DIVING MEDICINE & DECOMPRESSION SICKNESSA SELF DIRECTED LEARNING MODULE FOR MEDICAL STUDENTS

Mike McGovernSwansea College Of Medicine

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IntroductionDiving is an enjoyable recreational past-time for many, and is also an important commercial activity in the fields of engineering, research & fishing. Whilst usually a safe activity, the pressure of the underwater environment has many physiological effects on the body, which can lead to pathology. This SDL will introduce you to the basic physical principles underpinning diving, and give you a basic understanding of one pathology arising from them.

This SDL The aim of this SDL module is to allow medical

students to gain a basic understanding of the principles of diving medicine

No previous experience or knowledge of diving or its related medicine is required

This SDL will use an interactive slide show to teach the basic principles, and an interactive quiz to test learning

Learning objectives will be used throughout the SDL to guide your learning

An optional soundtrack is included for your enjoyment!

Navigating This SDLThis SDL is designed to be viewed in order. You should only need to click on the screen to continue learning. However, should you need to navigate, buttons are provided in the top right corner of most pages. These will allow you to return to the last slide you viewed, go back to the previous slide, go directly to a contents slide, or skip to the next slide respectively:

Hyperlinks are also provided in a number of places to allow you to refer back to relevant information covered earlier in the module. Hyperlinks look like this.

Contents

Introduction – This SDL - Navigating This SDL – Learning Objectives PART ONE – The Physics

Atmospheric Pressure – Pressure Underwater – The Liquid Phase & Pascal’s Principle – The Gas Phase & Boyle’s Law – Dalton’s Law – Henry’s Law

Re-cap PART TWO – Types Of Diving

Vessel Diving - Snorkelling – Breath Hold Diving – SCUBA Diving Re-cap

PART THREE – Decompression Sickness Decompression Sickness – Applying Henry’s Law – Pathophysiology – Presentation –

Preventing Decompression Sickness Re-cap

PART FOUR – Management Acute Management – Recompression Therapy – Co-morbidities & Complications Re-cap

TEST YOURSELF Question 1 – Question 2 – Question 3 – Question 4 – Question 5 – Question 6

APPENDICES Further Reading - References

This SDL is designed to be completed in order. However, this contents page gives you an idea of what is in store . Please click to the next slide to continue.

Learning ObjectivesThe learning objectives this SDL will cover are: To understand the basic physical laws

underpinning diving medicine To understand the different types of diving,

and how physical laws apply to them To understand the pathophysiology of

decompression sickness To know the principles of management of

decompression sickness, and link these to its pathophysiology

Part 1 – The Physics

Learning Objectives To understand the basic physical laws

underpinning diving medicine To understand the different types of diving,

and how physical laws apply to them To understand the pathophysiology of

decompression sickness To know the principles of management of

decompression sickness, and link these to its pathophysiology

Atmospheric PressureAt sea level, our bodies have ~8km of atmosphere above them. The air molecules that make up this atmosphere are acted upon by gravity, causing their mass to exert a pressure on our bodies below them. Therefore, one can imagine each of us to be supporting a tower of air molecules 8 km high.

Although atmospheric pressure does vary by small amounts dependent on climatic conditions, the weight exerted by the atmosphere on an area of ground at sea level, of area 1m2, is approximately 10,000kg. This value of pressure (10,000 kg m-2) is thus referred to as 1 atmospheric absolute (1 ATA), or 1 atmospheric pressure.

1 AT

A

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Pressure UnderwaterSea water is approximately 800 times more dense than air. Therefore, it exerts much greater pressure on the body of a diver.

For every 10m below the surface a person dives, he is subjected to an additional pressure of 1ATA. Therefore, at 30m, a diver will experience a pressure of 4 ATA (1 ATA exerted by the atmosphere, & 3 ATA exerted by the 30m of water above him).

This added pressure is of particular relevance to the diver’s respiratory system, where the (compressible) gas phase meets the (relatively incompressible) liquid phase.

1 AT

A1A

TA

Sea Level

10 m

The Liquid Phase & Pascal’s Principle

Liquids are made up of particles that are in close contact with one another, but that are able to move over one another, or flow. The close contact of these particles means that liquids are relatively incompressible, except under extremely high pressure.Pascal’s Principle states that ‘Pressure applied to a liquid will be transmitted equally throughout the liquid’.

