Residency · Residency · Anesthesiology

Pulmonary Function Testing and the Patient with Severe COPD or Asthma

Introduction

Patients with chronic obstructive pulmonary disease (COPD) and asthma represent a significant proportion of the surgical population. Understanding pulmonary function testing and the pathophysiology of obstructive lung disease is critical for perioperative risk stratification, anesthetic planning, and ventilatory management.

Pulmonary Function Tests (PFTs)

Spirometry

FEV1 (forced expiratory volume in 1 second) is the single most important value for preoperative assessment. FVC (forced vital capacity) represents the total volume exhaled during a maximal forced expiration. The FEV1/FVC ratio is normally greater than 0.70, and a ratio below 0.70 defines obstructive disease. The FEF 25-75% (mid-expiratory flow rate) is a sensitive marker of small airway disease but is highly variable. Flow-volume loops provide characteristic patterns that distinguish obstructive, restrictive, and upper airway lesions.

Lung Volumes

Total lung capacity is increased in COPD (hyperinflation) and decreased in restrictive disease. Residual volume is elevated in air trapping, and an RV/TLC ratio greater than 0.40 suggests significant gas trapping. Functional residual capacity is the equilibrium point between chest wall and lung elastic recoil. These volumes are measured by body plethysmography, helium dilution, or nitrogen washout.

Diffusing Capacity (DLCO)

DLCO measures gas transfer across the alveolar-capillary membrane and is reduced in emphysema, interstitial lung disease, and pulmonary vascular disease. A DLCO below 40% of predicted is a significant predictor of postoperative pulmonary complications. DLCO is not typically reduced in pure asthma or chronic bronchitis.

Preoperative Risk Assessment for Lung Resection

The predicted postoperative FEV1 (ppoFEV1) is calculated as the preoperative FEV1 multiplied by (1 minus the fraction of lung to be resected). A ppoFEV1 above 40% and ppoDLCO above 40% indicate acceptable risk for lung resection. If values fall below these thresholds, cardiopulmonary exercise testing (CPET) is indicated. A VO2max above 15 mL/kg/min represents acceptable surgical risk, while a value below 10 mL/kg/min indicates high risk.

COPD: Pathophysiology and Anesthetic Implications

Pathophysiology

COPD encompasses two phenotypes: chronic bronchitis (airway inflammation, mucus hypersecretion) and emphysema (alveolar destruction, loss of elastic recoil). Air trapping and auto-PEEP (intrinsic PEEP) result from premature airway closure during expiration. V/Q mismatch is the primary mechanism of hypoxemia. Chronic CO2 retention shifts the ventilatory drive to the hypoxic drive in some patients, a concept that is controversial but clinically relevant.

Preoperative Optimization

Inhaled bronchodilators (beta-2 agonists, anticholinergics) should be continued through the morning of surgery. Inhaled corticosteroids should be continued, and a short course of systemic steroids considered if there is an active exacerbation. Active respiratory infections should be treated before elective surgery. Smoking cessation ideally occurs more than 8 weeks preoperatively to reduce pulmonary complications, though even 24 to 48 hours of cessation improves carbon monoxide levels.

Intraoperative Management

Regional anesthesia is preferred when feasible to avoid airway instrumentation. If general anesthesia is required, high FiO2 should be avoided if possible to reduce absorption atelectasis. Ventilator settings should include a low respiratory rate, prolonged expiratory time (I:E ratio of 1:3 or 1:4), and lower tidal volumes. Auto-PEEP should be monitored by briefly disconnecting the circuit and observing for continued exhalation. Histamine-releasing drugs (morphine, atracurium, mivacurium) should be avoided. Sevoflurane is the preferred volatile agent due to its bronchodilating properties. Humidified gases should be used to prevent inspissation of secretions.

Emergence and Extubation

Patients should be extubated either deep or fully awake to minimize coughing and bronchospasm. Bronchodilators should be administered via the circuit before extubation. Complete reversal of neuromuscular blockade (TOF ratio above 0.9) must be ensured. Sugammadex is preferred over neostigmine because neostigmine can provoke bronchospasm via muscarinic effects.

Asthma: Pathophysiology and Anesthetic Implications

Pathophysiology

Asthma is characterized by reversible airway obstruction, airway hyperreactivity, and eosinophilic inflammation. Triggers include allergens, cold air, exercise, airway instrumentation, histamine-releasing drugs, and NSAIDs (in aspirin-sensitive asthma). Status asthmaticus is severe, refractory bronchospasm representing a life-threatening emergency.

Preoperative Assessment

Current symptom control should be assessed, including the frequency of rescue inhaler use, nocturnal symptoms, and recent exacerbations or hospitalizations. A peak expiratory flow rate below 80% of the patient's personal best suggests suboptimal control. All maintenance medications including inhaled corticosteroids and long-acting bronchodilators should be continued. Patients on chronic oral steroids or recent steroid courses may need stress-dose steroids.

Intraoperative Bronchospasm

Bronchospasm presents with rising peak airway pressures, wheezing, prolonged expiration, an upsloping capnography waveform, and decreased tidal volumes. The differential diagnosis includes endobronchial intubation, kinked ETT, mucus plugging, pneumothorax, anaphylaxis, and pulmonary edema.

The treatment ladder begins with deepening anesthesia (increasing volatile agent concentration). Inhaled albuterol (8 to 10 puffs via MDI with an in-line adapter) is administered. For severe bronchospasm, IV epinephrine at 10 to 20 mcg boluses is given. IV magnesium sulfate at 2 g over 20 minutes may help. IV ketamine at 0.5 to 1 mg/kg provides bronchodilating properties. IV hydrocortisone at 100 mg has a delayed onset but prevents the late-phase response.

Ventilatory Management in Severe Bronchospasm

Permissive hypercapnia is employed, accepting elevated PaCO2 to avoid air trapping and barotrauma. The respiratory rate is reduced to allow adequate expiratory time, and tidal volume is reduced to limit peak and plateau pressures. Dynamic hyperinflation should be monitored by observing for increasing plateau pressures and hemodynamic compromise. If cardiovascular collapse occurs, disconnecting the circuit allows complete exhalation and treats breath stacking.

Clinical Pearls

An FEV1 below 40% of predicted identifies patients at highest risk for postoperative pulmonary complications, and these patients warrant aggressive optimization and consideration of regional techniques. Auto-PEEP is a silent killer in COPD patients under general anesthesia; the clinician should always check for incomplete exhalation and allow adequate expiratory time. The most dangerous moment for the asthmatic patient is airway instrumentation, and adequate depth of anesthesia should be ensured before laryngoscopy, with IV lidocaine at 1 to 1.5 mg/kg considered to blunt airway reflexes. Sevoflurane and ketamine are the anesthesiologist's best bronchodilators and should be readily available when managing patients with reactive airway disease.

References

1. Lumb AB. Nunn's Applied Respiratory Physiology. 9th ed. Elsevier; 2022. Chapters 27-28.
2. Brunton LL, Hilal-Dandan R, Knollmann BC, eds. Drugs affecting airway smooth muscle. In: Goodman & Gilman's: The Pharmacological Basis of Therapeutics. 14th ed. McGraw-Hill; 2023.
3. Edrich T, Sadovnikoff N. Anesthesia for patients with severe chronic obstructive pulmonary disease. Curr Opin Anaesthesiol. 2010;23(1):18-24.
4. Woods BD, Sladen RN. Perioperative considerations for the patient with asthma and bronchospasm. Br J Anaesth. 2009;103(Suppl 1):i57-i65.

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