Hypobarism and Hyperbarism Flashcards
Hypobarism
- A decrease in atmospheric pressure
- Can result in pathology
- Altitude sickness
- HAPE
- HACE
- Decompression Sickness
Hyperbarism
A increase in atmospheric pressure
Can result in pathology
Can be used as a therapeutic intervention
Henry’s Law
States that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the solution and the solubility coefficient of the gas
Daltons’s Law of Partial Pressure
The total pressure of a mixture of gases is the sum of the pressure that each gas would exert if it were present alone
Pt= P1 + P2 + P3 + …+Pn
Consider air
- 21% oxygen and 79% nitrogen
- At sea level the total pressure is 760 mmHg
- The partial pressure of oxygen then is 0.21 x 760 mmHg = 160 mmHg
- The partial pressure of nitrogen is 0.79 x 760 mmHg = 600 mmHg
- We calculated PiO2 at 3ATA to be 480 mmHg (with FiO2 0.21). When 1.0 then the PiO2 is 2280 mmHg!
Dalton’s Law and HBOT
- If hyperbaric therapy is done with air (21% O2 and 79% N2) both the partial pressure of nitrogen and oxygen are increased in the tissues
- Nitrogen has narcotic effect at increased pressures “Rapture of the Deep”
- 2.5-3 ATA can result in mild euphoria
- 4-6 ATA can result in significant intoxication which could lead to procedures being performed erroneously
- Confirmation by someone outside the chamber may be required
Boyle’s Law
When temperature of a gas is constant, any increase in pressure causes a proportional decrease in the volume of the gas
Thus at a given temperature
P1V1=P2V2
Boyle’s Law and HBOT
- Changes in pressure lead to predictable changes in volume
- When the pressure is doubles then the volume is halved
- Tripling the pressure reduced the volume to 1/3 of original size
- Example- Pneumothorax and “Ascent”
- Consider what will happen to ETT cuff volumes and pressures
Altitude Sickness
- Altitude Sickness; Acute Mountain Sickness – AMS
- Generally speaking, not seen below 10,000 feet
- Generally seen in people who do not properly acclimatize when ascending to high altitudes
- Incidence depends upon three things:
- The altitude the individual is ascending to
- Their rate of ascent – how fast are they climbing
- The individual’s own susceptibility
- The etiology of AMS = Hypoxemia
Altitude Sickness Signs and Symptons
Generally start within the first 6 to 12 hours after entering altitude, increase in severity over first 48 hours, and lessen after 72 hours at altitude
Can include:
- Severe fatigue, progressive muscle strength loss, inability to walk
- Headache, dizziness, confusion, insomnia
- SOB, paroxysmal nocturnal dyspnea, periodic breathing
- May progress to pulmonary edema (HAPE) and cerebral edema (HACE)
- Symptoms are worse at night, when the respiratory drive is lessened
Altitude Sickness Treatment
- Three options: descend, acclimatize, or descend “artificially”
- With severe AMS the CNS is severely affected
- They need to descend immediately or they will die
- “Artificial” Descent
- Portable & inflatable hyperbaric tent (Gamow Bag)
Altitude Sickness
Proper Acclimatization
Generally takes the better part of a week (5 to 6 days ), where the individual ascends at a slow pace to allow for RBC’s due to stimulation of the hormone erythropoietin (due to ¯PaO2 ) and results in increased oxygen carrying capacity
Altitude Sickness Prevention
Ascend to altitude slowly
Climb high and sleep low
Don’t go up until symptoms go down
If symptoms get worse – descend
Keep well hydrated
Don’t over exert yourself
Altitude Sickness Protective Agents
- Diamox (acetazolomide)
- Carbonic anyhydrase inhibitor; diuretic
- Can decrease manifestations of mild edema (e.g. headaches)
- Can induce a relative metabolic acidosis and lead to further respiratory stimulation
- Dexamethasone
- Used prophylactically to prevent HAPE and HACE
- Must be started 5 days prior to ascent
Hyperbarism Indication for Acute Conditions
Decompression Sickness (the “bends”)
Air embolism
CO poisoning
Cyanide poisoning
Thermal injuries
Acute traumatic ischemia
Clostridial gangrene
Necrotizing soft-tissue infection
Ischemic skin graft/flap
Exceptional blood loss
Hyperbarism Indication for Chronic Conditions
Nonhealing wounds
Refractory osteomyelitis
Radiation necrosis
Hyperbarism Contraindications
High fevers
Hypercapnia (> 60 mmHg)
Obstructive airway disease
Optic neuritis
Seizure disorders
Pneumothorax (*Absolute)
Sinusitis
Upper respiratory tract infections
Viral infections
Hyperbarism Primary Physiological Effects
Hyperoxygenation
Reduction in bubble size
Hyperbarism Secondary Physiological Effects
Angiogenesis
Vasoconstriction
Increased leukocyte oxidative killing
Bactericidal/bacterial static for anaerobic/ facultative anaerobic bacteria
Hyperbarism Effects on the Cardiovascular System
HR: decreased
Cardiac output: decreased (due to decreased HR)
BP: possible minimal increase
SVR: Increased (vasoconstriction 2° increased oxygenation)
This results in increased afterload which could be detrimental to patients with CHF
Treatment for DCS/AE can sometimes be at up to 6 ATA for more severe cases, but, as always, depends on the institution’s protocols.
