Is there a correlation between susceptibility to fainting and blackouts?

[dawnzus21]

Hello! I’m a beginner free diver and a question has popped up in my sister’s mind, so I am going to ask it in here XD.
Women are more prone to fainting because they have significantly less blood to store oxygen in their bodies. Does it also make them more prone to blackouts?

 

[adrian.u]

I'm going to prefix this response by saying that I'm not a doctor, not a medical professional of any kind, so if you want you can take what I'm saying with a grain of salt, etc...

I'll also say that if anyone here on these forums *does* have some decent medical qualifications and could chime in with a response of some kind, that'd be helpful!


OK, so my understanding of fainting in general would be that, unless it's cardiac-related (i.e. something going on with the heart), the main cause would be due to lowering of blood pressure that leads to a drop in blood flow to the brain.

I'd think just about everyone will experience some form of this at some point in their life - you stand up quickly, and start to feel a little light-headed, etc. The typical cause of this is insufficient vasoconstriction in the legs, and gravity then means more blood ends up down there, and there's a sudden reduction in blood pressure higher up (i.e. head).


Now to your statement about fainting: "Women are more prone to fainting because they have significantly less blood to store oxygen in their bodies."

For a start, I don't know if that's correct...? I'm referring specifically to the link into the second part - the first part is true: women are, statistically, more prone to fainting - but I doubt blood volume plays (much of?) a role. I should probably take a look at medical references, but I'd suspect that changes in hormones over the month might play a more relevant role?


But I guess your (sister's?) main concern would be whether this could lead to it being easier (for you?) to reach the point of blackout when freediving. Am I right in thinking the fear would be that it could be more dangerous for you when freediving (or for her, if she is considering trying it)?


OK, so assuming that any fainting you might experience isn't related to some cardiac issue, then let me try to put your/her mind at ease...

First of all, I suspect that blackout in freediving is somewhat different from fainting. The former is due to hypoxia (i.e. low oxygen in the blood) rather than a sudden drop in blood pressure that leads to a reduction in blood to the brain. In particular, I'd expect the peripheral vasoconstriction that comes as part of the dive reflex (and which is intended to ensure blood supply to important organs, including the brain) would mean it's more difficult to suffer from the gravitationally-induced blood-pooling (from lack of vasoconstriction) that'd typically lead to fainting.

Secondly, if you're just starting your freedive journey, and you've not previously done breathhold training for some other reason, then it's unlikely you'll be going anywhere near the point of blackout because you shouldn't be pushing near your hypoxic limit. The vast majority of untrained apneists will terminate breathhold due to the discomfort of CO2 build-up well before reaching hypoxia. (If you have a fingertip pulse oximeter that measures spO2 level then you can try this for yourself... Hold your breath and see if you can get it to drop much below the normal level of 95-99% - it's actually quite hard for anyone who's not done lots of breathhold training...)

Furthermore, even if you do blackout, it's not a problem as long as you are with a buddy who knows how to deal with it (i.e. who can get you to open air and breathing again). This is one of the main things taught on a freediving course, so anyone who has done such a course should have some knowledge of this - and if you do have a regular buddy, I'd suggest it's good to re-practice blackout procedure on each other every so often.

Finally, I'd note that the majority of non-freedivers, and even beginner freedivers, make the mistake of thinking that freediving is all abouth breathhold, and that will be the thing that'd limit any freediving they may do. However, in my experience watching & talking with freedivers, the most frequent limiting factor actually tends to be equalisation. (And that can often even be true for experienced freedivers - it's not uncommon to see a world-class freediver 'turn early' during a competition dive because their equalisation wasn't quite working out.) Consequently, you might be surprised to find that the main reason you stop descending is not because you think can't hold your breath long enough, but rather because you find you've reached a depth where you can no longer equalise well enough.

Hope that helps!

 

[dawnzus21]

I'm going to prefix this response by saying that I'm not a doctor, not a medical professional of any kind, so if you want you can take what I'm saying with a grain of salt, etc...

I'll also say that if anyone here on these forums *does* have some decent medical qualifications and could chime in with a response of some kind, that'd be helpful!