As the human body is composed mainly of liquid, Pascal’s Principle explains its behaviour under pressure. A diver’s body transmits the pressure of the water acting upon it equally throughout its liquid phase, which does not compress. However, the body is also composed of a number of gas filled spaces, the largest of which is the lungs…

Liquid

Despite pressure being applied, the liquid is incompressible as the particles within it are so tightly packed together.

The pressure that is applied is spread evenly throughout the liquid.

Click the arrow to put the

liquid under pressure

The Gas Phase & Boyle’s Law

Gases are made up of particles separated from each other with large amounts of space between them. The particles have negligible interactions with each other, and move randomly. The energy with which they move (or their temperature) determines the pressure they exert on the walls of any container they are within.As a result of the large amount of space between particles, gases are easily compressed by even relatively low pressures.Boyle’s Law states that ‘at constant temperature, the volume of a gas varies inversely with the pressure’.

The lungs are the largest gas filled spaces within the body, and are surrounded by tissue composed mainly of matter in the liquid phase. As Pascal’s Principle makes clear, this tissue is relatively incompressible, and any pressure applied to it is distributed evenly throughout it. Thus, when diving, pressure exerted on the body is distributed throughout the tissue and exerted on gas, i.e. the lungs. If gas within the lungs remains at the same pressure, Boyle’s Law makes clear the lungs will reduce in volume at a rate inversely proportional to the pressure, which is itself proportional to the depth.

Gas

Gas compress easily due to the large amount of space between particles. This space is reduced with increased pressure.

Provided temperature remains constant, the volume of the gas will be inversely proportional to the pressure applied to it.

Click the arrow to put the

gases under pressure

Gas

Dalton’s LawTwo more laws are particularly important to diving medicine, especially for an understanding of respiration in divers.Firstly, Dalton’s Law states that ‘The partial pressure of a gas in a mixture is proportional to its percentage by volume in the mixture’.

Gas

= O2 molecule= N2 molecule

Q. If the diagram to the left illustrates a mixture of gas at a total pressure of 1kPa, what is the partial pressure of N2 in the mixture?A. As there are 5 N2 molecules & 5 O2 molecules in the mixture shown, the percentage volume of N2 is 50%.The partial pressure of N must therefore be the total pressure multiplied by the percentage by volume, or 1kPa x 0.5 = 0.5kPa

CLICK FOR THE ANSWER

Dalton’s Law and the concept of partial pressures are important for an understanding of how gases dissolve in liquids – a process which is governed by Henry’s Law...

Henry’s LawHenry’s Law states that ‘At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid’.

Therefore, the dissolution of respiratory gases in a diver’s blood is directly related to the partial pressure of those gases within the diver’s lungs (or specifically his alveoli). This will become important as we move on to discuss pathology in part 3.

Parti

al P

ress

ure

Of G

as

Amount Of Gas In Solution

Gas In Gaseous FormGas In Soltn

As the partial pressure increases, the gas in gaseous form is compressed. Thus more enters solution to decrease pressure, until an equilibrium is again reached. As pressure falls, the equilibrium moves back towards the right. Therefore gas leaves solution & returns to its gaseous form until the new equilibrium point is reached.

CLICK TO ANIMATE

Re-cap of Part 1- The Physics

So far, we have learned: 1ATA (Atmospheric Absolute) is roughly equivalent to the pressure exerted by the atmosphere at sea-level For every 10m below sea-level a person dives, pressure increases by a further 1ATALiquids, which the majority of matter in a diver’s tissue is composed of, are relatively incompressible & distribute pressure evenly throughout the liquidGases, which a number of spaces (including the lungs) within a diver’s body are filled with, are easily compressible

A gas’ volume is inversely proportional to the pressure exerted upon it

How to calculate the partial pressure of a gas in a mixture, and that the partial pressure of a gas within a diver’s lungs is related to the amount of that gas that will dissolve in his blood

Part 2 – Types Of Diving

Learning Objectives To understand the basic physical laws

underpinning diving medicine To understand the different types of diving,

and how physical laws apply to them To understand the pathophysiology of

decompression sickness To know the principles of management of

decompression sickness, and link these to its pathophysiology

Vessel DivingVessel diving refers to diving in thick-walled vessels, including submarines & rigid suits.Vessel divers are protected from ambient pressure (i.e. the pressure that would be otherwise exerted on them by the underwater environment) by the rigid walls of their vessels. These walls maintain a pressure of ~1ATA within the vessel no matter what the pressure of the outside environment may be. Therefore, vessel divers are not subject to the changes in pressure and their physiological effects that other divers are.

