing provides the clinician with important information regarding the potential for postoperative respiratory morbidity. If pulmonary function data are of value, which patients are candidates for such testing? Although no general agreement exists about which patients should be tested, the prime candidates are those in whom there is a reasonable expectation of abnormal pulmonary function. This broad list was outlined by Tisi^[21] ( Table 26-5 ).

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Conflicting data abound regarding these rather broad criteria. The American College of Chest Physicians has proposed a more stringent set of guidelines ( Table 26-6 ). Adherence to these stricter guidelines has been recommended as a means of decreasing unnecessary and costly spirometric testing.^[22]

Initial identification of most of these patients is accomplished by history, physical examination, and chest radiographic studies. The chest x-ray film is a particularly valuable clinical marker for clinically severe disease, especially if it identifies lung hyperinflation, which has been associated with a 33% rate of significant postoperative pulmonary complications.^[23]

Which pulmonary function studies are appropriate for preoperative evaluation? The objective of testing in the preoperative setting is not to detect mild, early lung disease but to predict the likelihood of pulmonary complications. The physician must ask whether the test results alter perioperative management or sufficiently affect risk estimates such that planned surgery is altered or even postponed. Other factors, such as the cost of testing and even the risk of testing, must also be considered. No single test appears to be the best predictor of risk, probably because none assesses all of the factors that are important regardless of whether complications may occur. The optimal scheme for evaluating patients preoperatively is by means of arterial blood gas analysis and the FEV₁ , FVC, FEV₁ /FVC, peak flow, and FEF_25%–75% , which can be obtained from a single spirometric study. Abnormalities on such spirometric tests seem to correlate with the incidence of postoperative pulmonary complications.

Although the evaluation of preoperative pulmonary function is largely aimed at predicting the risk of postoperative complications, the identification of abnormal lung function, particularly obstructive airway disease, is also important to reduce intraoperative morbidity. A reduced FEV₁ /FVC, for example, documents the existence of airway obstruction and suggests the likelihood of increased airway reactivity. The site and the nature of many surgical procedures often provide very little latitude for choosing between regional and general anesthesia. In patients with airway obstruction and heightened airway reactivity, airway instrumentation (e.g., laryngoscopy, tracheal intubation) is fraught with the hazard of provoking reflex bronchoconstriction, particularly under light planes of anesthesia. In many patients with

TABLE 26-6 -- Factors prompting preoperative pulmonary function testing: American College of Physicians Guidelines

Lung resection

Smoking history, dyspnea

Cardiac surgery

Upper abdominal surgery

Lower abdominal surgery

Uncharacterized pulmonary symptoms

From Hnatiuk OW, Dillard TA, Torrington KG: Adherence to established guidelines for preoperative pulmonary function. Chest 107:1294, 1995.

obstructive airway disease, the deeper levels of inhaled anesthesia required to blunt such airway reflexes are difficult to achieve because of poor V̇/ matching and, if achieved, the levels are poorly tolerated by the cardiovascular system. To minimize airway responses, it is important to other prophylactic measures, which may include anticholinergics and β-agonists inhaled as aerosols before surgery and intravenous opioids and lidocaine administered before airway instrumentation.

Patients with abnormally low FEV₁ values before surgery are likely to experience severe hypercapnia if they are allowed to breathe spontaneously under general anesthesia.^[24] The magnitude of the carbon dioxide increase is directly related to the degree of reduction in FEV₁ and develops largely because the rapid, shallow breathing pattern characteristic of anesthetized patients worsens V̇/ matching. It is essential to control ventilation in these patients. With controlled ventilation, low respiratory rates (<10 breaths/min) are desirable to minimize V̇/ mismatch, which arises largely because of the prolonged time required to move air in and out of the obstructed airways. Because of this obstruction, low inspiratory flow rates have long been advocated to lessen peak airway pressure and to presumably minimize barotrauma and circulatory disturbances. However, evidence suggests the opposite. In patients with chronic obstructive pulmonary disease who were ventilated with customary tidal volumes (10 mL/kg), respiratory system resistance decreased as inspiratory flow rates were increased from 0.25 to 1.5 L/s.^[25] The investigators postulated that this effect was caused by decreases in thoracic tissue resistance or was a reflection of improvement in time-constant inequalities or in the viscoelastic behavior of the respiratory system, or both. Other studies have shown that a high inspiratory flow rate in such patients produced improved gas exchange and was not complicated by barotrauma or circulatory depression.^{[26] [27]} An important consequence of the increased inspiratory flow rate was a reduced inspiratory time, which allowed increased time for exhalation. Increased expiratory time provides more complete emptying of alveoli, which must occur through high-resistance airways. With a shorter expiratory time, which inevitably occurs with the increased inspiratory time needed with a lower inspiratory flow, alveoli are not allowed to empty completely, and they receive less gas volume at the same distending pressures during inspiration.