OK, so my understanding of fainting in general would be that, unless it's cardiac-related (i.e. something going on with the heart), the main cause would be due to lowering of blood pressure that leads to a drop in blood flow to the brain.

I'd think just about everyone will experience some form of this at some point in their life - you stand up quickly, and start to feel a little light-headed, etc. The typical cause of this is insufficient vasoconstriction in the legs, and gravity then means more blood ends up down there, and there's a sudden reduction in blood pressure higher up (i.e. head).


Now to your statement about fainting: "Women are more prone to fainting because they have significantly less blood to store oxygen in their bodies."

For a start, I don't know if that's correct...? I'm referring specifically to the link into the second part - the first part is true: women are, statistically, more prone to fainting - but I doubt blood volume plays (much of?) a role. I should probably take a look at medical references, but I'd suspect that changes in hormones over the month might play a more relevant role?


But I guess your (sister's?) main concern would be whether this could lead to it being easier (for you?) to reach the point of blackout when freediving. Am I right in thinking the fear would be that it could be more dangerous for you when freediving (or for her, if she is considering trying it)?


OK, so assuming that any fainting you might experience isn't related to some cardiac issue, then let me try to put your/her mind at ease...

First of all, I suspect that blackout in freediving is somewhat different from fainting. The former is due to hypoxia (i.e. low oxygen in the blood) rather than a sudden drop in blood pressure that leads to a reduction in blood to the brain. In particular, I'd expect the peripheral vasoconstriction that comes as part of the dive reflex (and which is intended to ensure blood supply to important organs, including the brain) would mean it's more difficult to suffer from the gravitationally-induced blood-pooling (from lack of vasoconstriction) that'd typically lead to fainting.

Secondly, if you're just starting your freedive journey, and you've not previously done breathhold training for some other reason, then it's unlikely you'll be going anywhere near the point of blackout because you shouldn't be pushing near your hypoxic limit. The vast majority of untrained apneists will terminate breathhold due to the discomfort of CO2 build-up well before reaching hypoxia. (If you have a fingertip pulse oximeter that measures spO2 level then you can try this for yourself... Hold your breath and see if you can get it to drop much below the normal level of 95-99% - it's actually quite hard for anyone who's not done lots of breathhold training...)

Furthermore, even if you do blackout, it's not a problem as long as you are with a buddy who knows how to deal with it (i.e. who can get you to open air and breathing again). This is one of the main things taught on a freediving course, so anyone who has done such a course should have some knowledge of this - and if you do have a regular buddy, I'd suggest it's good to re-practice blackout procedure on each other every so often.

Finally, I'd note that the majority of non-freedivers, and even beginner freedivers, make the mistake of thinking that freediving is all abouth breathhold, and that will be the thing that'd limit any freediving they may do. However, in my experience watching & talking with freedivers, the most frequent limiting factor actually tends to be equalisation. (And that can often even be true for experienced freedivers - it's not uncommon to see a world-class freediver 'turn early' during a competition dive because their equalisation wasn't quite working out.) Consequently, you might be surprised to find that the main reason you stop descending is not because you think can't hold your breath long enough, but rather because you find you've reached a depth where you can no longer equalise well enough.

Hope that helps!

Let me start by saying that I truly appreciate your answer!

I can’t remember exactly who told it to me (it might even be my sister), but she told me that the main reason why the lack of blood to the brain causes fainting in the first place is because it deprives the brain of oxygen. Now, if females are using up their total amount of blood faster than males (in percentage to their body of course), doesn’t that mean they’re also using up their total amount of oxygen faster? (In another words, don’t they have a lower hypoxic limit?)

The fear is not about both of us, since I’m a guy and she’s not CIS, but she does consider it and she’s wondering if she’d have any advantages over other women.

