PADI IDC Study Notes: Air Spaces
Our previous blog, IDC Study Notes: Decompression Theory Part III briefly outlined the concepts of air spaces and lung over-expansion. In this post we’ll take a closer look at lung over-expansion as well as the other relevant air spaces involved in scuba diving.
Our post titled PADI IDC Theory: Physiology The Ear detailed the ins and outs of the ear, equalization, and the dangers presented (called barotrauma). There are, however, a number of other air spaces you should know about, and important terms that describe the dangers posed to them while scuba diving:
The Sinuses are spaces in your head that are connected to your nasal passages. They help moisturize and filter air before it reaches your lungs. Because they are connected to the nasal passages, they typically equalize very easily, but can have some difficulties (known as sinus squeeze) if you are congested. Sinus squeeze will, if a diver continues descending before the sinuses can equalize, will escalate to an injury where blood and fluid fills the space in the sinuses. This will feel like a sharp pain above the eyes, and upon surfacing, the expanding air will push the fluid back out. The diver will surface with blood in their mask. Typically, this condition is not serious and will heal on its own. This can easily be avoided by ending a dive if you encounter a sinus squeeze you cannot equalize.
Your mask is full of air (typically, for comfort reasons) and this space is also affected by changes in depth and pressure. As you descend, your mask will squeeze and feel tight as the air inside it is compressed. Exhaling slightly into your mask will easily fix the problem. If the space is not equalised, tissues will swell and capillaries in the skin and eyes will rupture. This injury appears quite unsightly (typically including blood-red sclera, the white part of your eyes) but usually heals on its own.
Dry suits are one large air space, excluding the air displaced by your body and clothes. As you descend, the water pressure pushes in on the suit. Add air to equalize the suit and avoid dry suit squeeze; dry suits can pinch and cause welts if not equalized properly.
Buoyancy Compensating Devices (BCDs) are, if you aren’t diving with a dry suit, the part of your equipment most drastically affected by depth. As you descend, you must add air to make up for the compression of your exposure suit and other spaces. As you ascend, that air can then expand to up to 5x its original volume, depending on the depth you reached during your dive. Be sure to vent it to maintain neutral buoyancy and prevent a runaway ascent.
Your lungs are the most important air space when scuba diving. In addition to affecting your buoyancy as you breathe, they are highly susceptible to injury when scuba diving. There are a number of life-threatening injuries that can occur if the proper care is not taken.
The air in your lungs, like in all other spaces in your body and equipment, compresses and expands as you change depths. If you do not allow this pressure to equalize, you risk serious injury.
When descending, if you do not inhale, your lungs are susceptible to lung squeeze, a condition that occurs when the volume of your lungs falls below its residual volume (the amount of air left in your lungs after a full exhale). Water pressure at depths in excess of 30m can compress the lungs to an unsafe volume and force fluids into the lungs. It is even more likely if you descend with only partially full, or completely empty lungs. If serious lung squeeze occurs, it can be life threatening. Always be sure to breathe normally during descent and ascent.
During a scuba ascent, air must be allowed to escape the lungs as it expands. In some situations, the passage to the lungs may be blocked due to a chest cold, or by a local blockage due to the loss of surfactant (a common result of smoking). There are four different types of injury that can occur from lung over-expansion. Of the four, air embolism is the worst, and often occurs alongside the other types lung over-expansion of injuries.
Air embolism, also called Arterial Gas Embolism (AGE) is when alveoli and pulmonary capillaries rupture, allowing air into the bloodstream. Because the air can flow into the arteries, this is immediately life threatening. The bubbles can get stuck anywhere in the body, but typically flow through the carotid arteries to the brain, causing stroke-like symptoms: dizziness, confusion, shock, paralysis, personality changes, unconsciousness and even death.
Pneumothorax is a severe injury where the lung ruptures and air is allowed to enter the chest cavity, causing a collapse of the lung. It is accompanied by chest pain and possibly coughing-up blood.
Mediastinal emphysema is when the rupture occurs and air accumulates in the centre of the chest over the heart. It can interfere with circulation, so a diver suffering from mediastinal emphysema may feel faint or short of breath.
Subcutaneous emphysema is when a rupture occurs and air accumulates underneath the skin in soft tissues, near the base of the neck. The skin may crackle like tissue paper when touched, and the diver’s voice may change. A sore throat, swelling, difficulty swallowing, and wheezing are all indicators of subcutaneous emphysema.
First aid treatment for all four types of decompression illnesses (DCI) is the same as Decompression Sickness. Lay the patient down, administer 100% oxygen (Nitrox is OK if it’s all that’s available), and arrange immediate professional medical attention. Read more about decompression sickness in our blog titled Nitrogen, Tables, and Decompression Illness Part III.
The appearance of symptoms of decompression sickness may be delayed (15 minutes to 24hrs) but they typically present within 35-40 minutes. Symptoms of lung over-expansion are usually immediately apparent.
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