11.4 Stress Testing and Myocardial Perfusion Imaging

Why Stress Testing Sits in the Monitoring and Diagnostics Domain

Test-plan item IV.A.5 covers stress testing and myocardial perfusion imaging. The CMC candidate is never asked to read the images. The candidate is asked to screen the patient for contraindications, hold the right drugs for the right reasons, recognise the responses that require the test to be terminated, and translate the report into a plan of care. Nearly every exam item in this area lands on one of those four things.

Exercise Treadmill Testing

Protocols

The Bruce protocol is the reference standard: three-minute stages, each raising both speed and grade.

The modified Bruce protocol inserts two preliminary three-minute stages at 1.7 mph, first at 0% grade and then at 5% grade, ahead of Bruce stage 1. It is used for deconditioned, elderly and early post-infarction patients who cannot begin at 4.6 METs. Because it shifts the workload curve, exercise time from a modified Bruce is not interchangeable with standard-protocol exercise time when a Duke score is calculated.

METs, Target Heart Rate, and What Makes a Test Non-Diagnostic

Metabolic equivalents (METs) achieved are the single most powerful prognostic variable an exercise test produces, and functional capacity outperforms the ST-segment response for predicting mortality. Fewer than 5 METs carries a poor prognosis; 5 to 9 METs is intermediate; 10 METs or more predicts an excellent outcome that is not improved by revascularisation in stable disease.

Target heart rate is 85% of the age-predicted maximum, where age-predicted maximum is 220 minus age. A test stopped for fatigue or leg pain before 85% is reached, with no ischemic ECG change and no symptoms, is non-diagnostic (submaximal), not negative. Reporting it as a negative test is a classic error, and beta-blockade is the most common reason a patient never gets there.

Interpreting the ST Response

Positive criteria: horizontal or downsloping ST-segment depression of at least 1 mm (0.1 mV) measured 80 ms after the J point, in two or more contiguous leads. Upsloping ST depression is non-specific. ST elevation of 1 mm or more in a lead without a Q wave (other than V1 or aVR) indicates transmural ischemia, localises the territory, is high risk, and is an immediate stop.

Other high-risk features on an exercise test:

• Ischemia appearing at a low workload (stage 1, or under 5 METs) or at a heart rate below 120
• ST depression in five or more leads, or ST depression persisting more than 5 minutes into recovery
• Exercise-induced hypotension (systolic pressure falling below baseline with increasing workload)
• Sustained ventricular tachycardia
• Chronotropic incompetence, and an abnormal heart-rate recovery (a fall of fewer than 12 bpm at 1 minute of active recovery)

The Duke Treadmill Score

Duke Treadmill Score = exercise time in minutes minus (5 x maximal ST deviation in mm, in any lead except aVR) minus (4 x angina index), where the angina index is 0 for no angina, 1 for non-limiting angina, and 2 for angina that stopped the test.

A high-risk Duke score is a referral for coronary angiography, not for another non-invasive test.

Contraindications and Termination Criteria

Contraindications

Absolute Indications to Stop the Test

These are the nurse's action list, and CMC items are written around them:

• ST elevation of more than 1 mm in leads without Q waves (other than V1 or aVR)
• A fall in systolic blood pressure of more than 10 mmHg below baseline despite increasing workload, when accompanied by any other evidence of ischemia — this signals an ischemic drop in stroke volume and is the most ominous hemodynamic finding on a treadmill
• Moderate to severe angina
• Central nervous system symptoms: ataxia, dizziness, near-syncope
• Signs of poor perfusion: cyanosis, pallor, cold clammy skin
• Sustained ventricular tachycardia, or any arrhythmia including second- or third-degree AV block that compromises hemodynamics
• Technical inability to monitor the ECG or blood pressure
• The patient's request to stop

Relative indications include marked ST depression of more than 2 mm horizontal or downsloping, systolic pressure above 250 or diastolic above 115 mmHg, increasing chest pain, severe fatigue or wheezing, and a new bundle branch block that cannot be distinguished from ventricular tachycardia.

Baseline ECG Abnormalities That Force Imaging

An exercise ECG cannot be interpreted when the resting ST segment is already abnormal. Send these patients directly for an imaging-based study:

Exercise remains preferred whenever the patient can walk and the ECG is interpretable, because it supplies functional capacity, symptom reproduction and heart-rate recovery data that no pharmacologic test provides.

