Dysbarism means problems associated with high pressure (as in scuba divers), or low pressure (as with high altitude) or due to rapid change in the pressure an individual is exposed to. I work at Prince of Wales Hospital in Sydney, which provides hyperbaric services for the state of NSW. I will give a brief introduction to the case mix of patients who require both HBOT and who need to be in ICU.
Several units for measurement of pressure are in common use, to the delight of examiners for primary Anaesthetic and ICU. It may be worth reviewing those. 1 atmosphere is equal to 101 Kpa, 14.7 psi, 760 mm Hg.
Most pressure gauge measure the difference between the atmospheric pressure, and place where the pressure is being measured. For example when we fill the tyre of a car to 32 psi, or approximately 2 atm, what the gauge is really telling us is that pressure inside the tyre is 2 atm above the atmospheric pressure, or 3 Atmospheric absolute. Similarly, if we have a flat tyre, the gauge reading is 0, but it does not imply that there is vaccum inside the tyre. The pressure inside is the same as outside, or = 1 ATAIn the discussion of Hyperbaric treatment, all pressures will be quoted as atmospheric absolute.ATA refers to the pressure at a place, when the ambient air pressure is added to the pressure. is the ambient pressure including the pressure of the air column above water. Remember, the gauge is usually reading the difference between the pressure at the tyre and the atmospheric pressure.
People who dive would know that at a depth of 10 m, the ambient pressure goes up to 2 ATA. , the pressure increases to 2 ATA.Marina trench is located in the Western Pacific Ocean. The deepest point is called Challenger Deep, which is over 11 km deep. Pressure of the water column there is >1100 ATA
At room temperature, the bottle of coke is pressurised to 2.7 ATA. At such high pressure, the solubility of CO2 in the coke is high, and all of it is in dissolved form. As soon as we open the bottle, the coke in the bottle is not exposed to atmospheric pressure. As a result the solubility of coke to CO2 decreases, and it starts to escape in the form of bubbes. Henry Law states that the solubility of a gas in liquid is directly proportional to the pressure the gas is exposed to. As a result, when a diver is a 20 m below the surface of water, large amount of N is dissolved in the blood and tissues. If the diver now ascends quickly, N2 solubility decreases and it forms bubbles in the blood and tissues, leading to characteristic symptoms of decompression illness.
This is the most beautiful ward in the whole hospital, with sunlight bathing the whole foyer during day time. You step out of the ward, and you are already in the cafeteria. Right behind the bridge, not shown in this picture, is the office of the hyperbaric registrar, perhaps the biggest registrar office in the hospital.Some of the pictures, that I am going to show have been provided the courtesy of Department of Hyperbaric Medicine at POWH. I do not work in the department. Prince of Wales Hospital HBOT unit provides service to the whole of NSW and parts of the southern Pacific.
Increased solubility of oxygen in blood: To put things in perspective, at 3 ATA, when breathing 100% oxygen, there will be enough oxygen dissolved in the blood for use by the tissues, that the mixed venous blood is still 100% saturated. During a routine session of HBOT ar 2-3 ATA, PaO2 of 1000-1500 are achieved.
There are two types of chambers. Monoplace and Multiplace chambers. Monoplace chamber is designed for one patient at any time. 100% oxygen is used to compress the chamber.
The multiplace chamber at POW is the largest in the southern hemisphere. Unlike the clothes we are wearing, and the chairs we are sitting on, this chamber is made in AustraliaRectangular shape for best economy of spaceThe covering ribs are the reinforcements to prevent deformation under high pressure`
The inside of the chamber looks very futuristic. You get the feeling as if you are at the sets of Star-TrekMultiplace chamber can have many patients at the same time. The chamber is compressed with airHyperbaric oxygenation is achieved with the help of supplemental oxygen, provided by Oxygen hoods (which are shown here) or with the help of masks, which are not so different from CPAP masks, and look like the mask used by Tom Cruise in ‘Top Gun’. TV encased in special cover to prevent high concentration of oxygen reaching, to reduce fire hazard. High pressure sprinklers
Compressed with airThe patient is brought to the ICU on a bed compatible with HBOT and the monitor is compatible with HBOT as well. Medical and nursing staff specially trained in HB treatment accompanies the patient at all times. Outlets for Oxygen, air and helioxVery high pressure room for divers with serious injury.Heliox used for treatment. Heliox is used because it is highly soluble in blood. He replaces N2 in the bubbles. Than during decompression, it readily dissolves in the blood and is cleared by the lungs in exhaled air.
