72. Principles of exercise testing (spiro-ergometry). Anaerobic threshold and maximal oxygen

Spiroergometry is a procedure in which a subject is subjected to progressively more difficult physical activity while their ventilation and cardiac function is monitored. The test subject usually walks on a treadmill which progressively increases the speed and incline to increase the difficulty. The functional capacity of the cardiac and respiratory systems can be measured.

The purpose of this procedure is to diagnose or assess the severity of certain heart or pulmonary conditions, especially heart failure and angina pectoris. This procedure can help determine exactly how much exercise a cardiac patient can tolerate before symptoms occur. Other indications include:

COPD
Restrictive pulmonary disease
Diagnosis of coronary artery disease

It can also be used to assess the fitness of healthy persons, which is often done by athletes.

Contraindications

Recent (< 5 day old) myocardial infarct
Recent unstable angina
Acute left ventricular failure
Complex arrhythmias
Pulmonary oedema
Severe hypertension
Recent asthma attack

Equipment

Several pieces of equipment are necessary:

Treadmill or exercise bicycle
12 lead ECG
A mask that covers the mouth and nose
A pulse belt
A cardiologist

The mask can measure the amount of O2 and CO2 passing through. A cardiologist is present in case sudden cardiac death occurs, in hopes of resuscitating the subject.

Procedure

A physical examination and ECG is performed beforehand, to determine whether the subject is suitable for the procedure.

The procedure begins with a warmup. After this the intensity is gradually increased by increasing the speed and incline. This increase is performed as described by a protocol called the “Bruce protocol”. Because the load is precisely defined according to the protocol it’s simple to compare the test subject’s performance to the performance of an equivalent healthy person.

During the procedure, several parameters are measured or estimated:

Blood pressure
ECG
Heart rate
Total ventilation
O2 consumption (VO2)
CO2 production (VCO2)
Arterial O2 (estimated from other parameters, not directly measured)
Arterial CO2 (estimated, not measured)
Lactate (estimated, not measured)
pH (estimated, not measured)

The procedure is continued until there is some indication that it should be stopped, like:

Reaching the maximal heart rate
Severe dyspnoea
Exhaustion
Angina
ECG abnormalities
Arrhythmias

Complications

Sudden cardiac death can occur. In young subjects this is often the result of hypertrophic cardiomyopathy or long QT-syndrome, while in elderly it is usually the result of acute coronary syndromes and fatal arrhythmias.

Results

By measuring the O2 consumption and CO2 production, we can calculate the respiratory quotient (RQ). The RQ is the ratio of CO2 produced to O2 consumed. It normally ranges from 0.7 to 1.0, but it can reach numbers above 1.0. The RQ increases as the intensity of the exercise increases.

The oxygen consumption VO2 is an important parameter, especially it’s max value. The VO2 increases as the intensity of the exercise increases, and it a good estimator of how much energy the body is currently producing by metabolism.

The VO2 max is reached when the oxygen consumption remains steady despite an increase in workload. This is the highest rate of oxygen consumption the body can have; at this point the body has reached the limit to how much oxygen it can consume. It therefore also shows the point where the body cannot increase its metabolism to produce more energy for exercising; the body has reached its limit.

We also collect information on which intensity of exercise is required for angina or dyspnoea to develop, whether the heart rate increases abnormally quickly and whether the blood pressure rises abnormally.

Interpretation

The respiratory quotient gives us information as to which energy source the body is currently metabolizing. When the RQ is 0.7, lipids are the major energy source. When it’s 0.8, proteins are the major energy source. When it’s 1.0, carbohydrates are the major energy source. This is because when lipids are metabolized less CO2 is produced than when carbohydrates are metabolized.

The respiratory quotient can be higher than 1, meaning that more molecules of CO2 are produced than molecules of O2 is consumed. This may seem impossible at first glance. The explanation lies in the bicarbonate buffer system in the blood. During anaerobic exercise lactic acid will be produced, which releases protons. These protons will be buffered by bicarbonate in the serum: H+ + HCO3– -> CO2 + H2O. This CO2 will then be exhaled, which is where the surplus CO2 comes from.

In other words, when the respiratory quotient reaches numbers higher than 1.0 the test subject has switched from doing mostly aerobic metabolism to doing mostly anaerobic metabolism. This becomes important later.

The VO2 max is the single most important parameter in determining fitness. A person with higher VO2 max than another person definitely has better fitness. VO2 is measured in L/min, indicating how many litres of oxygen the subject consumes per minute. It can also be measured in mL O2/(min · kg body weight), which normalizes the value across different body weights.

The VO2 max can be converted into another unit called a metabolic equivalent (MET). 1 MET is defined as 3.5 mL O2/(min · kg body weight). 1 MET is also equal to the oxygen consumed when sitting quietly.

The following table shows some examples of values for VO2 max:

Subject	VO2 max in L/min	VO2 max in mL/(min · kg body weight)	VO2 max in METs
World record holder	7.6	97.5	28
Average young man (20s)	3.5	45	13
Average young woman (20s)	2.0	38	10
Average older man (60s)	2.3	30	8
Average older woman (60s)	1.2	23	6

The world record holder is Oskar Svendsen, a Norwegian cyclist.

The VO2 max of a person depends on the gender, age and fitness of that person.

The anaerobic threshold is defined as the intensity of exercise required for the metabolism to switch from aerobic to anaerobic. We can measure it by looking at what the VO2 or MET of the test subject is the moment their RQ goes above 1.0. This usually occurs at around 50% of the VO2 max.

The higher your fitness (the more fit you are) the longer you can fuel your body by aerobic metabolism before the anaerobic system needs to take over, and therefore the anaerobic threshold will be higher.

The anaerobic metabolism uses substrates like glucose and glycogen, which are not abundantly stored. Depletion of carbohydrate stores will lead to exhaustion and occurs within 20 – 30 minutes of reaching the anaerobic threshold.

During anaerobic metabolism lactate will accumulate. The RQ will continuously increase as lactate is buffered. As lactate accumulates more CO2 is produced, so ventilation will be stimulated to get rid of the extra CO2.

The ventilatory threshold is reached when the RQ reaches around 1.15. At this point the limit of ventilation is reached, so the body can no longer get rid of the extra CO2. Eventually the CO2 load will be so large that lactic acidosis occurs. This severely limits and eventually stops the exercise as the subject will experience muscle weakness and fatigue.

After the procedure

Even after the exercise is stopped, VO2 and metabolism will remain high for several minutes. This is due to the oxygen debt, also called excess post-exercise oxygen consumption or afterburn. Even after the exercise is stopped there is a need for oxygen, for several reasons:

ATP and creatine-phosphate in the muscle should be replaced
The produced lactate should be transformed back to glucose
The core temperature is still elevated, which increases metabolism
Cardiac output should be shunted away from the muscles to visceral organs
O2 stored in myoglobin should be replenished
Electrolytes must be redistributed
The circulating catecholamines increase metabolism
Ventilation is still increased, so ventilatory muscles require energy

These processes require energy and therefore O2, which explains why O2 consumption (VO2) is elevated even after exercise.