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Wednesday, November 5, 2008

ABGs made easy: The Lexicon

Here are the basic terms you will need to understand in order to get a more detailed grasp of arterial blood gases. The explanation here is the pithy version, and this is to keep it easy for you.

Hyperventilation:  Rapid breathing (will blow off PaCO2)

Hypoventilation:  Slow breathing (will cause PaCO2 to rise)

Metabolic: The breakdown of foods within the cells of the body and its transformation to energy.

Respiratory: The exchange of oxygen (PaO2) and carbon dioxide (PaCO2) by means of the lungs. Humans breathe in oxygen and exhale carbon dioxide.

Homeostasis: The human body has the ability to maintain stability within the body. The human body is constantly trying to maintain homeostasis. When it comes to ABGs, the body alters PaCO2 and HCO3 to constantly work to keep pH within its normal range.

pH: A measure of the acidity or alkalinity of a fluid. In the human body this measure is normal between 7.35 and 7.45. The body is constantly trying to maintain pH homeostasis. A level of 7.30 to 7.50 is generally considered acceptable.

HCO3: (Bicarbonate) This acts as a buffer to maintain a normal pH in blood and other body fluids. The acidity is affected by foods or medications that we ingest and the function of the kidneys and lungs. A normal HCO3 is between 22 and 26. Bicarb has a symbiotic relationship with pH. When HCO3 increases, pH increases and becomes more alkalotic. When HCO3 decreases, pH decreases and becomes more acidotic.

PaCO2: (carbon dioxide): It's a byproduct of cellular metabolism released into the arterial bloodstream, carried on hemoglobin to the lungs where it is excreted from the body during respiration. PaCO2 has an inverse relationship with pH. When PaCO2 increases, arterial pH will decrease (become more acidic). When PaCO2 decreases, arterial pH will increase (become more alkalotic).

Oxygen: A colorless, odorless gas that makes up about 21% of the air we breathe. It is needed by the body for metabolism to occur. It is inhaled by the lungs and then carried through the bloodstream on hemoglobin to the cells, where metabolism occurs.

Acidosis: This occurs when there is too much acid in the body or not enough buffers (HCO3) in the blood to balance out the pH. This occurs when pH is abnormally low (less than 7.35). This can be caused by the lungs (not blowing off enough much PaCO2) or by the body's metabolic system (severe kidney disease, diabetic ketoacidosis,).

Alkalosis: This occurs when there is not enough acid in the body, or too many buffers (HCO3). This can be caused by hyperventilation (blowing off PaCO2), or excessive vomiting or diarrhea.

Respiratory Acidosis:  This is acidosis caused by the inability of the lungs to excrete PaCO2, and therefore PaCO2 levels in the blood rise, resulting in a decrease in pH.  The PaCO2 will be higher than 45 and the pH less than 7.35 (CO2 rises = pH to drop).  It is caused by hypoventilation. Treatment involves things that will ventilation (increase rate and depth of breathing), such as beta adrenergic medicine and positive pressure breaths.  Lacking intervention, the patient's body will try to compensate by the metabolic system increasing buffers to absorb the acid.  If the body is unable to compensate, intervention will be necessary. Possible resolution may include beta adrenergics, diuretics, or positive pressure breaths.

Respiratory Alkalosis:  This is alkalosis caused by the lungs blowing of too much PaCO2, and therefore PaCO2 levels in the blood drop, resulting in a rise in pH.  The pH will be less than 35 and the pH will be higher than 7.45 (CO2 drops = Ph rises).  It is caused by hyperventilation.  Treatment involves slowing the respiratory rate down, or decreasing the depth.   The body may try to compensate by slowing the rate of breathing to increase CO2.

Metabolic Acidosis: This is acidosis caused by the metabolic system. This can occur if a person excretes too many buffers (HCO3), and the kidneys are unable to generate enough HCO3, and therefore there aren't enough buffers in the body to balance out the acid.  HCO3 will be less than 22 and pH will be less than 7.35 (HCO3 drops = pH drops).  It can result from kidney failure, vomiting, diahrrea, or the administration of diuretics. The consequences of this can be severe, and may even result in coma or death if not treated. The respiratory system will try to compensate by increasing rate and depth of breathing.  This often results in kussmaul's breathing pattern (rapid and deep).

Ketoacidosis:  This is a special form of metabolic acidosis caused when a diabetic patient is unable to generate enough insulin.  Insulin is used to draw glucose into cells, and glucose is used for energy.  Lacking insulin, the body breaks down fat tissue for energy, and the result is the release of ketones, which is an acid.  The buildup of ketones in the blood causes acidosis that can be treated with the administration of insulin.  This often results in kussmaul's breathing pattern (rapid and deep).

Metabolic Alkalosis:  This is alkalosis caused by the metabolic system storing up too many buffers (HCO3).  The kidneys are producing too many buffers, or not excreting enough.  It is measured by HCO3 greater than 26 and pH greater than 7.45 (HCO3 rises = pH rises). This can result from poor kidney function (kidney failure).  It can also be caused by hypovolemia (loss of blood, shock), chloride depletion, hypokalemia, etc. It can be resolved by fixing the underlying problem (transfusion, administer chloride or calcium, or administering of magnesium).

Compensation: The body is constantly trying to maintain homeostasis. Compensation occurs when the pH is within the normal range of 7.35 to 7.45.
  • If respiratory acidosis occurs, the kidneys will work to excrete HCO3 into the bloodstream until the pH is within normal range.
  • If metabolic acidosis occurs, the lungs will try to increase the respiratory rate to blow off CO2 to balance the pH. Usually here you will see your rapid respiratory rate, almost like they are panting for air.
  • If respiratory alkalosis occurs (Co2 too low and pH too high), the kidneys will work to excrete HCO3 from the body through the urinary tract.
  • If metabolic alkalosis occurs, the lungs will try to compensate by slowing down the respiratory drive to increase CO2
Hypoxemia: A low level of oxygen in your blood. This means that less oxygen will be delivered to your tissues. A low level of oxygen to your tissues is called Hypoxia. According to the Mayo Clinic, "The main symptom of hypoxemia is shortness of breath, but depending on how quickly hypoxemia develops, you may experience a reduced capacity for exercise, fatigue and confusion."

Refractory Hypoxemia: This is when a patient has a low O2 that does not improve with increasing the oxygen. Generally, it is described as a PO2 of 60 torr or less with an FiO2 of 60% or greater.

SpO2: The percentage of oxygen in the inspired air that reaches the blood.

FiO2: Fraction of inspired oxygen. Room air is 21%. With supplemental oxygen, you can increase it up to 100%.

Further reading:

2 comments:

Scott said...

Thanks for the great resource. Can you clarify points 8 & 9? They seem contradictory.

8. Acidosis: ...This can be caused by the lungs (blowing off too much CO2)...

9. Alkalosis: ...This can be caused by hyperventilation (blowing off CO2), or excessive vomiting...

Thanks, Scott
srgravesucd@hotmail.com

Freadom said...

Thanks scott. #8 was a typo. The lungs cause acidosis by not blowing off enough CO2.