Hyperbaric oxygen is often mocked as a treatment modality searching for a disease. The Undersea and hyperbaric Medicine society in the USA publishes the indications for HB treatment, which are updated every few years, based on the available evidence. Many cases of arterial gas embolism are iatrogenic. One of the commonest reason is removal of CVC in sitting position, leading to large venous air embolism, which manages to reach the arterial side through PFO or through the lung. Patients typically present with collapse after removal of CVC. Other cases recently encountered in our unit included air embolism during CPB and accidental air injection through the flush line during cerebral angiogram.Necrotizing fasciitis and CO poisoning are next in the list. Occasionally, we have divers with decompression illness or arterial gas embolism.
CT scan is insensitive for picking up small quantity of air. CXR should be done to rule out pneumothorax in appropriate clinical setting. If transfer by air, the helicopter may have to fly at an altitude of < 1000 feet. Fixed wings chamber should pressurized to as close to atmospheric pressure as possible.
Differentiation between necrotizing soft tissue infection and simple cellulitis is not always obvious. Clues to diagnosis of necrotizing infection are:
send for early G stain and culture, before they dry up. Penicillin, Clindamycin, extended spectrum penicillin, Vancomycin.Clindamycin and Metronidazole for gas gangrene if penicillin allergy
Smokers: 10% is the uppeImportant to know the Time since exposureSupplemental oxygen since exposurer limit of normal in smokers.
Any air filled cavity will shrink when subjected to hyperbaric pressureThe volume will increase when decompressed to 1 ATA pressureEars are most vulnerable to pressure change. All of us must have experienced at one time or other, some pain in the ear at the time of ascend or descent of the aeroplane, particularly if we are suffering from upper respiratory infection.
Difficult oxygenation is not an absolute contraindication. If lung function is so poor that oxygenation is hard to maintain, than use of HBOT may not achieve tissue oxygen concentration high enough to be effective.
Sumesh Arora on Dysbarism
DYSBARISM Dr Sumesh Arora Staff Specialist Department of Intensive Care Medicine Prince of Wales Hospital, Sydney
Conversion between the units of pressure Kilo Psi Standard mm Hg Cm H2O Pascal Atmospher 1000 N/m2 e1 atm 101.325 14.69 1 760 1033.11 psi 6.895 1 0.068 51 70
Basic mechanism of action Increase in the solubility of all gases in blood at high pressure. Henry’s law Attainment of very high oxygen level Displacement of toxins like CO from haemoglobin Kills anaerobic organisms Anti-inflammatory action of hyperbaric oxygen
Monoplace Chamber •One patient at one time •Compressed with 100% oxygen •Greater fire hazard •Maximum pressure of approximately 2 ATA
Commonest indications forHBOT requiring ICU Arterial gas embolism Necrotizing fasciitis and gas gangrene Carbon monoxide poisoning Decompression illness
Arterial Gas EmbolismReferral and transfer for HBOT:Things to remember Diagnosis of AGE is clinical supine position. Lateral decubitus with head down increases ICP High concentration O2 is extremely important. If patient is intubated, use 100% If air transfer: Possibility of exacerbation of injury due to decompression and increase is size of bubbles
Necrotizing fasciitis and gasgangrenePain out of proportion to physical signsIn later stages, development of anaesthesia over the affected area indicate destruction of nervesEasy separation of tissue planes at the time of debridementFoul smelling exudate may suggest anaerobic infectionHypocalcemia may be indicative of extensive fat necrosis
Imaging to diagnose Necrotizingfasciitis?Really early in the course of disease, CT may identify gasMR may overestimate the extent of deep tissue involvement
Treatment before transfer for HBOT Early surgery and debriment. Commonest mistake is to refer for HBOT before debridement Obtain tissue samples Broad spectrum antibiotics
CO poisoning HbCO level Normal < 1% <1% Smokers: 10-12% Mild CO Poisoning: 10-20% Moderate CO poisoning 20-40% Severe poisoning >40%HbCO level does not correlate with severity of toxicity, response to therapy or prognosis
Indications for HBOLoss of consciousnessFocal neurological signsHbCO levels > 25%Pregnancy If neurological symptoms are present, the HbCO level is irrelevant Typical treatment schedule: 2.5 ATA, 90 min, 3 times
Immediate treatment with high concentration oxygen is more important than referral to a unit with HBOTBeneficial effect of HBOT may be unrelated to HbCOlevel, which is undetectable in most patients at start of treatmentEven minor toxicity in pregnant patient should be discussed with the hyperbaric unit due to slower fetal elimination
Practical issues when sendingintubated patient for HBOTGrommets for the tympanic membraneChest drain if pneumothoraxNG on free drainageReplace the air from the ETT cuff with saline. Change to air at the end of the treatment
Contraindications for HBOT Difficult oxygenation/ARDS Pneumothorax Agitation Seizures Congenital spherocytosis Drugs Bleomycin: Absolute contraindication Adriamycin with in previous one week. Cisplatin
Side effects of HBOTBarotrauma: Otic PulmonaryOxygen toxicityRefractory changesCataractAbdominal doscomfort
When in doubt,speak with your referral HBOT unit