Measures and Metrics: Take a Breath

In our Metrics and Measurements: Mania or Madness series, let’s take a breath.  For decades, yogis and meditation gurus have focused on breath for mindfulness and relaxation.  Is there merit to focusing on breath as a way to monitor exercise?  Yup.  There sure is, and while it may be trending in wearable devices, it isn’t new to exercise physiologists. Breathing as a measure of exercise intensity relates to cardiovascular and metabolic load. Breathing rate, as it turns out, is a key factor in your RPE. 

Before diving into breathing as a tool to monitor exercise intensity (topic of the next newsletter), it might be helpful to learn or review some respiratory anatomy and physiology. As we saw with RPE, there is more behind understanding the measure or metric than a number.  

 A BREATH BY ANY OTHER NAME

The physical process of breathing, moving air into the body through inhalation and out through exhalation, is called ventilation. The actual exchange of gas at the cellular level is called respiration.  The terms inspiration and expiration can be used to represent both ventilation and respiration (in and out of the body, in and out of the cell, respectively).

Ventilation or breathing frequency (breaths per minute) is a measurement if taken directly at the mouth, or a metric if derived from a wearable sensor.  Determining respiration at the cellular level requires a metabolic cart and is a metric, calculated from measuring gas concentrations at the mouth as a proxy for the cell.  

Structures of the Respiratory System

The respiratory system structures include the lungs, airways (upper and lower respiratory tracts) and diaphragm muscle. You’ve likely experienced an upper respiratory tract infection.  This refers to the large airways of the nose (nasal cavity), mouth (oral cavity), sinuses, throat (pharynx) and voice box (larynx). The lower respiratory system is made up of the wind pipe (trachea), 2 primary bronchi, and the respiratory tree (secondary and tertiary bronchioles) terminating in the air sacs (alveoli).  The phrase “it went down the wrong pipe” refers to food or drink making a wrong turn into the trachea rather than heading south to the stomach through the esophagus.

Fascinating facts about the structures:

  • The lung is the only organ to receive 100% of cardiac output and therefore is serviced by a very low resistance vascular system 

  • Throughout the entire respiratory tree, O2 and CO2 gas exchange only occurs between the blood-gas interface in the alveoli

  • Transit time for gas exchange between the alveoli and capillary blood is less than 1 second.  Even shorter with exercise (0.6 sec). Luckily, the structure is designed for function.

    • The blood-gas interface is very thin (~ 0.2-0.3 mm)

    • There are roughly 500-700 million alveoli and their surface area occupies half a tennis court (70-100 m2)

    • Capillaries that surround the alveoli can increase their capacity threefold during exercise, which protects the transit time for gas exchange

  • The diaphragm is a dome shaped muscle that contracts to create a negative pressure in the thorax, creating a vacuum to suck air in.  Ancillary inspiratory muscles, such as the intercostals, assist the diaphragm.

    • Negative pressure was the principle behind the Iron Lung, developed to treat respiratory muscle paralysis during the poliomyelitis epidemic in the 1920s and occasionally used in patients suffering from tuberculosis. 

    • Ventilators and CPAP machines used to treat sleep apnea use positive pressure rather than negative pressure (forcing air in versus sucking air in), which are more convenient than the Iron Lung but do not mimic natural breathing mechanics.

The Emerson Iron Lung introduced portholes to give medical staff access to the patient without having to open the cabinet, 1960. CDC Public Health Image Library

  • Exhalation is largely a passive process, relying on the elastic nature of the alveoli and springiness of the rib cage to expel air.  This is brilliant, allowing less energy required for exhalation.

  • The lung doesn’t adapt to training!  Athletes may say, “my legs were fine, but I need to get my lungs into shape”.  Is that accurate?  There is plenty of oxygen in the lungs waiting to be picked up. That feeling of breathlessness is due to lack of oxygen being delivered to their working muscles.  Training adaptations to the cardiovascular system allow oxygen to be picked up and delivered to working skeletal muscles, which also adapt to better utilize oxygen.  

Functions of the Respiratory System

The primary function of the respiratory system is gas exchange, bringing oxygen in to the body and removing carbon dioxide.  During the process of ventilation, several important secondary functions occur: 1) filtration and humidification of air and 2) sense of smell upon inhalation, 3) voice production through exhalation, 4) maintenance of acid-base balance, and to a smaller extent, 5) thermoregulation.

 Fascinating facts about the functions:

  • The nose and rest of the respiratory hose is your first line of defense against bad stuff.  Cilia (small hairs) and mucous work together to trap and move debris. The body produces up to 1L of mucous per day to get the job done. Go goobers!

  • The nose filters out particles larger than 0.5 µm, trapping it in mucous, which is then moved to the throat and into the gut by the cilia. During Covid, masks were implemented as they can filter out much smaller particles (N95 0.3 µm).

