The physiology of the bends

Gases and physiology go hand in hand. We breathe oxygen and exhale carbon dioxide. Several molecules that we usually come across in gas form act as signal molecules, for example carbon monoxide and nitric oxide. But sometimes, things go wrong when gases meet physiology. In this blog post, I will talk about one such situation: decompression sickness, also called the bends.

diving_free_stock

What, exactly, is the bends? The bends happens when a person has been under pressure and then that pressure is removed quickly, causing the gases that are normally dissolved in the body fluids to form bubbles. A typical situation is where someone has been diving deep underwater and then resurface too quickly*.

What causes it? The first thing we need to know is which gases are problematic, namely inert gases. There are inert gases in the body. An inert gas is a gas that, under the given conditions, does nothing – i.e. it doesn’t undergo chemical reactions. The inert gases in the body mostly sits there and does not take part in metabolism. Oxygen is not an inert gas in the body – it contributes to several physiological processes including respiration. Nitrogen, however, is an inert gas in the body – it does not contribute. It is the main inert gas in the air, and the most common culprit of the bends.

Nitrogen and other inert gases are typically dissolved in the bodily fluids. This process requires pressure. Henry’s Law describes the relationship between pressure and a gas dissolved in liquid. This law states that when pressure on a liquid is decreased, the amount of gas dissolved in the liquid will decrease to the same extent, and when pressure is increased, the amount of gas dissolved will increase. This means that if the human body is put under high pressure (e.g. when diving), the inert gases will be dissolved in the bodily fluids. This is not a problem. The gases are inert, doing nothing, so we can usually handle the extra amount. The problem occurs when the pressure is reduced, and it is reduced fast. If a diver is moving slowly from the high-pressure underwater environment to the surface, the gases being released may escape bit by bit as the person breathes out, which is fine. But if it happens fast, the lungs may not be able to keep up, and that causes trouble. The inert gases become un-dissolved in places where they really should not be: we get bubbles trapped in the body.

Bubbles are bad news. They disrupt blood flow and can cause damage. Bubbles can form in (or be transported to) any part of the body (although some places such as large joints are more at risk). This means that we can get a great range of symptoms, depending on where the bubbles form. The worst outcome is death, which can happen if the bubbles for example interrupt spinal cord function. Less severe outcomes, although still severely unpleasant, are seizures, paralysis, dizziness, pain, visual disturbances, breathlessness, nausea, incontinence and more. In short, we do not want bubbles of inert gas (usually nitrogen) interfering with our organ function.

The deeper and longer the dive, the more dangerous it can be. We know that pressure increases as you descend. At 10 meters, the pressure has increased from 1 atm (sea level) to 2 atm, and the same level of pressure will therefore be needed for the diving suit to remain inflated. For reference, one atmosphere is the pressure that the air exerts at sea level, and the pressure tends to increase with 1 atm for each 10 m increase in depth. In other words, each 10 m below the surface adds to the body of water pressing down on the diver. The further down, the more weight, and this weight is in addition to the weight of the air (1 atm at sea level). 

At 1 atm (sea level), the oxygen content is approximately 21%, and nitrogen content is approximately 78%. The oxygen pressure is therefore 0.21 * 1 atm =0.21 atm, and the nitrogen pressure is 0.78 * 1 atm = 0.78 atm. There are other pressure units that this can be measured in, such as mmHg or Torr (1 atm = 760 mmHg = 760 Torr) and kPa (1 atm = 101.325 kPa), but we will stick to atm for now.

If a diver is given normal air (with the percentage composition described above), at 10 meters the oxygen pressure will be 0.21 * 2 atm = 0.42 atm, and nitrogen pressure will be 0.78 * 2 atm = 1.56 atm. At this pressure, the blood will start to dissolve the nitrogen. This means that the fluids in the body will contain increasing amounts of nitrogen until an equilibrium is reached. How much nitrogen ends up where in the body depends on the tissue composition. Fat, for example, can dissolve a lot of nitrogen, and will take a bulk share.

Dissolving nitrogen is a slow process, as is releasing dissolved nitrogen. The longer the dive, the more nitrogen is dissolved, and the longer it takes to release it afterwards.

So why nitrogen? Why not oxygen, or carbon dioxide? As a rule of thumb, for the bends to happen, the diver has to experience the gas in question at a pressure of at least 2 atm. This excludes both carbon dioxide and oxygen. Even at depth, carbon dioxide pressure simply does not increase much above its normal level (about 40 mmHg, or around 0.05 atm), and oxygen is used up by the tissues so it does not rise as much either*. Unless the diver uses a gas mixture with a different inert gas (e.g. helium), nitrogen will be the villain of the piece.