And of course, both of us will never dive without a partner

 

[adrian.u]

From what I know, there doesn't seem to be a huge difference in the rate of oxygen decrease in the blood between (typical, healthy) men and women.
Here's an interesting reference I came across:

Sex differences in respiratory and circulatory cost during hypoxic walking: potential impact on oxygen saturation - PMC

Energy expenditure (EE) during treadmill walking under normal conditions (normobaric normoxia, 21% O2) and moderate hypoxia (13% O2) was measured. Ten healthy young men and ten healthy young women walked on a level (0°) gradient a range of speeds ...

www.ncbi.nlm.nih.gov

Also, this is worth reading:

Does gender affect human pulmonary gas exchange during exercise? - PMC

Women may experience greater pulmonary gas exchange impairment during exercise than men. To test this we used the multiple inert gas elimination technique to study eight women and seven men matched for age, height and V˙O2max (∼48 ml kg−1 min−1) ...

www.ncbi.nlm.nih.gov

However, that does not mean there are no gender differences (the above articles make that clear, as do others below) - it just seems that, even though there are differences, the way it all works out means the oxygen use tends to pretty much balance out overall.

Here's some more research that demonstrates both of these points:

Sex Differences in Pulmonary Function during Exercise : Medicine & Science in Sports & Exercise

ched men. Additionally, ovarian hormones, namely progesterone and estrogen, are known to modify and influence the pulmonary system. These differences may have an effect on airway responsiveness, ventilation, respiratory muscle work, and pulmonary gas exchange during exercise. Recent evidence...

journals.lww.com

Mechanics of Breathing during Exercise in Men and Women:... : Exercise and Sport Sciences Reviews

. Women use a greater fraction of their ventilatory reserve and have a higher metabolic cost of breathing....

journals.lww.com

Sex differences in the sympathetic neurocirculatory responses to chemoreflex activation - PMC

The purpose of this study was to determine whether there are sex differences in the cardiorespiratory and sympathetic neurocirculatory responses to central, peripheral, and combined central and peripheral chemoreflex activation. Ten women (29 ± 6 ...

www.ncbi.nlm.nih.gov

CO2 Chemoreflex Drive of Ventilation in Man: Effects of Hyperoxia and Sex Differences

Abstract. CO2 single breaths have been performed in 7 men and 7 women in conditions of normoxia (FICO2≈0.13; FIO2≈0.21; FIN2 6≈ 0.66) and of hyperoxia (FICO2≈0.13; FIO2≈0.87). Ventilatory responses of the subjects and modifications of breathing pattern in the course of the CO2 tests were also...

karger.com

Chemoreflex control of breathing during wakefulness in healthy men and women - PubMed

This study used a modified CO(2) rebreathing procedure to examine the effect of gender on the chemoreflex control of breathing during wakefulness in healthy men (n = 14) and women (n = 14). Women were tested in the follicular phase of the menstrual cycle. During rebreathing trials, subjects...

pubmed.ncbi.nlm.nih.gov

Anyway, I hope that's some interesting bedtime reading...


One further thing...
The other recent thread here on iron deficiency did make me realise that could be another link with fainting - so some kind of anaemic condition would likely also having some effect on susceptibility to blackout (since it's clearly linked to oxygen transport & delivery in the blood).

So I guess if you are anaemic, which could have fainting as a potential symptom, then I think it would be wise to be somewhat more careful when freediving.

I guess all of this means if you do find you are prone to fainting, you should see if there could be some kind of reason for it (e.g. cardiac-related, or anaemia, or something else I've not considered offhand).

But, to repeat what I said above, unless you're an experienced breathholder, and if you are in decent health (i.e. not suffering from a condition like above) it's unlikely you will hold your breath long enough to get so near the point of blackout. (Again, you can do some tests on that for yourself via some dry static breathholds with a pulse oximeter.)

And I'll also say again, for the benefit of anyone else who may happen to read this:
If you are diving with a well-trained buddy then a blackout isn't a particularly big issue. (Assuming it's not happening regularly!)
Conversely, though, if you don't then it is...

 

[adrian.u]

Sorry, wrote some of above in a bit of a rush…
Wasn’t meaning to suggest blackout isn’t something of concern at all - e.g. no more diving at least rest of day after blackout.

Just saying that if it really is just a blackout caused by pushing hypoxic limit a bit too much, and after some time resting, there’s no suggestion of something else going on, and it’s not a regular thing, then it shouldn’t be considered a big issue.

