Page 1533 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

1052 PART 10: The Surgical Patient

to carbon dioxide, as well as a blunted response to hypoxemia and the poorer the preoperative respiratory function, the more likely the
acidosis. In the postoperative period, narcotic analgesics may have patient is to have severe postoperative respiratory complications. Based
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undesirable effects. Whereas in optimal doses they decrease abdominal on cumulative experience, the following spirometric criteria for predict-
pain and increase the ability to cough and clear secretions, in larger doses ing morbidity and mortality in postoperative adult patients have been
they may depress the respiratory center, producing alveolar hypoventila- proposed. 73,74 If the FEV is <1 L, the FVC <1.5 L, the FEV /FVC is
1
1
tion as manifested by hypercapnia and secondary hypoxemia. <30%, or the forced expiratory flow [FEF 25%–75% ] is <0.6 L/s, and if the
The cough reflex is the main mechanism by which particles are maximum minute ventilation is <50% of the predicted value, then
cleared from the upper airway. The cough response is altered not only the risk of postoperative pulmonary complications is very high. In
by anesthesia, but also by narcotic agents. Clearance of particles from the patients whose respiratory function is below this threshold, the strategy
lower airways depends primarily on the mucociliary system, which can is to provide treatment that will improve respiratory function to a level
be disturbed by several factors in the postoperative period. Anesthetics above this threshold. Such treatment may involve cessation of smoking,
alter ciliary activity and mucus production, which leads to the produc- diaphragm muscle conditioning, weight loss, and the treatment of heart
tion of mucus plugs that may block the lower airways. In addition, the failure, fluid overload, and any identifiable reactive airway disease.
cellular defense mechanisms of the respiratory system may be altered by A patient who undergoes lung resection is at even greater risk of post-
anesthetic agents. 63 operative pulmonary complication, particularly if the excised lung tissue
■ ASPIRATION was functional. 73,74 In these patients, the effect of the lost lung volume
must be considered along with the factors discussed above. A quantita-
The supine position and depression of normal protective reflexes tive perfusion lung scan can help to predict the postoperative pulmonary
during general anesthesia predispose the surgical patient to aspiration of spirometric performance of these patients by indicating how much of the
gastric acid, which is one of the major causes of perioperative morbidity lung will remain after the planned procedure. The postoperative FEV is
1
and mortality. This event can first produce airway obstruction (from then calculated as the product of the preoperative FEV and the fractional
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1
aspirated debris and chemically induced bronchoconstriction), then a perfusion of the remaining lung. The usual rule for an adult patient is
chemical burn of the airway (with fluid loss into the injured area), an that the operative risk is prohibitive if the predicted postoperative FEV is
1
intense inflammatory response, and finally lung infection. The clinical ≤0.8 L. The prediction can be made more accurate by also measuring the
presentation of patients with gastric acid aspiration varies widely. Very diffusing capacity, which is an independent predictor of morbidity and
mild cases present only with transient coughing and minimal bron- mortality after major lung resection. A useful guideline is to exclude from
chospasm; the most severe cases exhibit a progressive downhill course major lung resection all patients whose diffusing capacity is <60% of the
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characterized by hypovolemia, hypoxemia, and finally fulminant bacte- predicted value, even if spirometric values are considered satisfactory.
rial pneumonia. Patients with only slightly impaired pulmonary function (FEV and
1
The management of acid aspiration is mainly supportive and includes: diffusing capacity ≥80% predicted) with no cardiovascular risk factors
(1) rapid removal of debris by immediate suction (endotracheal intu- can undergo pulmonary resections including pneumonectomy without
bation and fiberoptic bronchoscopy may be necessary at this stage) if further investigation. For others, exercise testing as well as pulmonary
there is particulate matter present; (2) placement of a nasogastric tube to split-function test studies are recommended. The symptom limited
evacuate the stomach and prevent further episodes; (3) oxygen adminis- cardiopulmonary exercise testing measures the maximum volume of
tration and mechanical ventilation if indicated by the degree of respira- oxygen utilization (V O 2 max) as an index of pulmonary and cardiovas-
tory failure; (4) bronchodilator therapy if bronchospasm is significant; cular reserve. A V O 2 max <10 mL/kg per minute is generally considered
(5) maintenance of normovolemia and normal perfusion by monitoring a contraindication to any resection, whereas a value >20 mL/kg per
and replacement of lost fluid, as well as vasoactive and inotropic support minute or >75% of predicted normal is considered safe for major resec-
where necessary. Antibiotics should be avoided unless there is a strong tions. Resections that involve no more than one lobe usually lead to early
convincing evidence of bacterial pneumonia rather than only chemi- functional deficit followed by recovery, and permanent loss in pulmonary
cal pneumonitis which is usually the case following most aspiration function is usually <10%. Generally, pulmonary function tests tend to
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events. Steroids have not been of any benefit in treating these patients. overestimate the functional loss after lung resection. Arterial blood gas
Preventive measures that can be taken in high-risk patients to prevent criteria may also be used to exclude patients from major lung resection
the aspiration of low-pH gastric contents include gastric decompres- because of the prohibitive risk of postoperative morbidity and mortality.
sion, positioning intubated patients in a semirecumbent position unless Patients who have a room-air partial pressure of arterial oxygen (Pa O 2 ) of
contraindicated, and continuous drainage of subglottic secretions. 65-67 <50 mm Hg or a partial pressure of carbon dioxide (P CO 2 ) of >45 mm Hg
■ PREDICTING AND PREVENTING PERIOPERATIVE LUNG DYSFUNCTION at rest are considered to have a prohibitive operative risk and should not
undergo major pulmonary resection. Other forms of surgical interven-
Many attempts have been made to correct the postoperative abnor- tion are justifiable in the presence of these blood gas criteria only if they
malities in lung function, using techniques such as incentive spirometry, are considered mandatory and lifesaving.
intermittent positive-pressure breathing (IPPB), and nasal continuous ■
positive airway pressure (CPAP). 68-70 Although incentive spirometry has TREATMENT PRINCIPLES FOR PERIOPERATIVE RESPIRATORY FAILURE
been reported to be ineffective in decreasing postoperative pulmonary At present, no specific therapy exists for underlying diaphragmatic
complications following cardiac and upper abdominal surgery, IPPB, dysfunction. Therefore, the principles of respiratory care in the surgical
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incentive spirometry, CPAP, and physiotherapy generally improve patient are as follows:
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postoperative respiratory function; IPPB offers no advantage over phys-
iotherapy when the latter is maximized in the postoperative period. 1. Maximization of the preoperative respiratory status. Meeting this
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Although nonyielding abdominal binders have a further restrictive goal may entail cessation of smoking, diaphragmatic conditioning
effect on lung volumes postoperatively, the elastic binders may pro- exercises, reduction in obesity, and treatment of any identified car-
duce some benefit. It must be recognized, however, that none of these diorespiratory disease, including congestive heart failure, broncho-
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methods completely reverses the postoperative respiratory dysfunction. pneumonia, or bronchospasm.
Attempts have been made to predict postoperative pulmonary mor- 2. Aggressive physiotherapy and early ambulation to overcome the
bidity by assessing respiratory mechanics preoperatively, as well as by effects of the supine position on changes in lung volumes, particu-
identifying risk factors such as age, obesity, smoking, and location of larly the relationship between closing volume and FRC. In patients
incisions. No individual respiratory parameter predicts respiratory with multiple fractures, early operative stabilization will decrease
morbidity or mortality in an individual patient. In general, however, the period of recumbence.

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