Given that patients with COPD often have several comorbidities, routine laboratory studies including a complete blood count, electrolytes, and an assessment of renal function should be ordered in most patients. Brain natriuretic peptide (BNP) appears to be tailor made to help differentiate patients with COPD from those with CHF. BNP levels less than 100 pg/mL have a very high negative predictive value for CHF, whereas most patients with CHF have levels >400 pg/mL. However, many patients have values that fall somewhere in between, and discordance between BNP values and patient symptoms occurs often enough that single measurements need to be interpreted carefully. If available, the patient's prior records should be sought out to compare current and past values to determine trends and to establish a baseline. Furthermore, some patients may have a mixture of presenting problems contributing to their dyspnea, so an elevated BNP does not exclude a concomitant COPD exacerbation.
Cardiac markers such as troponin are frequently ordered, but usually unnecessary. Because patients with severe COPD exacerbations often suffer from hypoxia and tachycardia, myocardial oxygen demand is increased, and many patients will have small troponin elevations owing to “demand ischemia.” In these patients, serial troponin measurements should be used to help exclude an acute coronary syndrome.
D-dimer levels may also be useful in patients with a presumed COPD exacerbation to help exclude PE. Given their comorbidities (CHF, a low flow state), sedentary lifestyle, history of smoking, and increased risk for an underlying malignancy, many patients with COPD are at increased risk for PE. Because d-dimer levels are also likely to be falsely elevated in this population, it is wise to limit d-dimer testing to those patients in whom there is a reasonable clinical suspicion of PE (abrupt onset, unilateral leg swelling).
Finally, arterial blood gases (ABG) have long been part of the routine evaluation of patients with severe COPD exacerbations. ABGs provide information about oxygenation (PaO2), ventilation (PaCO2), and overall acid–base status (pH). Blood gas readings in patients with significant COPD exacerbations will reveal a primary respiratory acidosis, with elevated CO2 levels (>40 mmHg) resulting in a decreased pH (<7.30).
The chest x-ray (CXR) primarily helps to diagnose pneumonia and to exclude alternative conditions such as CHF, a pneumothorax, or significant atelectasis or lobar collapse. The classic findings are hyperinflation and bullous changes (Figure 22-1). Vascular markings and heart size are often decreased in patients with emphysema pathology and increased in patients with chronic bronchitis.
Chest radiograph of a patient with chronic obstructive pulmonary disease.
As with the CXR, electrocardiograms are primarily useful to exclude alternative diagnoses, such as cardiac ischemia. In patients with pulmonary hypertension, peaked P waves in lead II may be present (p pulmonale), reflecting right atrial enlargement, whereas other patients may have signs of right ventricular hypertrophy (large R wave in v1 and v2 with prominent S waves in v5 and v6), a right bundle branch block, or right axis deviation. Multifocal atrial tachycardia (MAT) is the classic arrhythmia associated with COPD patients. MAT is an irregularly irregular rhythm, like atrial fibrillation (AF), but there are P waves of differing morphologies before every QRS complex, and it tends to be slower than AF.