There are advantages to using other gases. Helium, for example, is less soluble than nitrogen. This means it dissolves faster into the body fluids, but it is also removed faster, reducing the risk of developing the bends. For long dives, this is good, but for short ones (where nitrogen simply would not have time to dissolve much), it’s not so good. One may ask why use an inert gas at all, if they are so problematic, and the simple answer is that pure oxygen under pressure is toxic**.

What about diving animals? Plenty of animals have an aquatic mode of life, and the list includes a variety of species from the odd rodent, ungulate and marsupial to penguins, seals and whales. Many of these dive to incredible depth, and they ascend and descend quickly.

diving depths
Diving depths of marine mammals, not including the deepest diving whale (Cuvier’s Beaked Whale, 2992 m) or birds (deepest diving is Emperor Penguin at 565 m, and deepest diving flying bird is Brünnich’s guillemot at 210 m). The personal record of this particular mammal is 3 m, which was a proud, yet unpleasant, moment. *Record by Soviet submarine K-278 in 1984, data for current submarines classified. Image not to scale: the submarine was 117 meters long, making it around 4 times the size of a blue whale.

At depth, the lungs of diving animals are almost entirely collapsed and blood flow to the lungs limited. Some diving animals even have collapsible rib cages that forces the air out when they dive. This means that no nitrogen can enter the blood during the dive. Many diving animals have quite small lungs, possibly because of this (and because large lungs work as airbags, making diving difficult). The diving animals do not store oxygen in their lungs for their dives, but rather in their blood, and they are very particular in how they use what little oxygen they have.

Oxygen storage. Diving animals usually have much more blood relative to their body size than non-diving animals, and the blood itself has a greater oxygen capacity (almost 40 ml oxygen per 100 ml blood, twice that of humans). The increased oxygen carrying capacity is due to a greater amount of red blood cells (which contain haemoglobin, the molecule that binds oxygen), and a much higher level of haemoglobin in the muscles (called myoglobin). The sperm whale, for example, has a myoglobin content almost 10 times that of humans, allowing them to store a vast amount of oxygen. Theoretically, the high level should cause the myoglobin to clump together, causing disease, but diving animals’ myoglobin is positively charged and stays nicely separated even when the muscles are packed with it.

Oxygen use. This impressive store of oxygen, however, is not enough. In addition, most diving animals have a lowered heart rate during dives, and blood flow to most of their internal organs is lowered (the exception being the brain). Even the heart gets less blood (but then again, the heart rate is lowered, so less blood is needed). Muscles also get reduced blood flow despite being needed to swim – instead, they rely on anaerobic (non-oxygen) metabolism after the myoglobin oxygen storage is depleted. Many diving animals also reduce muscle use. For example, Weddell and Elephant seals, bottlenose dolphins and blue whales have all been shown not to use their muscles when descending: sinking rather than swimming. And diving animals also reduce their metabolic rate, meaning that they use less energy. This is made easier by many diving animals being large, because oxygen consumption relative to body size is smaller the larger the body size. In other words: a mouse uses much more oxygen per gram of its body (1.65 litre oxygen per kilo per hour) than does an elephant (0.07 litre oxygen per kilo per hour).

In short: diving animals avoid the bends through minimizing the amount of nitrogen entering their blood when they dive***. To stay submerged, they are very good at storing oxygen in their blood (through higher blood volume and greater amounts of red blood cells) and muscles (through greater myoglobin levels), and use less oxygen (through shutting off blood flow to non-important organ systems, reducing their heart rate and metabolic rate, letting their muscles work anaerobically or not work at all).

*Interesting fact: The bends can also happen when ascending rapidly in for example an aircraft, with the danger arising as the pressure is quickly reduced to 0.5 atm. The underlying principles remain the same. A pressurised cabin will prevent this entirely (as with deep-diving submarines).

**Oxygen at pressures above 1 atm is toxic. When diving deeper than 60 meters, the pressure is so high that the gas mix needs to contain less oxygen than at sea level (hypoxic mixture) to avoid oxygen poisoning. There are multiple studies on oxygen toxicity, but the wikipedia article on the topic is quite extensive and accessible: https://en.wikipedia.org/wiki/Oxygen_toxicity

***This does not mean that they cannot get the bends. For example, stranded whales have shown signs of gas-bubble tissue damage (Gas-bubble lesions in stranded cetaceans: Nature 425, 575–576 (2003) https://www.nature.com/articles/425575a). It is possible that this has to do with the animals being stressed (and therefore either diving for too long or ascending or descending too quickly), or having too high blood flow (again due to stress or possibly also cold). While we do not know the exact reason why and how this happens, we do know that the use of sonar has been linked to whales getting the bends.