4 ATA

4 AT

A

4 ATA 1 ATA

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SnorkellingSnorkelers use a long tube to allow them to breathe air from sea level whilst swimming a short distance below the sea’s surface. Snorkelers therefore breathe (& fill their lungs with) air at normal atmospheric pressure, but are subject to the increased pressure of the water around them.

The difference between the pressure of the air snorkelers breathe & the pressure of the water around them limits the depths they are able to reach to just a few metres :- As the pressure of the water around the snorkeler rises, it is increasingly difficult for him/her to enlarge the thorax. This makes it more & more difficult to reduce pressure in the lungs to less than 1ATA - the level required to suck in air from the surface & inspire.

Breath-hold DivingBreath-hold diving is the practice of diving whilst holding one’s breath. Recreational free-divers & pearl divers are able to reach depths of over 70m on one breath, helped by the diving response; an oxygen-conserving reflex on immersion of the face that triggers apnoea, bradycardia & vasoconstriction. As breath hold divers descend, the increased pressure of the water around them causes their lungs to shrink in volume, and the diaphragm is sucked upwards into the thorax. Thus, the air inside their lungs is compressed, and its pressure increases to that of the environment around them.

Depth1 ATA1 ATA

3 ATA

5 ATA

3 ATA

5 ATA

CLICK TO ANIMATE

0m

40m

20m

SCUBA DivingSCUBA Diving utilises special breathing apparatus that supplies air (or other gas mixtures) at ambient pressure. This allows scuba divers to maintain a constant lung volume underwater; as they descend, their breathing apparatus allows them to increase the pressure of air in their lungs to the level of the water around them every time they take a breath.

However, as the pressure of the air inside the lungs increases, the partial pressures of the gases within them also increase proportionally, in accordance with Dalton’s Law. This in turn effects how these gases dissolve in blood, as made clear by Henry’s Law. This can lead to pathology, as we will learn in the next part of the SDL.

Re-cap of Part 2- Types Of Diving

In part 2, we have learned about different types of diving, including: Vessel Diving in which divers are not subjected to pressure changes, as the rigid walls of the vessel containing them protects divers from ambient pressure. Snorkelling where divers breathe air at 1 ATA from the surface, but are subjected to higher pressures by the water around them. This makes inspiration impossible at depths greater than a few metres. Breath-hold diving where divers hold a constant amount of air in their lungs. As divers descend, the ambient pressure increases. The volume of the diver’s lungs therefore decrease, with a corresponding increase in pressure of the air inside them to match ambient pressure. Scuba diving where divers breathe air compressed to ambient pressure. This allows scuba divers to maintain a constant lung volume, with the pressure of the air inside the lungs varying with changes in ambient pressure.

Part 3 – Decompression Sickness

Learning Objectives To understand the basic physical laws

underpinning diving medicine To understand the different types of diving,

and how physical laws apply to them To understand the pathophysiology of

decompression sickness To know the principles of management of

decompression sickness, and link these to its pathophysiology

Decompression SicknessDecompression sickness (DCS) arises in persons moving from a high pressure to a low pressure environment, and is caused by gas (principally nitrogen) dissolved in the body leaving solution and forming bubbles. Therefore, in divers it occurs on or shortly after ascent.

Decompression sickness principally occurs in SCUBA divers, who spend long periods at depth. However, the disease may also occur in breath hold divers who, as we have seen previously, also expose their respiratory system to high pressure. Decompression sickness may also occur at altitude, most commonly when flying in unpressurised aircraft.

Decompression sickness may also be referred to as:The BendsThe term arose from the strange shape sufferers are thrown into by the resulting musculoskeletal pain, & its resemblance to ‘The Grecian Bend’, a Victorian dance move.

Caisson DiseaseThe term arises from the first cases of DCS, which were noted amongst bridge builders who had worked in pressurised caissons –

pressurised vessels used to allow the building of bridge supports on river beds.