Pharmacologic Stress Agents

Vasodilators

Mechanism to understand, not memorise. Vasodilators triple to quadruple flow through normal coronary arteries. A vessel distal to a significant stenosis is already maximally dilated at rest and cannot increase flow further. The result is flow heterogeneity, not true ischemia in most patients — the tracer distributes preferentially to normal territory and the stenotic territory appears as a relative defect. Coronary steal is the exception: when a territory depends on collaterals, dilating the donor bed diverts flow away from it and produces genuine ischemia with chest pain and ST-segment change.

Contraindications to vasodilator stress: second-degree Mobitz II or third-degree AV block, or sinus node dysfunction, without a functioning pacemaker; systolic blood pressure below 90 mmHg; acute coronary syndrome or unstable angina within 48 hours. Adenosine and dipyridamole are additionally contraindicated in active bronchospasm or severe reactive airway disease because A2B and A3 stimulation constricts airways. Regadenoson is A2A selective and is the usual choice in stable COPD or mild asthma, but bronchoconstriction has still been reported, so the bronchodilator stays at the bedside.

The caffeine rule. Methylxanthines — caffeinated coffee, tea, cola, energy drinks, chocolate, and drug products including theophylline and aminophylline — are competitive adenosine-receptor antagonists. They blunt the hyperemic response and produce false-negative studies. Withhold all caffeine for at least 12 hours (many laboratories require 24) and theophylline preparations for 12 to 48 hours per protocol. Also hold dipyridamole-containing products such as aspirin-dipyridamole for 24 to 48 hours before a dipyridamole or adenosine study. Expect flushing, chest tightness, dyspnea, headache, metallic taste and transient AV block; these are drug effects, not automatically ischemia, and they resolve within minutes.

Dobutamine

Used when vasodilators are contraindicated (severe bronchospasm, unavoidable recent caffeine, high-grade block without a pacer) and as the standard stressor for viability assessment on stress echocardiography. Start at 5 to 10 mcg/kg/min, escalate by 10 mcg/kg/min every 3 minutes to a maximum of 40 mcg/kg/min, and add atropine 0.25 to 0.5 mg IV in divided doses (commonly to 1-2 mg total) if the target heart rate is not reached. Risks are atrial and ventricular arrhythmia, hypotension from a dynamic left ventricular outflow gradient, and severe hypertension. Because the half-life is about 2 minutes, stopping the infusion usually suffices; a short-acting beta blocker such as esmolol is the antidote. Beta blockers blunt the response, so they are typically held before a dobutamine study.

Imaging Modalities and How to Read the Report

Reading a Perfusion Study

• Reversible defect — present on stress images, absent or improved at rest = ischemia.
• Fixed defect — present on both = scar, or an attenuation artifact. Check gated wall motion: if the wall in question moves and thickens normally, it is artifact, not scar.
• Partially reversible — infarct with surrounding peri-infarct ischemia.
• Transient ischemic dilation (TID) — the left ventricular cavity appears larger on post-stress images; a ratio above roughly 1.2 signals severe multivessel or left main disease.
• Post-stress ejection fraction drop of more than 5%, a stress EF below 45%, or increased lung uptake — high risk.
• Balanced ischemia is the trap. Relative perfusion imaging compares one territory against another. In left main or triple-vessel disease every territory is underperfused, so the relative images can look uniform and be called normal. TID, the post-stress EF fall, an abnormal PET flow reserve and the clinical picture are what catch it.

Nursing Responsibilities

• NPO 3 to 4 hours (some protocols 4 to 6). No caffeine for 12 to 24 hours. Confirm the hold verbally on arrival — patients frequently forget decaffeinated is not caffeine-free.
• Medication holds. Beta blockers, non-dihydropyridine calcium channel blockers and long-acting nitrates are held 24 to 48 hours before a diagnostic exercise test so the patient can reach target heart rate. Do not hold them when the purpose of the test is to evaluate the adequacy of anti-anginal therapy, after recent acute coronary syndrome, in aortic dissection or thyrotoxicosis, or where abrupt beta-blocker withdrawal risks rebound ischemia — and never hold a cardiac medication without an order.
• IV access, baseline 12-lead ECG and blood pressure, continuous ECG with blood pressure at every stage and at intervals through recovery, and a checked defibrillator and emergency drugs immediately available.
• Screening: pregnancy and lactation before any radiotracer, weight limits on the table, ability to walk, recent contrast and renal function, and current glycemic management (diabetic patients typically take half their usual insulin with the NPO order).
• Recovery is not downtime. ST-segment depression that appears only in recovery is a genuine positive result, and arrhythmia and hypotension frequently occur after exercise stops as venous return falls. Keep the patient monitored until heart rate, blood pressure and the ECG return to baseline, encourage fluids and voiding to clear the tracer, and document symptom onset and resolution times.