A Brief Discussion on Minute Ventilation
As we all know, ventilation is composed of two phases: one that brings air into the lungs, called inspiration or inhalation, and one that lets air into the environment, called expiration or exhalation. Minute Ventilation refers to how much air we either inspire or expire (but not both) in one minute. Most often it refers to the amount expired(VE) rather than inspired (VI). This amount can be determined by knowing (a) the tidal volume (TV) (i.e., how much air we expire in one breath) and (b) the respiratory frequency (f) (i.e., how many breaths we take in one minute). In other words:
VE= TV x f
Ventilation at Rest
Under normal resting conditions, minute ventilation varies considerably from person to person. Usually, we ventilate between 4 and 15 liters per minute (BTPS) at rest. This varies with body size and is smaller in women and larger in men. Tidal volume and respiratory frequency vary even more than minute ventilation. This is easy to understand, because there are many combinations of tidal volume and frequency that yield the same minute ventilation. At rest, typical values for tidal volume and frequency are 400 to 600 milliliters (ml) and 10 to 25 breaths per minute, respectively. The respiratory control mechanism play a major role in determining the combination of frequency and tidal volume.
Ventilation during Exercise
Minute ventilation increases during exercise. In general, the increase in ventilation volume is directly proportional to increases in the amount of oxygen consumed and carbon dioxide produced per minute by working muscles. Only at the extremes of exercise intensity do we see that minute ventilation (VEBTPS) is disproportional to oxygen consumption (VO2). However, this in not the case with carbon dioxide production (VCO2). This indicates that minute ventilation is perhaps regulated more to the need for carbon dioxide removal than to oxygen consumption, at least under maximal exercise. The fact that ventilation increases much more than VO2, also tells us that minute ventilation does not normally limit the capacity (max VO2) of the cardiorespiratory system.
One other point to note is that trained subjects tend to have lower minute ventilation during exercise at given work loads or oxygen consumption (VO2) and at given carbon dioxide production (VCO2). This lower ventilatory response to exercise, although common in most athletes, is most pronounced in endurance athletes. The physiological reason for this is not entirely known; however, it is suggested to be related to diminished peripheral chemoreceptor function and genetic and familial influences. Regardless of its cause, the low ventilatory response to exercise may be linked to outstanding endurance athletic performance.
Maximal ventilation (max VE) due to exercise can reach values as high as 180 and 130 liters per minute (BTPS) in male and female athletes, respectively. This represents about a 25-30 fold increase over resting values. Such large increases are made possible by increases in both depth (tidal volume) and frequency of breathing. In untrained men and women, where VO2, VCO2 and working capacities are lower, max VE is also lower. Along with this lower max VE is a lower ventilatory efficiency, in other words, as previously indicated, untrained men and women have greate VE at a given VO2 than trained men and women.
Ventilation varies not only with work load, but also before, during and after exercise at any given work load.
Changes before Exercise
Immediately before exercise begins, ventilation increases. This increase obviously cannot be due to anything resulting from the exercise. Therefore, it is most likely due to stimulation from the cerebral cortex resulting from anticipation of the ensuing exercise bout.
Change during Exercise
During exercise, there are two major changes in ventilation.
- A very rapid increase within only a few seconds after the start of exercise. This is probably related to nervous stimulation arising from the joint receptors resulting from movement generated by the working muscles.
- The rapid rise in ventilation soon ceases and is replaced by a slower rise, which in submaximal exercise tends to level off (i.e, reach a steady state value). In maximal exercise, this leveling off, or steady state, does not occur; rather, ventilation continues to increase until the exercise is terminated. These changes are thought to be stimulated be chemical stimuli, mainly from the carbon dioxide in the blood produced during exercise.
Changes during Recovery
During recovery from exercise, there are again two major changes;
- As soon as exercise is stopped, there is a sudden decrease in ventilation. This is because motor activity has stopped, and so has the nervous stimulation arising from receptors located in the muscles and joints.
- After the sudden decrease in ventilation, there is a gradual or slower decrease toward resting values. The more severe the work, the longer it takes for ventilation to return to resting values. This change is probably related to the decrease in stimulation resulting from a decrease in stimulation resulting from a decrease in carbon dioxide production.
Daryl Conant, M.Ed