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Breathing and the Menstrual cycle

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April 2021

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The average breathing rate has a wide normal range of 10 – 20 breaths per minute. Decades of research indicates that better physical and mental health is associated with the lower end of the range. At rest, breathing slower is better. Sex differences are apparent in many aspects of biology.  Female hormones, broadly estrogen and progesterone, have predictable variability within the menstrual cycle.

 

Research shows that breathing rates increase in the second half of the normal menstrual cycle (days 15 – 21). As the breathing rate increases, more carbon dioxide is exhaled, leading to a reduced concentration of carbon in the blood. Low levels of carbon dioxide lead to suboptimal oxygenation of tissues with symptoms of irritability, fatigue.

 

Increased breathing rates stimulates the part of the nervous system called ‘fight, flight and freeze’ and decrease the influence of the ‘rest and digest’ part of the autonomic nervous system. This has a negative impact on heart rate variability (HRV) and leads to a heightened state of vigilance with increasing levels of stress and anxiety experienced. Ovarian hormones, specifically progesterone, could partly drive this increased breath rate.

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A brief overview of breathing

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Regulation of breathing is a complex interplay of neuronal, biochemical and hormonal influences. Breathing allows for the exchange of oxygen (O2) and carbon dioxide (CO2) in the lungs: oxygen is transferred from the inhaled air to the blood and carbon dioxide is delivered from the blood and exhaled. The normal level of oxygen is measured as a saturation of 94 – 99% and normal carbon dioxide is measured as a partial pressure of 35 - 45mmHg. It is not the level of oxygen that drives the urge to breath. The concentration of carbon dioxide regulates breathing rates, increasing when the level is too high and vice versa.  

 

Carbon dioxide is a by-product of many processes in the cells. It is commonly regarded as a waste product, but it also has many important regulating functions for example blood pressure. Our bodies are designed to operate best with blood pH at a slight acidic range of 7.35 – 7.45, where a lower number is more acidic. A higher level of carbon dioxide increases the acidity of the blood. The red blood cells are sensitive to the acidity in the blood and release oxygen more easily when the acidity is higher.

 

During exercise, more carbon dioxide is produced by working muscles as it converts the glucose and fat into energy. Special sensors called chemoreceptors, recognise this rise in blood carbon dioxide and pH. The brain sends signals to increase the breathing rate in order to exhale this excess. This ensures that the acidity of the blood is kept in a narrow band where our bodies operate best.

 

A common misinterpretation is that all carbon dioxide needs to be exhaled. As with most things, there is the Goldilocks middle ground of just enough – too little carbon dioxide and too much carbon dioxide have negative effects on the body. Constant ‘over-breathing’ leads to adaptations by the chemoreceptors and they get triggered at a lower level of carbon dioxide. The body gets used to a ‘lower normal’ of carbon dioxide and will increase the breathing rate when that threshold is reached.

 

What is relevant here is that a higher breathing rate (more breaths per minute) leads to more carbon dioxide being exhaled, lowering the concentration of carbon dioxide in the blood. This is a good thing during increased metabolic demand (exercise) to maintain the normal pH of 7.35 but is less optimal when metabolic demand did not change (during rest).

 

 

 

Overview of the female hormone cycle

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Estrogen and progesterone have rhythmic fluctuations over the course of the menstrual cycle, typically 28 days (figure 1). The four distinct time periods are menstruation (days 1-4), follicular phase (days 1-13), ovulation (days 13-15) and luteal phase (days 15-28). Each phase has a characteristic hormone profile.

 

Both estrogen and progesterone are at their lowest level during menstruation. Estrogen rises quite steeply during the follicular phase to a high peak just before ovulation. Estrogen falls after day 13 and rises at a slower rate to have a second, but lower, peak in the middle of the luteal phase, day 21.

 

Progesterone is typically at a low level during the first half of the menstrual cycle. It rises during ovulation, peaking in the middle of the luteal phase. It then falls slowly to levels at the beginning of the cycle.