Breathlessness & the Brain

Breathlessness can be many things. For example, it can be the shortness of breath after exercise – short-lived and laced with endorphins – or it can be the frightening gasping for breath experienced by patients with a range of diseases from cardiac failure and cancer to respiratory disease. From a physiological point of view, these may look quite similar, but they are not really the same thing. Breathlessness is the sensation produced by sensory input evaluated within the context of psychological and environmental factors. In short, the quality of breathlessness depends on why, who and where it is experienced – it’s situational and subjective. This means it cannot be approached, from a medical point of view, with a one size fits all solution. It’s important to understand its nuances.

So, which parts of the brain are involved in processing breathlessness? The answer is quite simple: we don’t know yet. To date, studies are few and far between, with the majority being on breathlessness in healthy controls which may or may not translate to various patient groups. While we don’t know anything for certain, particularly for patients, we do however have a few likely suspects.

usual

The usual suspects. The first is the insular cortex. This is a region of the brain that has been identified in the majority of neuroimaging studies on breathlessness. It plays a role in the conscious awareness of body state and is involved in the perception of other unpleasant sensations, such as pain. The role of the insula is two-fold, with the posterior (towards the back) part processing the physical aspects and the anterior (towards the front) part processing the affective (relating to moods and feelings) aspects of the sensation 1. This anterior-posterior division may also be present in breathlessness, as the anterior insula appears to be more associated with the unpleasantness of breathlessness.

The next on the list of likely suspects are somatosensory and motor regions (e.g. 2), which are also activated in a number of breathlessness studies. This is not surprising, given the added work of breathing harder than normal when you’re breathless. In addition to cortical somatosensory/motor regions, we also see activation in the brainstem (particularly in the brainstem respiratory centres) and in the cerebellum (a region associated with motor function). Some of these areas may be mostly involved with breathing itself and not with breathlessness specifically, as they appear frequently in studies looking at simple breathing responses without any breathlessness.

Then there are regions that sometimes appear and sometimes don’t: the prefrontal cortex, the periaqueductal grey (PAG), the amygdala and the anterior cingulate cortex (ACC). In the case of the PAG, which is a structure in the midbrain involved in pain processing and fight-or-flight responses, this may simply be because it is small and hard to image. Recent studies using high resolution imaging have found that the PAG is involved with unpleasant respiratory sensations, and that parts of the PAG have different roles in processing respiratory threat (see Figure 1 below) 3. Looking further into its sub-divisions, the lateral PAG is downregulated during restricted breathing, and the ventrolateral PAG is upregulated. This is interesting as the lateral PAG has been associated with active coping strategies for stressful situations that can be escaped, and the ventrolateral PAG with passive coping strategies for stressful situations that cannot be escaped. In short, it seems as if different parts of the PAG are involved in different aspects of breathlessness.

elife-12047-fig2-v3-480wFig 1. 7T FMRI of respiratory threat in the PAG. Faull et al. 2016.

The prefrontal cortex has been implicated in the processing of breathlessness cues (i.e. a non-physical stimulus) in COPD patients (medial prefrontal cortex, mPFC), as has the ACC (see Figure 2 below) 4. The latter has also been identified in many studies on breathlessness in healthy volunteers, and the former is an area of the prefrontal cortex involved in emotion processing, and particularly associated with responses to fear and threat. In the above study, both patients and controls showed activation in the insula (labelled ‘Conjunction’). Opioids, which can cause make breathlessness less unpleasant, dampens brain responses to breath holding in the prefrontal and anterior cingulate cortices as well as in the insula 5. So, it’s a good guess that the prefrontal cortex and ACC are involved in the modulation of breathlessness.

chestpaperFig 2. COPD patients and control. Response to breathlessness cues. From Herigstad et al. 2015.

Similarly, the amygdala, which is frequently identified in a whole range of studies relating to emotion, threat appraisal and fight-or-flight responses, is also activated in some studies of breathlessness. The amygdala is strongly connected to the anterior insula, and breathlessness studies that have seen activation in the amygdala also show large activation in the insula.

There are also another few regions that are occasionally found and we don’t quite know how they fit in. These include the before-mentioned cerebellum, the dorsolateral prefrontal cortex and the precuneus.