Hope that helps clarify!

 

[dawnzus21]

From what I know, there doesn't seem to be a huge difference in the rate of oxygen decrease in the blood between (typical, healthy) men and women.
Here's an interesting reference I came across:

Sex differences in respiratory and circulatory cost during hypoxic walking: potential impact on oxygen saturation - PMC

Energy expenditure (EE) during treadmill walking under normal conditions (normobaric normoxia, 21% O2) and moderate hypoxia (13% O2) was measured. Ten healthy young men and ten healthy young women walked on a level (0°) gradient a range of speeds ...

www.ncbi.nlm.nih.gov

Also, this is worth reading:

Does gender affect human pulmonary gas exchange during exercise? - PMC

Women may experience greater pulmonary gas exchange impairment during exercise than men. To test this we used the multiple inert gas elimination technique to study eight women and seven men matched for age, height and V˙O2max (∼48 ml kg−1 min−1) ...

www.ncbi.nlm.nih.gov

However, that does not mean there are no gender differences (the above articles make that clear, as do others below) - it just seems that, even though there are differences, the way it all works out means the oxygen use tends to pretty much balance out overall.

Here's some more research that demonstrates both of these points:

Sex Differences in Pulmonary Function during Exercise : Medicine & Science in Sports & Exercise

ched men. Additionally, ovarian hormones, namely progesterone and estrogen, are known to modify and influence the pulmonary system. These differences may have an effect on airway responsiveness, ventilation, respiratory muscle work, and pulmonary gas exchange during exercise. Recent evidence...

journals.lww.com

Mechanics of Breathing during Exercise in Men and Women:... : Exercise and Sport Sciences Reviews

. Women use a greater fraction of their ventilatory reserve and have a higher metabolic cost of breathing....

journals.lww.com

Sex differences in the sympathetic neurocirculatory responses to chemoreflex activation - PMC

The purpose of this study was to determine whether there are sex differences in the cardiorespiratory and sympathetic neurocirculatory responses to central, peripheral, and combined central and peripheral chemoreflex activation. Ten women (29 ± 6 ...

www.ncbi.nlm.nih.gov

CO2 Chemoreflex Drive of Ventilation in Man: Effects of Hyperoxia and Sex Differences

Abstract. CO2 single breaths have been performed in 7 men and 7 women in conditions of normoxia (FICO2≈0.13; FIO2≈0.21; FIN2 6≈ 0.66) and of hyperoxia (FICO2≈0.13; FIO2≈0.87). Ventilatory responses of the subjects and modifications of breathing pattern in the course of the CO2 tests were also...

karger.com

Chemoreflex control of breathing during wakefulness in healthy men and women - PubMed

This study used a modified CO(2) rebreathing procedure to examine the effect of gender on the chemoreflex control of breathing during wakefulness in healthy men (n = 14) and women (n = 14). Women were tested in the follicular phase of the menstrual cycle. During rebreathing trials, subjects...

pubmed.ncbi.nlm.nih.gov

Anyway, I hope that's some interesting bedtime reading...


One further thing...
The other recent thread here on iron deficiency did make me realise that could be another link with fainting - so some kind of anaemic condition would likely also having some effect on susceptibility to blackout (since it's clearly linked to oxygen transport & delivery in the blood).

So I guess if you are anaemic, which could have fainting as a potential symptom, then I think it would be wise to be somewhat more careful when freediving.

I guess all of this means if you do find you are prone to fainting, you should see if there could be some kind of reason for it (e.g. cardiac-related, or anaemia, or something else I've not considered offhand).

But, to repeat what I said above, unless you're an experienced breathholder, and if you are in decent health (i.e. not suffering from a condition like above) it's unlikely you will hold your breath long enough to get so near the point of blackout. (Again, you can do some tests on that for yourself via some dry static breathholds with a pulse oximeter.)

And I'll also say again, for the benefit of anyone else who may happen to read this:
If you are diving with a well-trained buddy then a blackout isn't a particularly big issue. (Assuming it's not happening regularly!)
Conversely, though, if you don't then it is...