CLICK TO DISCOVER THE TERMS’ HISTORIES

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Applying Henry’s LawReference to Henry’s Law tells us that the solution of a gas in a liquid is dependent on:• The partial pressure of that gas• The solubility of that gas in that liquid• Time, which must be allowed for any new equilibrium to be reached once a

new pressure has been established.These three factors are important in explaining how decompression sickness occurs. Click to find out more:Partial

PressureAt high partial pressures of N2, (such as on deep dives), large amounts of N2 dissolve

in the blood & distribute through the body in a

dissolved state. When the diver ascends, pressure on

the liquids of the body decreases, and N2 is able to leave solution. If the rate of

pressure change is too rapid, N2 will accumulate

before it can be eliminated by the lungs. It may form

bubbles in cells, in the circulation & in other

spaces, causing pathology.

SolubilityNitrogen is the main gas

responsible for decompression sickness rather than oxygen, due to its high solubility in

blood and the large fraction of air it makes up . Therefore, more nitrogen dissolves in

blood & the body’s liquid compartments at high pressure, and more is able to leave solution

and form bubbles/emboli.

Divers are more likely to suffer from

decompression sickness after long

dives at high pressure, as nitrogen has more time to dissolve in the

high pressure conditions

experienced. This explains why

decompression sickness is more

common in SCUBA divers, where divers

may spend hours under water, as opposed to breath-hold divers,

where dives are very short in duration.

Time

PathophysiologyThe formation of nitrogen bubbles within the body cause pathology through a number of mechanisms. These include:

EmboliBubbles forming

in the blood can disrupt blood flow

Nitrogen BubblesIschaemiaBlood flow

obstruction can lead to ischaemia

& infarction of tissue/organs

Cell DisruptionBubbles forming within cells can damage their

internal structure & lead to loss of

function

Large bubbles forming in the vasculature may cause

vessels to rupture. Similarly, bubbles forming in other spaces (e.g. The

lungs) may cause them to rupture

Tissue Rupture

CoagulationThe gas:blood interface

can activate clotting factors, leading to

intravascular coagulation

Mechanical CompressionLarge bubbles

may compress surrounding structures,

causing damage & leading to loss

of function

InflammationThe gas: tissue interface

can activate complement, other

inflammatory pathways & inflammatory cells, leading to an acute

inflammatory response

CLICK TO DISCOVER EACH PATHOLOGICAL MECHANISM

PresentationSymptoms of decompression sickness may develop from minutes to 48 hours (or occasionally longer) after an ascent, and their progression may be continuous or relapsing & remitting.

Due to the multiple pathological mechanisms at play, symptoms are numerous & varied. Some of the most common symptoms are listed below. Click on them to find more information.Musculoskeletal

PainThe most common symptom, occurring in up to 80% of patients. May range from ‘niggling aches’ to severe

joint pain & muscle splinting. Most commonly affects the

upper limbs.

Skin RashesMottling & marbling of the skin

may occur, occasionally progressing to an ‘orange peel’-

like discolouration. May be associated with itching or burning

sensations.

Neurological SymptomsSymptoms are varied, and

may be dynamic. Spinal symptoms are most common, including lower back pain &

lower body paraesthesia /paralysis. Other symptoms

include psychological changes, intellectual/visual

impairment and ataxia.

Respiratory SymptomsChest pain & non-productive

coughing are the most common respiratory

symptoms, and may progress to severe dyspnoea & respiratory distress.

Haemoptysis may also occur.

Aural SymptomsLabyrinthine symptoms,

including nausea, vertigo & nystagmus, may be combined

with hearing symptoms, including hearing loss &

tinnitus

Preventing Decompression Sickness

Divers can avoided decompression sickness by taking regular decompression stops when ascending from depth. Divers ascend a few metres, then remain at that depth for a period of time. This allows nitrogen to come out of solution at a rate slow enough to prevent it accumulating as bubbles within the body. The nitrogen is then eliminated by the lungs, as shown in the animation to the right.

Divers use decompression tables to calculate the number, depth & length of decompression stops required after a dive. The deeper & longer a dive has been, the more nitrogen will be dissolved in the body. Hence, more and longer stops are required to allow the larger volumes of nitrogen dissolved in the body to be eliminated.

