Exercise cardiac testing, or functional stress testing, has been an important assessment tool for many decades now. Most often employed in the medical follow-up of known heart disease or screening for potential heart disease in those with known risk factors, cardiac testing is a common protocol. As life underwriters, we analyze stress tests on a monthly if not more frequent basis.
Nevertheless, familiarity with stress test interpretation has become a luxury in today’s underwriting world because stress tests are no longer ordered routinely outside the large case market. The main source of these test results is medical records, and chances are that a case with non-routine cardiac testing will be referred to a medical director for interpretation anyways. But as they say, the journey is half the fun, and robbing yourself of a chance to exercise your brain power is no fun at all.
To familiarize yourself with stress tests, we’ve prepared a brief primer for you to enhance your understanding of common (and not-so-common) stress test irregularities. An ongoing myth is that ST segment depression on the electrical tracing is the only important parameter to monitor on a stress test. While important, it’s far from the only notable finding.
#1 The Protocol
Bruce protocol is the standard protocol, and consequently what most underwriters are accustomed to seeing. The patient typically works until arriving at complete exhaustion or until cardiac symptoms are induced, with the workload increased every three minutes. Modified Bruce protocol may be undergone with elderly or sedentary patients but with a slower wind-up time. The red flag to watch for though is the Naughton protocol, which is commonly employed in patients with a history of heart attacks.
#2 The Target Heart Rate
Each age bracket will have its own target heart rate (THR) that the patient is aiming to meet or exceed. How well they do relative to that target has a significant effect on the test quality. Achieving 80% of the THR is a good test result, and achieving >90% of the THR is an excellent result. With below 80% THR achievement, the sensitivity of the stress test begins to drop dramatically, causing a high rate of false negatives.
#3 The Effort
A patient’s effort on a stress test can be measured in metabolic equivalent tasks (METs). Certainly, the % THR is an important measurement, but how hard the patient had to work before achieving >90% THR is equally important. Effort is a great measure of one’s fitness level. A healthy, reasonably fit individual should pull at least 8 METs on a standard Bruce protocol. However, someone who can’t even manage 6 METs is unfavorable from a fitness standpoint.
#4 The Termination
One question that should be asked with every stress test is, “Why was it stopped?”. In the best scenarios, the answer should be due to exhaustion or THR achievement (exercise to beyond 100% THR is of little value). However, THR achievement is not always the cause. Less favorable scenarios may see a test terminated due to induced symptoms, such as chest pain or significant arrhythmias that could pose a health risk if the patient were to continue protocol. A test terminated due to shortness of breath (SOB) needs some close examination, too, as the cause of termination could simply be fatigue. If the SOB is correlated with electrical changes or drops in blood pressure though, it’s a red flag.
#5 ST Segment Changes - During Exercise
Here it is! This is the magnet that typically captivates a medical underwriter’s attention because ST segment changes over the course of exercise are often correlated with blockages in the coronary arteries and result in ischemia in the heart tissue. Nevertheless, this correlation is less black-and-white than some believe.
ST segment changes are grouped into horizontal and down-sloping depression, slowly up-sloping depression, or rapidly up-sloping depression. ST depression, also known as down-sloping or horizontal, is more closely linked to ischemia than up-sloping ST depression. The cut-off for significance of down-sloping depression is usually 1.0 mm lasting for 0.08 seconds or longer. Although we can give greater significance to down-sloping ST segments depressed by 2-4 mm, they aren’t necessarily a marker of increased severity of disease – just increased probability.
ST segment changes with upsloping depression are often non-specific, that is, not closely correlated with ischemia. Yet this finding isn’t necessarily a write-off: slowly upsloping depression is more significant than rapidly upsloping depression, which is why we separate these two groups. The cut-off for speed is usually around 1.00 mm per second.
Various other factors can influence whether an ST segment change is more or less likely to reflect ischemia. ST depression is often said to be more specific if it occurs in several leads rather than just one. Furthermore, ST depression occurring in leads V4-V6 is more significant than if occurring in just leads 3 and/or aVF.
