Indirect calorimetry is the only accurate and clinically feasible method of measuring energy expenditure. It is called “indirect” because the caloric burn rate is calculated from a measurement of oxygen uptake. Direct calorimetry implies a measurement of heat released by the body, which is technically difficult and clinically impractical.

Indirect calorimetry relies on the fact that burning 1 calorie (Kilocalorie) requires 208.06 milliliters of oxygen. Because of this very direct relationship between caloric burn and oxygen consumed, measurements of oxygen uptake (VO2) and caloric burn rate are virtually interchangeable. Oxygen uptake requires a precise measurement of the volume of expired air and of the concentration of oxygen in the expired air. The following is a physiological review of metabolic pathways and measurements:

INTRODUCTION TO METOBLIC TESTING PRINCIPLES

Introduction

Metabolic testing uses the method of “indirect calorimetry” to determine the quantity and type of fuel being used during a given state of exercise or rest.  In order to understand the principles behind indirect calorimetry we will review the 2 types of metabolism and the 4 different fuel sources during exercise and what fuel sources are preferred during different states of exercise.  We will also examine what VO₂ is, and what it tells us about an individual’s cardio-respiratory fitness level.  Finally we will review the concepts behind the “respiratory quotient” and explain what information we can derive from its value.  Mobile Metabolics uses Korr CardioCoach CO2 metabolic equipment.  Your examples will come from data generated from the “New Leaf” programs. 

The Fick Principle

The Fick Principle correlates the volume of O₂ consumed (VO₂) with our ability to deliver O₂ (Cardiac Output (CO)) and our ability to extract oxygen (arterial –venous O₂ difference (a-v)O_2diff)).  In other words

VO₂ = CO (a-v)O_2diff

The Fick principle states that our ability to consume oxygen is a function of our hearts ability to pump blood (CO) and our muscles ability to extract the oxygen from the blood ((a-v)O_2diff).  We can now take a closer look at what each component of this equation means.

Cardiac Output (CO) is a function of how many times our heart beats in a minute (HR), and how much blood is expelled from the heart each time it beats (Stroke Volume SV).  We can summarize this by the following equation:

CO = HR x SV

We can see that CO will increase if either HR or SV increase.  HR and SV can increase as a result of nervous input or hormonal influence both of which change during exercise.  SV can change depending on the End Diastolic Volume (EDV), End Systolic Volume (ESV), or Contractility.  The EDV is the amount of blood that has filled the right atrium and ventricle before contraction; the higher the EDV the greater the SV.  ESV is the amount of blood remaining in the left ventricle after systole; the higher the ESV the lower the SV.  Contractility is the force with which the heart contracts; the greater the contractility to higher the SV.  Proper training will help to increase EDV and contractility, while decreasing ESV thereby giving an increased CO.  Creating an increased CO means we are multiply the right side of the Fick Equation by a higher number and therefore increase VO₂.  CO will vary with varying age, gender, disease state, and current fitness level.  There is a point where SV can no longer increase and further increases in CO come from HR alone. 

One can also increase their VO₂ by training their muscles to be more efficient at extracting O₂.  The part of Fick Equations that reads (a-v)O_2diff refers to the difference between O₂ concentration from the arterial blood and the venous blood.  As blood flows past active muscle, oxygen is extracted to facilitate aerobic metabolism.  Naturally, as oxygen is extracted from the blood the concentration of oxygen in the blood decrease as blood returns to the heart.  The difference between oxygen concentration in the blood leaving the heart and blood returning to heart is called the extraction of oxygen and referred to as the (a-v)O_2diff in the Fick Equation.  The less oxygen there is in the venous blood, the more the oxygen was extracted by the working muscle tissue.  A lower venous oxygen concentration means we are subtracting a smaller number from the arterial oxygen concentration and therefore multiplying the right side of the Fick Equation by a larger number.  Multiplying by a larger number means an individual’s VO₂ will increase. 

To summarize we can see that there are two different ways to increase an individual’s ability to consume oxygen.  One is by increasing the hearts ability to deliver oxygen to the tissues, and the other is by increasing the tissues ability to extract oxygen from the blood.  This information will be important for you to know as you explain to your clients why VO₂ measurements are important and what correlations they have to their cardio-respiratory health.

UNDERSTANDING METABOLIC PATHWAYS

Introduction:

There are several different metabolic pathways one uses to perform exercise.  The metabolic pathway our body uses is determined by the intensity of the exercise we perform.  As we will see, our body uses each metabolic pathway for a specific purpose and we will discuss why each is optimal for certain exercise. 

Our body converts Carbohydrates (CHO) and Fat to energy in the form of Adenosine Tri-Phosphate (ATP).  ATP is our body’s energy currency.  Below we will discuss the various ways in which we metabolize CHO’s and Fat to produce ATP.

Aerobic Metabolism

When our body utilizes oxygen to burn energy we are using “Aerobic Metabolism”.  During aerobic metabolism our body can burn CHO’s and Fat.  The mechanisms for aerobic metabolism are contained within a tiny organelle in our cells called the mitochondria.  Although the energy released from aerobic metabolism is considerably greater than that or aerobic metabolism, it is a considerably slower process.  Our “Aerobic Base” is considered the HR interval where our intensity is such that we burn a maximal quantity of fat in kcal/min.