That was surely an interesting bed time reading xD

All of these are very interesting! Thank you so much! However, none of these really include breath holding so it’s a bit hard to know for sure. For example: it’s impossible to adapt your breathing rate underwater, because you’re basically don’t breathe underwater xD.

And yes, we know that untrained individuals have no reason to worry about BOs. I guess a simpler way to give answer for it is to ask two groups to hold their breath for the same amount of time and see how much their SpO2 decreases.

 

[adrian.u]

All of these are very interesting! Thank you so much! However, none of these really include breath holding so it’s a bit hard to know for sure. For example: it’s impossible to adapt your breathing rate underwater, because you’re basically don’t breathe underwater xD.

Indeed, that is conspicuously absent from above references. However, the aim was more to demonstrate some of the potential gender differences in oxygen use, so I'd think there's likely some relevance there somewhere.

In the past I've had a look for references concerning gender differences in O2 use during BH (from answering other questions here -should be straightforward to find those posts if you're interested), and there's actually not as much really clear research as might have been expected. Also, the few that did look like they could be most relevant either only had abstract available rather than full text (and the abstract was not enough to get a clear idea what's going on), and/or they were mainly testing with 'elite apneists' (so some of the findings may not necessarily apply so well to the rest of us mere mortals - see 'bonus article' at the bottom for some interesting comparisons...)

Here are a couple more that do some tests during hypoxia (or hypoxemia, but that's effectively what leads to hypoxia in context of BH), but unfortunately not actually BH itself:
Sex differences in the vascular response to sympathetic activation during acute hypoxemia

Sex differences in the vascular response to sympathetic activation during acute hypoxemia - PMC

Activation of the sympathetic nervous system causes vasoconstriction and a reduction in peripheral blood flow. Sympathetically-mediated vasoconstriction may be attenuated during systemic hypoxia to maintain oxygen delivery; however, in predominantly ...

www.ncbi.nlm.nih.gov

On the hemodynamic consequence of the chemoreflex and muscle mechanoreflex interaction in women and men: two tales, one story

On the hemodynamic consequence of the chemoreflex and muscle mechanoreflex interaction in women and men: two tales, one story - PMC

The cardiovascular response resulting from the individual activation of the muscle mechanoreflex (MMR), or the chemoreflex (CR), is different between men and women. Whether the hemodynamic consequence resulting from the interaction of these ...

www.ncbi.nlm.nih.gov

For actual breathhold, here is one I found that does have full text available:
Effect of gender on maximal breath-hold time

https://www.researchgate.net/profile/Evgenia-Cherouveim/publication/233788276_Effect_of_gender_on_maximal_breath-hold_time/links/09e4150e8b5a6ca92f000000/Effect-of-gender-on-maximal-breath-hold-time.pdf

Finally, here are a couple of abstracts (consequently only giving quite minimal overview - you need to register with the journal to get the full text):
Maximum effort breath-hold times for males and females of similar pulmonary capacities during sudden face-only immersion at water temperatures from 0 to 33 °C

https://cdnsciencepub.com/doi/abs/10.1139/h06-050

Effects of sex differences on breath-hold diving performance

Effects of sex differences on breath-hold diving performance - PubMed

It was suggested that % forced vital capacity contributed more significantly to performance than diving response and CO₂ sensitivity. Furthermore, the higher performance of male divers compared to female divers may be due to the % forced vital capacity rather than the diving response...

pubmed.ncbi.nlm.nih.gov

However, going back to the original question concerning fainting, you're right - it's still not totally clear how much of what these do say might be linked with that (again, really just demonstrating some of the potential differences in oxygen use...)

I guess the bottom line is that the way our bodies work is really complicated, with lots of factors that can be slightly different for different people (let alone different genders), and you'd need someone with plenty of experience & research in this area to take numerous measurements (several of them somewhat invasive) to get a half-decent idea of how & why any two individuals (any genders) might differ in their abilities.