#6 ST Segment Changes - During Recovery
Similarly, we should consider how ST segment changes are represented in the recovery period when the patient stops exercising. An ST segment depression that appears in exercise but resolves rapidly (<30 seconds) is less likely to be significant. ST segment changes persisting beyond 60 seconds into recovery are more likely to represent coronary artery disease.
#7 ST Segment Changes - Considering Workload
Finally, consider the patient’s workload (i.e., Bruce stage, METs) at the time that the ST segment first became depressed by 1.0 mm. ST segment depression appearing late into Bruce protocol could be non-specific in nature, but depression appearing early in exercise is another significant red flag.
#8 The Rhythm
Three words we like to see frequently on electrocardiographs are normal, sinus, and rhythm (written as “NSR” on many interpretations). Normal sinus rhythm is the rhythm of a healthy heart, meaning that the electrical impulse from the sinus node is being properly transmitted throughout the myocardium. The accompanying heart rate is usually 60-100 beats per minute (bpm) at rest. During exercise, it sometimes goes slightly awry. These abnormalities vary in terms of severity. Although rare premature ventricular contractions (PVC’s) may be acceptable, increased frequency is a concern, and this finding should be correlated with the overall study.
A more severe finding is any sort of exercise-induced fibrillation. It may not necessarily point to ischemia, but it could be a sign of structural heart disease. Finally, beware of the new finding of bundle branch block with exercise, especially on the left side of the heart. If bundle branch block is present at rest, it can distort the electrical findings over exercise, particularly in cases of left bundle branch block (LBBB), which renders the entire electrical tracing with exercise uninterpretable for ischemia. A Wolff-Parkinson-White (WPW) arrhythmia has a similar effect on the tracing - unless this pattern disappears with exercise. LBBB appearing only during exercise (i.e., not seen at rest) is highly correlated with ischemic blockage.
#9 The Blood Pressure Response
Blood pressure (BP) should change linearly with increasing exercise workload. Watch for two related results : the peak blood pressure reading at max exercise and any blood pressure drops. A max systolic BP reading over 200 mmHg is considered a hypertensive response and could indicate undiagnosed high blood pressure, or at worst, structural heart damage. On the other hand, be mindful of sudden drops in the systolic BP over exercise, as it can be representative of heart failure.
#10 False Positive Tip-Offs
While it’s rarely wise to discount a positive stress test in its entirety without first consulting a medical director, do keep a few things in the back of your mind, should you need to advocate aggressively for a favorable rating.
Pre-test probability (PTP) is a significant factor in gauging the reliability of the treadmill stress test. The PTP is the likelihood that the patient undergoing stress testing actually has coronary artery disease. Typically, the PTP is higher in men than in women, higher with age, and definitely higher if the test itself is prompted by symptoms of chest pain typical of angina. The higher the PTP, the greater the specificity of the test; the lower the PTP, the greater the risk of false positives. Consider this then: why was the test ordered in the first place? Was it due to family history alone, based on symptoms, or based on risk factors (e.g., diabetes)? A stress test performed on a male over age 50 with a history of smoking and complaints of chest pain makes sense: the PTP is probably high. In contrast, a stress test done on a middle-aged female with nothing more than adverse family history is of dubious value.
Finally, watch out for the mention of exaggerated atrial repolarization waves in the electrical tracing since they are a well-known cause of false positive ST segment depression.
Interpretation of stress tests is a skill that demands ongoing practice, additional research, and knowledge of the person’s overall health picture. If you are looking for more information, I recommend consulting the latest edition of Medical Selection of Life Risks (Brackenridge, Elder) as well as the links below for some light reading and education:
- Myocardial Ischemia
- The electrocardiographic differential diagnosis of ST segment depression
- ST segment depression in myocardial ischemia and differential diagnoses
- Clinical Significance of Upsloping ST Segments in Exercise Electrocardiography