Anaerobic Metabolism

Anaerobic metabolism has three different sources which include ATP pools, Creatine Phosphate, and Anaerobic Glycolysis.  We will discuss each of these more completely below.

ATP is actively stored in the muscle to be used if needed to begin a bout of activity.  While energy we get from these immediately available ATP pools is great, they only last a matter of seconds (2-3sec) during maximal exertion.  This energy source is short lasted and must be replenished.

Creatine Phosphate (CP) is also present in the muscles and is used to recycle the byproducts of ATP hydrolysis (use) very quickly.  The quantity of CP in the muscles is limited and therefore the energy supplied by CP only lasts 8-12 seconds during maximal exertion. 

Anaerobic Glycolysis uses sugar (glucose) to produce energy without oxygen.  Glucose is metabolized to produce ATP and the byproduct lactic acid.  Individuals have different capacities for maintaining energy demands by anaerobic glycolysis but in general, this energy source can last approximately 1-2min during maximal exertion.  Since lactic acid is produced during this type of metabolism and the rate limiting enzyme of this process is pH dependant, exercise duration will be limited. 

During exercise we never use only one source of energy, but rather a combination of energy sources to meet our energy demands.  Since energy is produced at an extremely fast rate by anaerobic metabolism it is used for exercises of high intensity.  Exercises that demand energy as a slower rate allow us to use oxygen for the metabolism of Fat.  Glycolysis produces only 2 ATP per unit of glucose but the burn rate is extremely high while aerobic metabolism uses fat and produces 132 ATP per unit of fat but the burn rate is extremely low.  Keeping this in mind, it makes sense for our bodies to use glycolysis when our energy demands are high and aerobic metabolism when our energy demands are low. 

Excess Post Exercise Oxygen Consumption (EPOC)

EPOC is a period after steady state exercise in which HR and respirations are out of proportion to our metabolism.  During the initial stages of exercise our body uses immediately available ATP and CP to meet energy demands.  These sources of energy need to be replenished for the next time we begin an exercise bout.  This increase in HR and respiration accommodates increased metabolic activity to replace these depleted stores of energy.  We will discuss this phenomenon in greater detail later.

CALCULATING VO₂ & THE METHOD OF INDIRECT CALORIMETRY

Introduction:

In this section we will discuss the method of indirect calorimetry and how we measure VO₂.  When our bodies do work they produce heat as well as work.  The heat we produce is directly proportional to the amount of energy we are using.  Since it is impractical to measure heat production from an individual exercising directly, we use indirect calorimetry instead of calorimetry (measuring heat) to measure work.

Calculating VO₂:

VO₂ is most commonly calculated in standard conditions of temperature, barometric pressure, and humidity (dry) (STPD).  VO₂ means the volume of oxygen consumed; in order to calculate the volume of oxygen we need to separate out oxygen as a fraction of air.  Oxygen is always 20.93% of standard air.  In order to determine the amount of oxygen we consume, the difference between inspired volumes of oxygen and expired volumes of oxygen must be determined.  With this information we generate the following equation for VO₂:

VO₂ = Volume_inspired(Fraction_inspiredO₂) – Volume_expired(Fraction_expriedO₂)

VO₂ = V_i(F_iO₂) – V_e(F_eO₂)

This equation will give us the amount of oxygen extracted from tissues before it expelled.  If you remember the Fick Equation, you will see that this is a similar figure to the (a-v)O_2diff.  In this equation we are assuming that the amount of oxygen extracted at the lungs is the same as the amount of the oxygen extracted at the muscle tissue.  In most cases this assumption can be made without error.  Since it impractical to measure oxygen concentration in the blood outside a laboratory setting, our method of indirect calorimetry will suffice. 

In order to measure the VCO₂, or the volume of VCO₂ produced during exercise we will use the same principles.  Since carbon dioxide is not used in metabolism there should be no difference from the volume of inspired carbon dioxide to the volume of expired carbon dioxide (fraction of CO₂ in air is 0.0314%).  Therefore we can use the following equation to measure VCO₂:

VCO2 = (Ve)(FeCO2)

Although the fraction of inspired carbon dioxide will not change the volume expired can.  If a person begins to metabolize anaerobically they will begin to expel excess carbon dioxide in order to buffer acid produced from anaerobic glycolysis.  We will discuss this stage of exercise in greater detail later.

Absolute vs. Relative Intensity

Absolute and relative intensity are two different ways to measure a persons effort level during exercise.  Absolute intensity is a result of persons work without relation to their height, weight, age, or gender.  For instance, Sarah could have a VO₂ of 4.0 L/min and Josh could have a VO₂ of 4.8 L/min.  Josh has a higher VO₂ than Sarah.  Does this mean that Josh is in better condition than Sarah?  This number shows us that Josh is able to consumer 0.8 L more oxygen than Sarah.  This could be as a result of having increased lean body mass, a higher fitness level, larger lungs, height difference, and because he is male.  This only tells us that one person has a greater capacity to consume oxygen than the other. 

In order for this term to be relevant and comparable from individual to individual we need to make it relative to their body.  Let us take the following individuals:

Relative VO₂ is expressed in terms of ml of oxygen consumed per kilogram of body weight each minute or ml / (kg)(min).  In order to find relative intensity we will have to convert L to ml and divide by kilograms of body weight.  Examples follow: