Interpreting & Reporting Results (ECG response, RPP, claudication, Duke Treadmill Score, VO₂/RER/AT)

Interpreting & Reporting Results: ECG Response, RPP, Claudication, Duke Treadmill Score & VO₂/RER/AT

Once a test is complete, the CEP's job shifts from monitoring to interpretation — turning raw waveforms, vitals, and symptom reports into a clinical picture the referring physician can act on. Several standardized tools structure that interpretation.

ECG Response

An exercise ECG is generally read as positive (ischemic), negative, or equivocal/non-diagnostic. The classic ischemic pattern is horizontal or downsloping ST-segment depression of 1 mm or more, measured 60-80 ms after the J point, in a patient without confounding baseline abnormalities; upsloping depression is a less specific finding. Tests are non-diagnostic in patients with baseline findings that already distort the ST segment — left bundle branch block, a ventricularly paced rhythm, left ventricular hypertrophy with strain, or digoxin effect — because ischemic changes cannot be reliably distinguished from the underlying abnormality.

Rate-Pressure Product (RPP)

RPP = heart rate × systolic blood pressure. RPP is a widely used estimate of myocardial oxygen demand and cardiac workload. Because it reflects the heart's actual demand rather than just external workload, RPP is often used to compare a patient's tolerance before and after an intervention — for example, a lower RPP at the point angina or ischemia appears after starting a beta-blocker or after revascularization reflects reduced myocardial oxygen demand for a given level of exertion, even if the achieved workload (mph, watts) looks similar.

Claudication Grading

For patients tested for suspected or known peripheral artery disease, claudication pain is tracked on a graded scale throughout the test — commonly a 4-point scale progressing from initial onset of mild discomfort, through moderate pain, to pain severe enough that the patient must stop. Claudication severity that reaches this stopping threshold is itself one of the relative indications for test termination covered in the previous section, so claudication grading and termination decisions are directly linked.

Duke Treadmill Score

The Duke Treadmill Score (DTS) combines exercise time on the Bruce protocol, ST-segment deviation, and angina during the test into a single prognostic number:

DTS = Exercise time (minutes) − (5 × maximum ST deviation in mm) − (4 × angina index)

The angina index scores 0 for no angina, 1 for non-limiting angina, and 2 for exercise-limiting angina. Resulting scores stratify risk:

The DTS is intended for patients without known coronary artery disease undergoing a standard Bruce-protocol test and should not be applied outside that context.

VO2, RER & Ventilatory (Anaerobic) Threshold

Measured vs. estimated VO2peak. Measured VO2peak, obtained directly from expired-gas analysis (metabolic cart) during cardiopulmonary exercise testing (CPET), is more accurate than VO2 estimated from workload/MET prediction equations, which assume a standard mechanical efficiency that any individual patient may not match.

Confirming maximal effort. Because a literal VO2 plateau despite increasing workload is often not observed clinically, "VO2peak" (the highest value actually achieved) is generally preferred over "VO2max." Supporting evidence of a true maximal or near-maximal effort includes a respiratory exchange ratio (RER) at or above approximately 1.10 at peak exercise, attainment of age-predicted maximal heart rate, and a peak RPE in the 18-19+ range.

Ventilatory (anaerobic) threshold (VT/AT). VT marks the point where ventilation begins increasing disproportionately relative to VO2 as anaerobic metabolism contributes more to energy production and lactate rises. Because it can be identified from a submaximal effort, VT is a valuable, effort-independent marker of aerobic fitness in patients who cannot or should not be pushed to a true peak — including many older, heart-failure, or pulmonary patients.

VE/VCO2 slope. This ventilatory-efficiency measure reflects how much ventilation is required to eliminate a given amount of CO2; values above roughly 30 indicate abnormal ventilation-perfusion matching and are seen in heart failure, pulmonary arterial hypertension, interstitial lung disease, and COPD, carrying prognostic significance in these populations.

Putting It Together

None of these tools is interpreted in isolation. A physician review typically weighs the ECG response, hemodynamic response (including RPP), symptom pattern, and — when available — the DTS or CPET indices together to arrive at a risk classification and next step, whether that's referral for further cardiac workup, clearance for a structured exercise program, or direct input into the FITT-VP prescription decisions covered in Chapter 8.