---

Bonus article:

Cardiovascular adjustments in breath hold diving comparison between divers and non divers in simulated dynamic apnoea

Cardiovascular adjustments in breath-hold diving: comparison between divers and non-divers in simulated dynamic apnoea

Cardiovascular adjustments in breath-hold diving: comparison between divers and non-divers in simulated dynamic apnoea

www.academia.edu

 

[dawnzus21]

Indeed, that is conspicuously absent from above references. However, the aim was more to demonstrate some of the potential gender differences in oxygen use, so I'd think there's likely some relevance there somewhere.

In the past I've had a look for references concerning gender differences in O2 use during BH (from answering other questions here -should be straightforward to find those posts if you're interested), and there's actually not as much really clear research as might have been expected. Also, the few that did look like they could be most relevant either only had abstract available rather than full text (and the abstract was not enough to get a clear idea what's going on), and/or they were mainly testing with 'elite apneists' (so some of the findings may not necessarily apply so well to the rest of us mere mortals - see 'bonus article' at the bottom for some interesting comparisons...)

Here are a couple more that do some tests during hypoxia (or hypoxemia, but that's effectively what leads to hypoxia in context of BH), but unfortunately not actually BH itself:
Sex differences in the vascular response to sympathetic activation during acute hypoxemia

Sex differences in the vascular response to sympathetic activation during acute hypoxemia - PMC

Activation of the sympathetic nervous system causes vasoconstriction and a reduction in peripheral blood flow. Sympathetically-mediated vasoconstriction may be attenuated during systemic hypoxia to maintain oxygen delivery; however, in predominantly ...

www.ncbi.nlm.nih.gov

On the hemodynamic consequence of the chemoreflex and muscle mechanoreflex interaction in women and men: two tales, one story

On the hemodynamic consequence of the chemoreflex and muscle mechanoreflex interaction in women and men: two tales, one story - PMC

The cardiovascular response resulting from the individual activation of the muscle mechanoreflex (MMR), or the chemoreflex (CR), is different between men and women. Whether the hemodynamic consequence resulting from the interaction of these ...

www.ncbi.nlm.nih.gov

For actual breathhold, here is one I found that does have full text available:
Effect of gender on maximal breath-hold time

https://www.researchgate.net/profile/Evgenia-Cherouveim/publication/233788276_Effect_of_gender_on_maximal_breath-hold_time/links/09e4150e8b5a6ca92f000000/Effect-of-gender-on-maximal-breath-hold-time.pdf

Finally, here are a couple of abstracts (consequently only giving quite minimal overview - you need to register with the journal to get the full text):
Maximum effort breath-hold times for males and females of similar pulmonary capacities during sudden face-only immersion at water temperatures from 0 to 33 °C

https://cdnsciencepub.com/doi/abs/10.1139/h06-050

Effects of sex differences on breath-hold diving performance

Effects of sex differences on breath-hold diving performance - PubMed

It was suggested that % forced vital capacity contributed more significantly to performance than diving response and CO₂ sensitivity. Furthermore, the higher performance of male divers compared to female divers may be due to the % forced vital capacity rather than the diving response...

pubmed.ncbi.nlm.nih.gov

However, going back to the original question concerning fainting, you're right - it's still not totally clear how much of what these do say might be linked with that (again, really just demonstrating some of the potential differences in oxygen use...)

I guess the bottom line is that the way our bodies work is really complicated, with lots of factors that can be slightly different for different people (let alone different genders), and you'd need someone with plenty of experience & research in this area to take numerous measurements (several of them somewhat invasive) to get a half-decent idea of how & why any two individuals (any genders) might differ in their abilities.

---

Bonus article:

Cardiovascular adjustments in breath hold diving comparison between divers and non divers in simulated dynamic apnoea

Cardiovascular adjustments in breath-hold diving: comparison between divers and non-divers in simulated dynamic apnoea

Cardiovascular adjustments in breath-hold diving: comparison between divers and non-divers in simulated dynamic apnoea

www.academia.edu

That’s really interesting too!

I wonder if oxygenation is actually balanced between the sexes, because as one of the researches said-there isn’t a gender difference in CO2 sensitivity but there is a difference in VC, and VC contributed more significantly than CO2 sensitivity. I wonder if it’s due to men being more extreme or if they have actually a physical advantage that untrained individuals still can’t reach, which’s why there isn’t a big difference when they’re untrained. I guess some mysteries are better left unsolved

Thank you so much for making my day interesting once again and for a really great answer!