Hyperbarism Effects on Blood Gases
other than the obvious increase PaO2!
Transport of CO2 in the blood occurs as bicarbonate (~75%), carbaminohemoglobin (~20%) and dissolved in the plasma (~5%)
As described by the Haldane Effect, in the presence of oxygen, hemoglobin has less affinity for CO2, thus CO2 is not carried on hemoglobin in a hyperbaric environment
This results in increased levels of CO2 dissolved in the blood
HBOT: Principles of Application
- Uses standardized protocols
- E.g. Gas Bubble Disease
- Eg. Decompression sickness (the “bends”) or iatrogenic gas embolism
- Patient is compressed to 2.8 ATA and then decompressed to 1.9 ATA
- Periods of 100% O2 breathing are interspersed with short “air breaks”
- Minimizes oxygen toxicity
- This treatment schedule proposed for mild DCS
Carbon Monoxide Poisoning
Hemglobin binds with CO 200-250 times more readily than with O2
So patient commonly pressurized to 203 ATA times vary depening on the protocol
Gas Ambient Pressure and Mean Half Life
Air
- Ambient Pressure
- 1
- Mean Half Life
- > 5 hours
Oxygen
- Ambient Pressure
- 1
- Mean Half Life
- 80 min
Oxygen
- Ambient Pressure
- 3
- Mean Half Life
- 23 min
Hyperbarism and Barotrauma
- Can occur in any gas containing organ or cavity in the body
- Middle ear (Most common)
- Sinuses
- GI tract
- Teeth
- If infected or restored
- Lungs
- Patients coached during decompression to avoid breath holding
Hyperbarism and Oxygen Toxicity
Effects may occur at any point in the treatment
Correlated more to the duration of the treatment rather than the number of treatments
Can avoid by giving “ait breaks”
Hyperbarism and CNS Signs and Symptons
Nausea/vomiting
Sweating
Visual changes
Hallucination
Decreased LOC
Seizures
Hyperbarism and Respiratory Signs and Symptons
Dry cough
SOB
Chest pain
Sub sternal
Pulmonary edema
The “Bends”
A mild form of decompression sickness
Caused by the formation of gas bubbles in the body during rapid ascent (normal ascent allows nitrogen to slowly move out of the body)
The “Bends” Signs and Symptons
- Consists of “pain only” manifestations
- The pain occurs in the joints
- Due to bubble accumulation
- Patients avoid or are unable to bend affected joints
- Can also include skin mottling and itching
Decompression Sickness
Potentially lethal
Signs and symptoms will appear gradually
First symptom will tend to appear at 20 min and the second at 2 hours
Decompression Sickness Signs and Symptons
Pain
Mild headache
Blurred vision
Paresthesia
Shortness of breath
Cough
Vertigo
Paralysis
Shock
DCS and Arterial Gas Embolism
Thus, these bubbles generally become trapped in the pulmonary circulation
Arterial gas embolism occurs when there are air bubbles in the arterial system
These usually arise due to pulmonary expansion barotrauma! (eg. From a breath hold ascent)
Can be lethal depending on location of air embolus
Decompression Sickness the Four Cs
Another way to look as the signs and symptoms: The four “C’s“:
Creeps: sensation of tiny insects moving underneath the skin – bubble formation
Cramps: the bends: pain localized in the large joints of the body – severe
Chokes: rare: substernal chest pain, SOB, a dry, non-productive cough – caused by pulmonary air emboli
Collapse: the end stage, death
Nitrogen Narcosis: “Rapture of the Deep”
Pathologydue to the hyperbaric environment (i.e. not during removal from)
Only at risk if breathing air (not oxygen) in a hyperbaric environment (\deep sea diving as well!)
Due to the high partial pressure of N2 in the tissues (specifically in the brain)
Nitrogen Narcosis Clinical Manifestations
Euphoria
Impaired judgment
Decreased coordination
Light-headedness
Inattention
Hallucinations
Coma
Death
Nitrogen Narcosis Treatment
= Decompression
Fully, or at least to a depth where symptoms disappear.