 

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Figure 1. Graphic representation of the menstrual cycle showing the menstruation (days 1-4), follicular phase (days 1-13), ovulation (days 13-15) and luteal phase (days 15-28). Estrogen peaks to its highest level prior to ovulation, falls sharply and then rises to a lower second peak in the middle of the luteal phase. Progesterone is low during the follicular phase, rising during ovulation to peak in the middle of the luteal phase before falling to original levels.

Hormone influence on breathing

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The effect of hormones on breathing rates were investigated as far back as 1981 by Schoene et al. and more recently followed up by others including Behan et al., Jensen et al. and Slatkovska et al.

 

In the test method used by Jensen et al., participants are asked to hyperventilate to decrease their blood carbon dioxide levels. Then they breath at a normal pace into a bag of air. With every breath the amount of oxygen available will decrease and the concentration of carbon dioxide in the blood will rise. As explained above, the brain is constantly evaluating the level of acidity (pH) and carbon dioxide and will increase the breathing rate to exhale the excess carbon dioxide and restore the pH level to normal. This point is called the ventilatory recruitment threshold (VRT) – in other words, the level that the body perceives as the ‘normal’ level of carbon dioxide that it wants to maintain. In this study researchers found no difference between men and women with respect to sensitivity to carbon dioxide, the chemoreflex with respect to carbon dioxide threshold was the same in both sexes. Female hormones to not affect the sensitivity to carbon dioxide, the concentration of carbon dioxide in the blood that is tolerable before breathing rates increase to reduce it.

 

In a small study of 14 women, a similar protocol was followed by Slatkovska et al., but this time they also measured estrogen and progesterone concentrations in addition to carbon dioxide. They confirmed the findings by previous studies that the chemoreflex sensitivity to carbon dioxide does not change.

 

They did find distinct differences between the follicular and luteal phase of the menstrual cycle. In the luteal phase, carbon dioxide concentration was lower and breaths per minute were increased compared to the follicular phase. The differences coincided with progesterone level fluctuations. The breathing response to the lower levels of carbon dioxide was greater in the luteal phase by approximately 1L/min in the luteal phase – leading to a reduction in carbon dioxide.

 

  • Rising progesterone in the second half of the menstrual cycle leads to increased breathing rates that decrease the concentration of carbon dioxide in the blood.

  • At the same low level of carbon dioxide, the breaths per minute increases more in the luteal phase.

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Changes in heart rate variability (HRV)

 

Heart rate variability can be used to measure ability to recover from stress and is frequently used to evaluate readiness for intense exercise. Heart rate variability (HRV) is an indicator of autonomic cardiovascular control [8] and divided into the sympathetic (fight, flight or freeze) and parasympathetic (rest and digest) nervous system. The difference between heart beats is not uniform, for example a heart rate of 60 beats per min (bpm) some beats are 1.02 seconds apart while others are 0.98 seconds apart and so on. HRV is used to indicate the level of stress experienced or how well stress is tolerated. When we are stressed the rest and digest part of the nervous system is suppressed creating a sympathetic dominance resulting in a lower HRV. In other words, the difference between heart beats become more uniform. Using the above example, all heart beats are either 1.01seconds or 0.99 seconds apart – a uniform difference of 0.02 seconds. In contrast, in a more relaxed state, the parasympathetic tone of the nervous system is dominant and the HRV increases. The heart beats can be 1.02, 0.99, 1.03 or 0.94 seconds apart in a random fashion.

 

Tenan et al. found that HRV is highest just before ovulation and then falls (day 16-22) until the new menstrual cycle begins. Rising progesterone during days 16-21 suppress the parasympathetic nervous system increasing the overall sympathetic tone – a higher level of stress is experienced. This results in a lower HRV and a higher heart rate as well as an increased breathing rate. At mid-luteal phase, HRV is lowest, heart rate and breath per minute highest. In contrast, the lowest heart rate and lowest breaths per minute is found during menstruation (days 1 to 4).