The cerebellum is probably mostly involved in the motoric response to breathlessness, as it is a centre for coordination of motor function, but as the cerebellum has also been associated with cognitive function, we can’t yet determine its role in breathlessness. The dorsolateral prefrontal cortex may be part of the cognitive evaluation of breathlessness, as this is a structure that is associated with attention and working memory. The precuneus could be involved in both sensory and cognitive aspects of breathlessness, depending on which part of the precuneus is active. It is a poorly mapped structure, but in general it has a sensorimotor anterior region which links to sensory/motor areas of the brain and the insula, and a cognitive central region which links to prefrontal regions, including the dorsolateral prefrontal cortex. It also links to the thalamus, which is a subcortical structure that relays a vast range of information between the brainstem and the higher brain areas.

Linking it all together. We don’t know how this all ties together. However, one could speculate ways in which breathlessness is processed in the brain:

Brainstem breathing control. We know that respiratory centres in the brainstem receive peripheral input (input from the rest of the body) and adjust the breathing pattern in response to this. The brainstem is crucial for breathing – without it, no breathing occurs – and it almost certainly plays a part in the actual breathing response to breathlessness. The cerebellum, which is probably involved in the motor response to breathlessness, connects to the higher brain areas through the brainstem. So far so good.

PAG as a gateway. Peripheral input is also believed to be received in the PAG, which could act as a gateway of sorts. The lateral PAG relays signal directly to primary motor/sensory cortices and the (posterior?) insula, and this path may be processing breathlessness intensity. The ventrolateral PAG relays signal directly to the prefrontal cortex, (anterior?) insula and also to motor regions of the brain, and this path may be processing the threat and/or possible responses to the breathlessness. The PAG probably also connects with the thalamus, which in turn acts as a hub (see below).

Thalamus as a hub. Finally, signal could also be relayed via the PAG (or directly) to the thalamus, which has been identified in a range of breathlessness studies and is a common hub for cortical communication. From the thalamus, signal is transmitted to a range of cortical areas (including prefrontal regions and the amygdala via the medial/frontal thalamus, and somatosensory and motor regions via the ventroposterior thalamus). Breathlessness is likely modulated by several of these cortical areas and how they interact.

The complexity of the interactions may be best explained by an example:

Anxiety relating to threat is modulated by activation in both the medial prefrontal cortex (mPFC) and the amygdala. The mPFC possibly influences threat by dampening activation in the amygdala. So far, so good. The prefrontal cortex receives signal from the PAG, and may also receive input from the ACC. Both the ACC and PAG are in turn linked to the anterior insula. The anterior insula also receives input from the thalamus, which relays signal to the amygdala. The thalamus also receives input from the PAG. So now we have a network of interconnected regions which may all influence each other and work to fine-tune the anxiety response. Furthermore, the anterior insula may also receive and integrate information on the physical sensation from the posterior insula, which is linked to the ventroposterior thalamus. This part of the thalamus is connected with somatosensory and motor cortices. These are also influenced by the PAG. The thalamus and somatosensory/motor cortices also relay signal to the precuneus, which in turn could connect to the dorsolateral prefrontal cortex, thus incorporating cognitive processing. And so on. It can get a bit complex.

A suggested network is presented in the figure below.

brain3 Fig 3. Possible breathlessness network. Purple=connected cortical regions, black= signal to/from periphery, blue=signal from PAG. ‘Hubs’ are shown in green. PFC = prefrontal cortex (encompasses both medial (emotional/threat processing) and dorsolateral (cognitive processing) PFC)

In short, various studies have shown that a range of brain regions are activated during breathlessness, but we don’t yet know exactly how they are involved in processing the sensation. We may still only guess at the full picture, based on breathlessness work as well as studies on other unpleasant sensations (pain, mostly), threat or emotional processing. Much of the same processing mechanisms are likely to be found in these similar sensations. While the above figure outlines some possible networks for the processing of breathlessness, based on our current understanding of breathlessness and related conditions, (much) more information is needed.

References:
1. Oertel, B., Preibisch, C., et al. Clin Pharmacol Ther 2008; 83:577–588.
2. Hayen, A., Herigstad, M., et al. NeuroImage 2012; 66: 479-488.
3. Faull, O., Jenkinson, M., et al. eLife 2016; 5: e12047
4. Herigstad, M., Hayen, A., et al. Chest 2015; 148(4): 953-61
5. Harvey, A., Pattinson, K., et al. J Magn Reson Imaging 2008; 28:1337–1344.

Brainstem? Brainstem!

Revisiting brain regions, courtesy  of Pinky and the Brain.

 

(Incidentally, I named my first neuroscience review paper “Dyspnoea and the Brain” partly in honour of this comic, fully expecting the title to be dismissed by colleagues and journal alike. It wasn’t. Nobody has spotted the connection yet, sadly.)

