All exercise stress test protocols progressively increase
myocardial work and aim to assess the degree of ischaemia,
dysrhythmia and ventricular dysfunction. The outcomes are
assessed clinically and electrocardiographically.
A standard way to compare exercise protocols is to measure
metabolic equivalents (METs). One MET is equal to an oxygen
consumption of 3.5 ml O2 kg-1 min-1 and represents resting
oxygen uptake. The usual method is the Bruce protocol, which
consists of five graduated three minute stages on a treadmill that
differ in both grade and speed. In stage 1, the treadmill speed is
1.7 miles/hour on a 10% gradient (5 METs), whilst in stage 5 the
speed is 5 miles/hour on an 18% gradient (16 METs). The test is
continued until a pre-defined end-point is reached. A standard
end-point is when a percentage of predicted maximum heart rate
is attained (i.e. 90% of age predicted maximal heart rate) or when
there are electrical or clinical signs of myocardial ischaemia.
Ischaemia is assessed by a 12-lead ECG before, during and
after the exercise protocol. The classical change associated with a
positive exercise test is a 1 mm (0.1 mV) depression that persists
for 80 msec beyond the J point in any lead.
The amount of ST segment depression (>2 mm), the time of
appearance (before 6 METs), the duration of depression (persisting
for >5 minutes into recovery), and the number of ECG leads
involved are all predictive of multi-vessel coronary artery disease
and may indicate an increased risk of perioperative ischaemic
events. The appearance of dysrhythmias and the inability to
exercise to >5 METs are poor preoperative prognostic factors.