Exercise Tolerance Tests and the Unmasking of Coronary Artery Disease!
What is an Exercise Tolerance Test (ETT)?
An Exercise Tolerance Test is a simple Cardiology test to assess the behaviour of a persons heart when it is stressed (working harder). To stress the heart the patient is asked to walk/run on a treadmill to increase their heart rate.
To monitor the hearts behaviour a 12 Lead ECG is attached to the patient for the duration of the test. Changes in the ECG allow us to make assumptions about the condition of the heart.
– The most widely used protocol sees the treadmill start very slowly but get faster and steeper every three minutes. This is designed to bring you safely to your maximal heart rate – 220 minus your age! The test is stopped when a patient is exhausted, a significant result occurs, target heart rate is achieved or the test is unsafe to continue.
As described in my previous post on Angina and Coronary Heart Disease
, the blocked arteries associated with Ischaemic Heart Disease
can quite happily cope with the demand of a sedentary lifestyle. The demand of the heart for blood supply is less at rest and sometimes even arteries with severe blockages can cope with the demands of a resting heart.
When a person exercises the heart has to work harder and requires more fuel (blood) to cope with its work load. Much like a car engine that uses more fuel when it accelerates and moves faster.
This is why most cases of Angina
are exertional, because the blockage in the coronary arteries
(the fuel line to the heart) often become problematic when the heart is working harder and requires more blood.
Remembering the Motorway (freeway) analogy…
Picture a motorway with 3 Lanes closed for road works at 4am on a Sunday morning – hardly any traffic has to pass and the blockages is not flow limiting. This is like a resting heart.
Imagine the same motorway with 3 lanes blocked, but at 5pm on a Friday evening (rush hour) the narrowed road cannot cope with the demands of rush hour traffic and is severely flow limiting. This is like an exercising heart.
HOW does an Exercise Tolerance Test Work?
The exercise part of the test does not record or detect any blockages in the hearts blood vessels
, it simply makes the heart work harder, it is the ECG that picks up on any disease. For those of you who don’t know an ECG (EKG) is a tracing of the hearts electrical behaviour over a certain period of time.
You will probably have seen one, it looks like this…
The different parts of the tracing are explained in my previous post on ‘How the Heart Beats
‘ and that post will give you a better understanding of Exercise Tolerance Tests.
|Each ‘complex’ represents a Heart Beat
In each heart beat you have a P Wave, a Q Wave, an R Wave, an S Wave and a T Wave.
When we exercise the ECG recording of our heart changes. Some changes, like an increase in heart rate, are quite normal. Other changes are not ‘normal’ and indicate that one of the arteries supplying the heart may be slightly blocked. One change that is particularly of interest involves the ST Segment!
No guesses for figuring out which part of the heart beat is the ST Segment….
When the blood supply to the heart is obstructed it becomes starved of sufficient oxygen and glycogen when this occurs the ST Segment can change. The images so far show a pretty regular and ‘healthy’ ST Segments. In a heart with restricted blood supply ST Segments can become ‘depressed’ by 2mm or more or down sloping.
|Horizontal ST Depression
|Down sloping ST Depression
Exercise tests normally start by recording a normal ECG but when the heart is ‘stressed’ with exercise the ST Segments change and start to unmask a blockage in the blood vessels! This is a major change we look for.
See if you can make out the significant ST Changes in this real life example below… it is a ‘positive’ exercise test – one that suggests ischaemic heart disease
Pre-Exercise (Normal ST’s)
Note the flat ST Segments
Peak Exercise (ST Depression)
You can see the ST Segments have clearly become ‘depressed’ – ST Depression
Recovery Phase (Down Sloping ST’s)
In recovery this patient demonstrates the other form of ST Segment that we are interested in, down sloping.
ST ELEVATION :/
If a blood vessel becomes completely occluded like in a Heart Attack – you can get ST Elevation… you do not want to see this during an exercise test!
Other signs of Ischaemic Heart Disease
such as Ectopy, T Wave Inversion and Arrhythmias, also tend to show during exercise – with the trigger being the Oxygen and nutrient deficit in the heart that occurs more readily during exertion.
This is how Exercise Tolerance Tests work!
As a side note depending on which part of the ECG has changes (they probably will not occur all over the ECG) can tell us what part of the heart is being affected.
Time for a Cuppa!!
Thanks for reading
Series of ECG’s courtesy of http://cardiophile.org/