 

[lunkskilbitpurl]

I'm going to prefix this response by saying that I'm not a doctor, not a medical professional of any kind, so if you want you can take what I'm saying with a grain of salt, etc...

I'll also say that if anyone here on these forums *does* have some decent medical qualifications and could chime in with a response of some kind, that'd be helpful!


OK, so my understanding of fainting in general would be that, unless it's cardiac-related (i.e. something going on with the heart), the main cause would be due to lowering of blood pressure that leads to a drop in blood flow to the brain.

I'd think just about everyone will experience some form of this at some point in their life - you stand up quickly, and start to feel a little light-headed, etc. The typical cause of this is insufficient vasoconstriction in the legs, and gravity then means more blood ends up down there, and there's a sudden reduction in blood pressure higher up (i.e. head).


Now to your statement about fainting: "Women are more prone to fainting because they have significantly less blood to store oxygen in their bodies."

For a start, I don't know if that's correct...? I'm referring specifically to the link into the second part - the first part is true: women are, statistically, more prone to fainting - but I doubt blood volume plays (much of?) a role. I should probably take a look at medical references, but I'd suspect that changes in hormones over the month might play a more relevant role?


But I guess your (sister's?) main concern would be whether this could lead to it being easier (for you?) to reach the point of blackout when freediving. Am I right in thinking the fear would be that it could be more dangerous for you when freediving (or for her, if she is considering trying it)?


OK, so assuming that any fainting you might experience isn't related to some cardiac issue, then let me try to put your/her mind at ease...

First of all, I suspect that blackout in freediving is somewhat different from fainting. The former is due to hypoxia (i.e. low oxygen in the blood) rather than a sudden drop in blood pressure that leads to a reduction in blood to the brain. In particular, I'd expect the peripheral vasoconstriction that comes as part of the dive reflex (and which is intended to ensure blood supply to important organs, including the brain) would mean it's more difficult to suffer from the gravitationally-induced blood-pooling (from lack of vasoconstriction) that'd typically lead to fainting.

Secondly, if you're just starting your freedive journey, and you've not previously done breathhold training for some other reason, then it's unlikely you'll be going anywhere near the point of blackout because you shouldn't be pushing near your hypoxic limit. The vast majority of untrained apneists will terminate breathhold due to the discomfort of CO2 build-up well before reaching hypoxia. (If you have a fingertip pulse oximeter that measures spO2 level then you can try this for yourself... Hold your breath and see if you can get it to drop much below the normal level of 95-99% - it's actually quite hard for anyone who's not done lots of breathhold training...)

Furthermore, even if you do blackout, it's not a problem as long as you are with a buddy who knows how to deal with it (i.e. who can get you to open air and breathing again). This is one of the main things taught on a freediving course, so anyone who has done such a course should have some knowledge of this - and if you do have a regular buddy, I'd suggest it's good to re-practice blackout procedure on each other every so often.

Finally, I'd note that the majority of non-freedivers, and even beginner freedivers, make the mistake of thinking that freediving is all abouth breathhold, and that will be the thing that'd limit any freediving they may do. However, in my experience watching & talking with freedivers, the most frequent limiting factor actually tends to be equalisation. (And that can often even be true for experienced freedivers - it's not uncommon to see a world-class freediver 'turn early' during a competition dive rena monrovia when you transport something by car ... because their equalisation wasn't quite working out.) Consequently, you might be surprised to find that the main reason you stop descending is not because you think can't hold your breath long enough, but rather because you find you've reached a depth where you can no longer equalise well enough.

Hope that helps!

Susceptibility to fainting (syncope) and blackouts (transient loss of consciousness) are often correlated, as both conditions can share common underlying causes. Fainting typically results from a temporary reduction in blood flow to the brain, which can occur due to dehydration, standing up too quickly, or a drop in blood pressure. Blackouts, on the other hand, can be triggered by similar factors, but may also be caused by more severe conditions like seizures, heart arrhythmias, or neurological disorders.