  • Heat and humidity.  We might not appreciate the need for heat and humidity in July, but winter is coming.  From the nose to the throat, air is warmed (average room temp 20°C) to 35°C (95°F) and humidified.  By the time it reaches the delicate alveoli, is has reached body temperature 37°C and 100% relative humidity, preventing damage and inflammation to those delicate tissues.

  • Breathing requires little energy cost, ~5% of resting oxygen consumption at rest.

  • The lungs work in concert with the kidneys to maintain acid-base balance of pH 7.4.  A rise or drop of 0.1 can have devastating effects on function.

  • Many animals pant to maintain body temperature.  Humans can’t pant, so fortunately we have sweat glands for more robust thermoregulation.

Stimulus to Breath

There is more to breathing than you might think. If you have a healthy respiratory system, you likely DON’T think about breathing. If you have asthma (or many other diseases of the respiratory tree) you most likely think about your next breath. It is both an autonomic and conscience function.

Similar to our beating heart or digestive tract, breathing is largely an automatic function.  There are inspiratory and expiratory centers located in the brain which receive chemical and mechanical sensory information, and use that to drive the diaphragm and other ancillary inspiratory muscles.

Chemoreceptors detect and ensure chemical balance (pH) and homeostasis.  Surprisingly, perhaps, low oxygen is not the primary driver to take a breath. Recall that carbon dioxide is an acid: too much CO2 is our major driver to breath.  

Have you ever tried to hold your breath (hypoventilate)?  What happens?  You take a breath before anything really bad happens! CO2 builds up, chemoreceptors tell the brain to breath and you do, despite your best effort to hold out for a few more seconds. You can’t override the body’s built in safe guard to keep you breathing. 

You CAN override the system to breath more than necessary (hyperventilate). You may have experienced this during a period of high anxiety, before the start of a race or other stressful episode. Again, your body has a safeguard against expelling too much carbon dioxide just as it does retaining it.  Low CO2 levels causes your blood vessels to constrict, decreasing blood flow to the brain until, leading to dizziness and ultimately fainting.  One way or another, your body will succeed in slowing down breathing to restore acid-base homeostasis!  Pretty cool.

There are also mechanoreceptors in skeletal muscles that drive the inspiratory center, and stretch receptors in the lungs that drive the expiratory center.  Makes sense, right?  Each pedal stroke triggers a mechanical sensor that feeds that information forward to your inspiratory center.  Hey, my quads are working hard, send oxygen!  Your lungs fill, and trigger a breath out (remember, the lung gets stiffer as it gets fuller, so is makes sense energetically not to fully inflate).   Brilliant.

Breathing Response to Exercise

Minute ventilation (VE) is the amount of air breathed in liters per minute.  Minute ventilation is the product of tidal volume (volume per breath) and breathing rate (breaths per minute). This is very similar to cardiac output, which is the product of stroke volume and frequency.   

Average values for a 70 kg human

As the demand for CO2 removal and O2 increase with exercise, minute ventilation increases by increasing both tidal volume and frequency.  The body is very smart about how to titrate the increase in these two variables.  Note in the graph below how volume increases first, followed by rate.  Like most everything in the body, there is a reason for this sequence.

Tidal volume: volume of air per breath. Tidal volume at rest is approximately 500 ml (based on a 70kg human).  With exercise, this may increase up to 3 L/breath. Increasing minute ventilation by increasing tidal volume is a smart business move. There is a “tax” to pay with each breath called dead space.  Regardless of the volume of the breath, there is an amount that cannot be exchanged.  This is similar to a fee to make a withdraw from an ATM machine.  If you take out $10 or $100, you pay a fee of $2 which is “lost”.  Most people take out a larger sum to make the fee relatively smaller.  

The lung is seemingly overbuilt, as max exercise yields ~80% of total lung capacity. You might wonder, why not take the biggest breath possible? 

Mechanically, the stiffness of the lung becomes greater at higher volumes, making the metabolic cost of the work of breathing higher. There is also the ventilatory to perfusion ratio (V:Q)  to consider.  This basically says that each part of the lung is not equally perfused by blood vessels. The same volume of air in one portion of the lung may not be perfused as well as another, such that more air to that region will yield less gas exchange.  Complicated, right?

Breathing frequency: breathes per minute. During exercise, frequency can more than double from ~12-20 breaths/minute to 60 bpm.  Why not keep pushing the frequency? The metabolic cost of breathing can become significant.  Similar to pedaling cadence, a faster breathing rate becomes metabolically inefficient.  In revolutionary work by University of Wisconsin’s world-renowned exercise physiologist, Jerome Dempsey, the concept that the lung is not a limiting factor in exercise was challenged (Classical Perspectives). While undeniably true that its capacity is not reached during maximal exercise, the work of breathing at some point becomes significant enough to “steal” blood from working muscles (those pushing the pedals).

Digest those bits of respiratory anatomy and physiology.  Understanding a basic breath is no simple task. Next month, we will explore using breathing as a tool to monitor exercise intensity and discuss some of the research around training breath to enhance performance.

 

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Measures and Metrics: Rated Perceived Exertion