2.3 Labs, ECG, Imaging, and PFT Clues
Key Takeaways
- Labs explain respiratory symptoms: hemoglobin sets oxygen content, the WBC pattern suggests infection type, and electrolytes drive weakness or arrhythmia.
- ECG clues in CRT scenarios point to right-heart strain, electrolyte effects, ischemia, or rhythm-related oxygen-delivery problems.
- Read chest imaging for pattern and volume effect, not only for the presence of an opacity.
- Start PFT interpretation with FEV1/FVC for obstruction, TLC for restriction, bronchodilator response for reversibility, and DLCO for diffusion.
Lab Clues That Change the Respiratory Picture
Laboratory data explain why the same SpO2 means different things in different patients. Hemoglobin sets oxygen content, white-blood-cell patterns help separate bacterial infection from other causes, and electrolytes alter respiratory-muscle strength, mental status, and cardiac rhythm. Use labs as support, not as standalone answers: a high WBC count does not prove pneumonia without matching symptoms or imaging, and a low hemoglobin does not make the lungs fail but reduces oxygen delivery, explaining dyspnea, tachycardia, and poor exercise tolerance despite an acceptable saturation.
| Lab clue | Exam meaning | Why it matters |
|---|---|---|
| Low hemoglobin or hematocrit | Low oxygen content | SpO2 may look fine while delivery is poor |
| Elevated WBC with neutrophilia | Bacterial pattern | Supports infection when symptoms and imaging fit |
| Low potassium (<3.5 mEq/L) | Weakness, dysrhythmia risk | Can impair ventilation and stability |
| High potassium (>5.0 mEq/L) | Conduction and arrest risk | ECG changes make it urgent |
| Elevated BNP | Heart-failure pattern | Supports edema with matching CXR and exam |
| Elevated lactate | Poor perfusion or severe stress | Raises shock or sepsis concern |
| Elevated D-dimer | Possible clot burden | Sensitive, not specific; needs clinical context |
ECG Signals Respiratory Therapists Should Recognize
The TMC does not expect cardiology-level interpretation, but it does expect pattern recognition. Right-axis deviation, a tall R wave in V1, or peaked P waves (P pulmonale) support chronic lung disease, pulmonary hypertension, or right-heart strain. ST-segment changes with dyspnea can point to cardiac ischemia rather than a primary lung problem, reminding you that not every breathless patient is having a pulmonary event.
Electrolytes also surface on the ECG. Hyperkalemia produces tall, peaked T waves and a widening QRS; hypokalemia produces flattened T waves, ST depression, and U waves. When a rhythm threatens perfusion, oxygen delivery falls even when the lungs themselves are normal.
| ECG or monitor clue | Linked patient data | Interpretation habit |
|---|---|---|
| New rapid atrial fibrillation | Palpitations, dyspnea, hypotension | Consider reduced cardiac output |
| ST depression with chest pressure | Dyspnea plus ischemic symptoms | Do not assume all dyspnea is pulmonary |
| Tall peaked T waves with renal failure | Hyperkalemia risk | Escalate; rhythm may deteriorate |
| Right-heart strain pattern | Hypoxemia, clot risk, pulmonary HTN | Correlate with oxygenation and perfusion |
Chest Imaging: Pattern Plus Volume
Read imaging descriptions for both pattern and volume effect. Consolidation fills airspaces and usually does not shift the mediastinum. Atelectasis causes volume loss and may pull structures toward the affected side. A space-occupying pleural process (large effusion or tension pneumothorax) pushes structures away. The direction of mediastinal shift is the single most useful image cue for separating volume loss from pleural pressure.
| Image finding | Think | Key clue |
|---|---|---|
| Air bronchograms | Consolidation | Air-filled bronchi within an opacity |
| Shift toward the opacity | Atelectasis | Volume loss pulls structures in |
| Absent peripheral lung markings | Pneumothorax | Pleural air outside the lung edge |
| Shift away plus shock | Tension pneumothorax | Pressure effect with instability |
| Blunted costophrenic angle / meniscus | Pleural effusion | Dependent fluid |
| Perihilar bat-wing opacities | Pulmonary edema | Cardiac/fluid clues support it |
| ETT tip near or below the carina | Mainstem intubation risk | Compare breath-sound symmetry and depth |
PFT and Monitoring Clues
Begin pulmonary function test (PFT) interpretation with the FEV1/FVC ratio. A reduced ratio (commonly below 0.70, or below the lower limit of normal) supports obstruction. Restriction requires a reduced total lung capacity (TLC), not merely a low forced vital capacity (FVC), because FVC can drop in both patterns. A mixed defect needs obstruction plus a reduced TLC.
| PFT data | Pattern | Exam interpretation |
|---|---|---|
| Low FEV1/FVC, normal/high TLC | Obstructive | COPD, asthma, airflow limitation |
| Normal/high FEV1/FVC, low TLC | Restrictive | Lung volume reduced |
| FEV1 rises >=12% and >=200 mL post-bronchodilator | Reversible obstruction | Asthma component supported |
| Low DLCO | Diffusion or vascular defect | Emphysema, interstitial, or pulmonary vascular disease |
| Peak flow improves after therapy | Better large-airway flow | Useful asthma response trend |
Trend recognition spans every data type. A falling peak flow after repeated bronchodilator therapy is worse than one low reading. A chest radiograph that shifts from low-volume atelectasis to a new unilateral hyperlucent field after line placement suggests a fresh complication. A lactate that climbs while blood pressure falls makes a stable-looking saturation far less reassuring.
Correlation Rule
One supporting test rarely stands alone. Pair labs with symptoms, imaging with volume clues, ECG with perfusion, and PFT data with the patient's history. The strongest CRT interpretation usually explains why several findings converge on the same respiratory or oxygen-delivery problem, and the keyed answer is the one that ties the cluster together rather than reacting to a single value.
A patient with acute dyspnea has bilateral crackles, ankle edema, jugular venous distension, BNP 980 pg/mL, and a chest radiograph showing perihilar fluffy opacities. Which interpretation is best supported by the combined data?
Pulmonary function testing shows FEV1/FVC 0.84, FVC 61% predicted, TLC 58% predicted, and DLCO 54% predicted. Which pattern is most consistent with these results?
A renal-failure patient reports weakness and palpitations. The monitor shows a widening QRS and tall peaked T waves, and potassium is 6.8 mEq/L. What is the most important interpretation for a CRT candidate?