 

In the luteal phase, progesterone is the main driver of an increased breath rate. Breathing rates impact HRV substantially, as the breaths per minute increase, the HRV will decrease. Lower breaths per minute (6-10bpm) can increase HRV, potentially overriding the hormonal influence [1].

Conclusion

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Increased breathing rates are normal when exercising. This ensures that more oxygen is delivered to working muscles and that the excess carbon dioxide produced is exhaled and blood acidity is kept at optimum levels.

 

When breathing rates increase at rest – where the demand for oxygen does not change – it leads to more carbon dioxide being exhaled than is required to keep pH within optimal ranges. This ‘hyperventilation’ lowers the level of carbon dioxide and decreases the oxygenation of tissues and organs, including the brain. Symptoms of low carbon dioxide include mental fatigue, headaches, excessive sighing or yawning and increased susceptibility to anxiety.

 

  • Rising progesterone in the second half of the menstrual cycle increases breathing rates, lowers carbon dioxide concentration and suppresses HRV.

 

  • From a performance perspective, the lower HRV recorded during the second half of the menstrual cycle should be interpreted with caution especially when the focus is to prevent overtraining. This research points to the benefit of establishing a personal baseline for each phase of the menstrual cycle. Only then can fluctuations of HRV be accurately interpreted.

 

Slow breathing has many health benefits including improved ventilation efficiency and oxygenation, lower blood pressure, increased exercise performance, shift towards parasympathetic dominance and increased HRV, and reduced susceptibility to anxiety.

 

Notice when your breath rate increase – at any point during your cycle – and make a conscious effort to slow down.

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Reference:

  1. Behan, M., Zabka, A. G., Thomas, C. F., & Mitchell, G. S. (2003, Jul 16). Sex steroid hormones and the neural control of breathing. Respir Physiol Neurobiol, 136(2-3), 249-263. https://doi.org/10.1016/s1569-9048(03)00086-7

  2. Guyenet, P. G. (2014, Oct). Regulation of breathing and autonomic outflows by chemoreceptors. Compr Physiol, 4(4), 1511-1562. https://doi.org/10.1002/cphy.c140004

  3. Jensen, D., Wolfe, L. A., O'Donnell, D. E., & Davies, G. A. (2005, Mar). Chemoreflex control of breathing during wakefulness in healthy men and women. Journal of applied physiology (Bethesda, Md. : 1985), 98(3), 822-828. https://doi.org/10.1152/japplphysiol.01208.2003

  4. Russo, M. A., Santarelli, D. M., & O’Rourke, D. (2017). The physiological effects of slow breathing in the healthy human. Breathe, 13(4), 298-309. https://doi.org/10.1183/20734735.009817

  5. Schoene, R. B., Robertson, H. T., Pierson, D. J., & Peterson, A. P. (1981). Respiratory drives and exercise in menstrual cycles of athletic and nonathletic women. Journal of Applied Physiology, 50(6), 1300-1305. https://doi.org/10.1152/jappl.1981.50.6.1300

  6. Slatkovska, L., Jensen, D., Davies, G. A. L., & Wolfe, L. A. (2006, 2006/12/01/). Phasic menstrual cycle effects on the control of breathing in healthy women. Respiratory Physiology & Neurobiology, 154(3), 379-388. https://doi.org/https://doi.org/10.1016/j.resp.2006.01.011

  7. Tenan, M. S., Brothers, R. M., Tweedell, A. J., Hackney, A. C., & Griffin, L. (2014). Changes in resting heart rate variability across the menstrual cycle. Psychophysiology, 51(10), 996-1004. https://doi.org/https://doi.org/10.1111/psyp.12250

  8. Wolfe, L. A., Kemp, J. G., Heenan, A. P., Preston, R. J., & Ohtake, P. J. (1998, Sep). Acid-base regulation and control of ventilation in human pregnancy. Can J Physiol Pharmacol, 76(9), 815-827. https://doi.org/10.1139/cjpp-76-9-815

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