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

Page created on May 21, 2019. Last updated on December 18, 2024 at 16:57

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.

Purpose: 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.