How do eggs breathe?

Bird’s eggs contain everything that the chick needs, except oxygen. So how exactly does the egg ‘breathe’?

First, the shell needs to be permeable to gases. The shell consists of a calcium carbonate outer layer, with two shell membranes beneath. A typical outer shell of a hen’s egg holds about 10,000 pores, each less than 0.02mm (approximately 0.017mm) in diameter, about 2.3mm2 pore area in total. Through these pores oxygen is taken in and carbon dioxide (CO2) released. However, the pores also allow water vapour to escape. Over the 21-day incubation period, a hen’s egg typically takes in 6 litres of oxygen, releases 4.5 litres of CO2 and loses 11 litres of water vapour. The rate of water loss depends on the porosity of the eggshell and the water vapour pressure gradient between the nest and egg.

Water vapour pressure is the pressure at which water evaporates at the same rate that it condenses. We can change this equilibrium point by changing temperature. At lower temperatures, the vapour pressure is less (fewer molecules have the kinetic energy to evaporate), and at higher temperatures the vapour pressure is higher (more molecules have the kinetic energy to evaporate). A liquid with a higher vapour pressure will evaporate more readily, and it will boil when its vapour pressure reaches the external, atmospheric pressure. For example, water has, at 100°C, a saturated vapour pressure equal to the atmospheric pressure at sea level (760 Torrs). If the atmospheric pressure decreases below 760 Torrs, the saturated vapour pressure of water and the atmospheric pressure would match at lower temperatures. For example, on Mount Everest, the low pressure associated with altitude means water will boil at about 70°C.

To avoid the egg losing too much water, whilst not jeopardising oxygen uptake, birds employ a number of strategies. The water vapour pressure gradient between nest and eggs is regulated by nest construction (insulation, humidity) and incubation patterns. The pore area of the egg is kept within certain biological limits: large enough for the egg to get enough oxygen, and small enough for the egg not to dry out. Species that lay their eggs at lower atmospheric pressures (higher altitude) tend to have lower pore area as the diffusion coefficient for gases is increased at altitude. This means that while more oxygen is delivered (good), more water vapour escapes (bad), and the lowered pore area at altitude is therefore necessary to prevent the egg from drying out. Some pollutants (such as aluminium) can increase shell porosity, causing water loss that is too great for the embryo to survive.

A pocket of air can be found between the inner and outer shell membrane on the blunt side of the egg, and this increases in size during the incubation as water is lost to the environment. This is believed to be the principal site of gas exchange, and there tends to be more and larger pores in this area. In the final day before hatching, the chick breaks through the barrier to this air space, and begins breathing with its lungs (pulmonary breathing).

Before pulmonary breathing is possible, gas exchange occurs via a membrane called the corioallantoic membrane. This is inside the inner membrane of the shell. Air diffuses through the shell and outer membrane into the area between membranes, where the capillary-rich corioallantoic membrane allows diffusion of oxygen into the blood and CO2 out of the blood. The blood is transported from the embryo to this membrane through corioallantoic arteries to the corioallantoic membrane, and oxygenated blood returns from the membrane to the embryo through allantoic veins.

The resistance to diffusion is mostly in the hard shell. The membranes have low permeability at first, but they increase in permeability quickly within the first few days after the egg is laid. After this, the egg doesn’t change its diffusion characteristics, and so the concentration of gases depends on the air composition outside of the egg and the metabolism (oxygen uptake, carbon dioxide production) of the embryo inside.

During the incubation period, the embryo grows and its metabolic demands increase. That means it uses more oxygen and produces more CO2 as it grows. As the partial pressures of gases outside of the egg don’t change, this means the gradients of gases between the inside and outside of the shell begins to increase. In short, oxygen concentration goes down and CO2 concentration goes up inside the egg over time. This change in pressure gradients of the gases across the shell and enables more oxygen to flow in (and more CO2 to flow out) as the embryo’s metabolism increases. Despite this, oxygen is reduced and CO2 increased inside the egg in a gradual change that continues almost until hatching. In this way, the ability of gas to diffuse through the shell places an upper limit on the metabolism of the growing embryo.

(Reptile eggs are a different kettle of …err fish, and tends to be more variable than bird’s eggs. Typically, reptiles use underground nests which have reduced oxygen availability, increased CO2 availability and increased humidity compared to the open nests seen in most birds. As discussed above, gas exchange depends in part on atmospheric conditions, and so reptile eggs’ respiration often depends greatly on the properties of the nest (and its interaction with the environment) as well as the properties of the egg itself. )