Decompression tables are calculated based on models of gas solution in various tissue compartments ultimately derived from Henry’s Law. Staying within the limits of the tables will, in the vast majority of cases, prevent decompression sickness. However, the tables cannot predict all physiological factors, and decompression sickness may still occur in individuals who have obeyed the tables.

Depth

CLICK HERE TO ANIMATE

0m

40m

20m

N2 N2

Re-cap of Part 3- Decompression Sickness

In part 3, we have learned about the pathophysiology & presentation of DCS, including: That nitrogen bubbles form in blood & tissues Snorkelling where divers breathe air at 1 ATA from the surface, but are subjected to higher pressures by the water around them. This makes inspiration impossible at depths greater than a few metres. Breath-hold diving where divers hold a constant amount of air in their lungs. As divers descend, the ambient pressure increases. The volume of the diver’s lungs therefore decrease, with a corresponding increase in pressure of the air inside them to match ambient pressure. Scuba diving where divers breathe air compressed to ambient pressure. This allows scuba divers to maintain a constant lung volume, with the pressure of the air inside the lungs varying with changes in ambient pressure.

Part 4 – Management Of DCS

Learning Objectives To understand the basic physical laws

underpinning diving medicine To understand the different types of diving,

and how physical laws apply to them To understand the pathophysiology of

decompression sickness To know the principles of management of

decompression sickness, and link these to its pathophysiology

Acute ManagementSevere decompression sickness can be a medical emergency. Prompt resuscitation measures can therefore save lives.Basic principles of acute management are listed below. Click on each to discover more.Administration Of

O2100% oxygen should be administered via a non-

rebreathe mask. This will improve O2 saturations and thus supply to any

ischaemic tissues.

Fluid ResuscitationHypovolaemia often

occurs in divers suffering decompression sickness

due to dehydration, increased capillary

permeability caused by inflammation & increased

diuresis due to cold. IV crystalloids should therefore be given .

Lie Patient HorizontallyLying patient horizontally

limits air emboli rising, and therefore may reduce

the chances of emboli reaching the brain. The recovery position should be utilised in the event of

vomiting.

WarmingWetsuits should be

removed & passive re-warming of patients

should be commenced to prevent hypothermia.

However, the definitive treatment for decompression sickness is recompression therapy. Patients should therefore be evacuated to a recompression chamber as soon as is possible. If flying, care should be taken to maintain low altitude, so that the disease is not compounded by a further drop in pressure.

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Recompression TherapyRecompression therapy is the definitive treatment for decompression sickness.

Patients are placed in a chamber, and pressure is increased to that of the deepest part of the dive, or until symptoms are relieved. This causes nitrogen bubbles within the vasculature & tissues to first shrink and then dissolve.

Pressure is then slowly returned to 1ATA, at a rate that allows excess nitrogen to be eliminated by the lungs. This process may take hours or even days.

During the treatment, patients usually breathe 100% oxygen delivered at the pressure within the chamber. This prevents further nitrogen (or other inert gases) entering the bloodstream & tissues, and creates a diffusion gradient across the alveolar membrane that accelerates excess nitrogen elimination as the controlled ‘ascent’ occurs.

N2 N2

N2

N2

CLICK TO ANIMATE

Pressure1 ATA

5 ATA

3ATA

Co-morbidities & Complications

Pulmonary Over-Pressurisation

SyndromePOPS occurs on fast ascents, when divers who

have lungs full of compressed air fail to exhale. As ambient

pressure reduces, the lungs expand and rupture, leading to

pneumothorax, pneumopericardium & surgical emphysema.

Large air emboli may also pass into the pulmonary

arteries & on into the arterial circulation. These usually pass cephalad if the diver is upright, &

may occlude the arteries supplying the brain

producing devastating neurological symptoms.

Oxygen ToxicityBreathing high partial

pressures of O2 for prolonged time periods can produce acute CNS

toxicity. Symptoms include behaviour

disturbance, hallucinations, syncope &

convulsions. Oxygen toxicity may be fatal

when underwater, and so care must be made to

reduce O2 content of gas mixtures for deep dives.

In recompression chambers, O2 toxicity can

be treated by withdrawing O2 &

administering benzodiazepines to

prevent convulsions.

DehydrationDivers often become

dehydrated due to salt water, the warm climates preferred by divers, and increased diuresis due to low water temperatures.

This may aggravate symptoms & contribute to

any hypovolaemia that may occur as a result of

DCS.

SIRS & DICAs previously discussed,

nitrogen bubbles may activate inflammatory &

clotting mechanisms. These can lead to

widespread inflammation & coagulation as occurs

in Severe Respiratory Response Syndrome &

Disseminated Intravascular Coagulation

Numerous co-morbidities & complications can occur in conjunction with, or as a result of decompression sickness. Some examples are included below. Click on each to find out more.

Re-cap of Part 4 – Management Of DCS

In part 4, we have learned about the management of DCS, including: Its Acute Management – Lying the patient flat, administration of oxygen, fluid resuscitation & passive re-warming Recompression Treatment – Increasing ambient pressure results in nitrogen bubbles shrinking & eventually dissolving. Slowly reducing the pressure as the patient breathes oxygen then allows nitrogen to be eliminated from the lungs without bubbles accumulating Some of the Co-morbidities & Complications that may exist in divers suffering from decompression sickness, including POPs, DIC, SIRS & oxygen toxicity.

Test Yourself

Test Yourself

The following section will allow you to check your learning with a number of

interactive questions

Question 1

At a depth of 50m below sea level, what pressure will a diver experience?

CLICK ON THE CORRECT ANSWER

1 ATA

7 ATA 6 ATA

5 ATA

Question 2

In a gas mixture of 10% Oxygen, 20% Nitrogen and 70% Helium at a total pressure of 1000kPa, what is the partial pressure of

Oxygen?CLICK ON THE CORRECT ANSWER10kPa

300 kPa

100 kPa

150 kPa

Question 3

In which type of diving does a diver hold a fixed amount of air in their lungs which varies in volume according to depth?

CLICK ON THE CORRECT ANSWER

Breath-hold Diving

Snorkelling Scuba Diving

Vessel Diving

Question 4

A diver is most likely to suffer from decompression sickness if he has...

CLICK ON THE BEST ANSWER

Made a breath-hold dive to 75m with no decompression stops

on ascentMade a 2 hour SCUBA dive to 40m with no decompression stops

on ascent

Made a 6 hour dive in a submarine with no decompression stops

on ascent

Made a 20 minute SCUBA dive to 50m

with no decompression stops

Question 5

When treating a patient with decompression sickness, what intervention is NOT

appropriate?CLICK ON THE CORRECT ANSWER

Administer 100% O To

Maintain SatsPassively Re-

Warm The Diver To Avoid

Hypothermia

Keep The Diver Sat Up To Aid

Breathing

Give IV Fluids To Counter

Hypovolaemia

Question 6

Symptoms of Decompression Syndrome May Include...

CLICK ON THE BEST ANSWER

Urinary & Faecal Incontinence

Vomiting & Dizziness

Visual Disturbance

Shoulder Pain

Further ReadingShould you wish to find out more about

diving medicine: Dave Williams’ article provides a useful introduction to the topic The Scottish Diving Medicine Website contains a host of useful information Bennett & Elliott’s ‘Physiology & Medicine Of Diving' provides an in-depth look at all aspects of diving medicine

Thank You

Thank you for taking the time to complete this SDL.

I hope you have enjoyed it.

References Sources Of Information:

Pulley, SA. ‘Decompression Sickness’. Medscape Reference. 2009. Available at http://emedicine.medscape.com/article/769717-overview - Accessed 3/6/11

Ward, JP; Ward, J; Leach, RM; Wiener, CM. The Respiratory System At A Glance (2nd ed.). 2006; Blackwell, Oxford

Williams, D. ‘Bubble Trouble: An Introduction To Diving Medicine’. Bristish Journal Of Anaesthesia 2002; 2(5) pp.144-147

References (cont.) Images – All images from clip art/self-designed

except: ‘Red Bubbles I’ by Ting Yi Chan ‘The Grecian Bend’ by Thomas Worth Caisson Schematic by Yk Times ‘Decompressed’ by Lens Adventurer

N.B. All images referenced are free of copyright restrictions Music

‘Under Pressure’ by David Bowie & Queen ‘Apply Some Pressure’ by Maximo Park ‘Pressure Drop’ by Toots & The Maytals ‘Doctor Robert’ by The Beatles