Blood gas analysis is an important diagnostic tool and an essential part of managing a patient’s oxygenation status and acid-base balance. An understanding of the basic components of blood gas analysis will help you on several areas of the exam, including the anatomy and physiology section, and the medical equipment function and operation section.
Blood gas analysis is important as it can indicate a problem long before vital signs become affected. It also shows respiratory and kidney function and helps determine the method of treatment. The basic measurements of arterial blood gas analysis are pH (alkalinity or acidity of the blood) and the gaseous components of the blood, pCO2 and pO2 (partial pressures of carbon dioxide and oxygen in the blood). Plasma bicarbonate (HCO3-) is usually derived from a calculation. Some modern analyzers also measure electrolytes and hemoglobin components, but those will not be covered in this article.
|Illustration courtesy of Stephen Lower/commons.wikimedia.org/~|
Most BMETs are somewhat familiar with the term pH as it pertains to blood. It is the measurement of the relative concentrations of hydrogen (H+) ions in the blood. In fact, the pH number itself is inversely proportional to the number of hydrogen ions in the blood. The more hydrogen ions present, the lower the pH number will be. This is because the pH number itself is derived from an exponent based on the molar concentration of hydrogen ions. In other words, lemon juice, with an approximate pH of 2, has a higher concentration of hydrogen ions (10-2) than ammonia with an approximate pH of 11 (10-11). The pH scale ranges from 0 (very acidic) to 14 (very alkaline). As you can see from the illustration, distilled water has a neutral pH of 7 (neither acid nor base) and blood has a normal pH range of 7.35 to 7.45, making blood slightly alkaline.
The body accomplishes the regulation of pH in three ways: through the kidneys, lungs, and chemical buffers. Chemical buffers bind to or release hydrogen ions depending on the pH level of the blood. The lungs regulate pH by controlling how much CO2 is in the blood. When pH starts to increase or decrease out of range, the brain either depresses or stimulates the respiratory system so more CO2 and hydrogen accumulate in the blood (breathe slower—hypoventilation) or more CO2 is expelled (breathe faster—hyperventilation). In the lungs, CO2 combines with water (H2O) to form carbonic acid (H2CO3). The blood pH will change according to the level of carbonic acid present.
The kidneys also help regulate blood pH by excreting or retaining plasma bicarbonate (HCO3-). As blood pH decreases, kidneys retain bicarbonate; and as pH rises, the kidneys excrete bicarbonate through the urine. While the kidneys have the most effect on blood pH, their effect also take the longest.
Two other main measurements involved in basic blood gas analysis are pCO2 and pO2—partial pressures of carbon dioxide and oxygen. These pressures are measured in millimeters of mercury (mmHg). Partial pressures relate to Dalton’s Law, which states that in a mixture of gases, the pressure of one gas is proportional to its concentration. So, the total pressure in a mixture of gases is equal to the pressures of the individual gases: Ptotal = P1 + P2 + P3 +. An important point to remember is that at the point of measurement, gases are saturated with water vapor at 37°C (temperature-dependent measurements). The pressure of water vapor at 37°C is 47 mmHg, and this amount of pressure has to be deducted from the total amount before dry gas partial pressure can be derived. So partial pressure of a gas is equal to the barometric pressure minus the water vapor pressure times the percentage of gas concentration.
The partial pressure of carbon dioxide (pCO2) is the acid half of the acid-base balance. It is the amount of carbon dioxide dissolved in arterial blood. The normal range for pCO2 is 35 to 45 mmHg. Combined with the pH measurement, the pCO2 measurement is a key diagnostic tool in determining metabolic and respiratory disorders.
Plasma bicarbonate (HCO3-) is regulated by the kidneys and is the base half of the acid-base balance. It has a normal range of 22 to 26 mEq/liter.
The partial pressure of oxygen (pO2) is the amount of oxygen that is dissolved in arterial blood. It has a normal range of 80 to 100 mmHg. An important measurement, pO2 can help respiratory therapists determine a course of treatment.
Backing up just a bit, it is important to go over a few terms such as acidosis and alkalosis. When the pH of arterial blood is less than 7.35, the blood is considered to be too acidic and a condition called acidosis exists. When the pH is above 7.45, blood is considered to be too alkaline and a condition called alkalosis exists. The key measurements that determine whether the condition is caused by a respiratory disorder (respiratory acidosis or respiratory alkalosis) or a kidney disorder (metabolic acidosis or metabolic alkalosis) are the pCO2 and pH measurements (along with HCO3– and BE calculations).
For example, let’s say we have a pH measurement of 7.25, a pCO2 measurement of 60 mmHg, and an HCO3– value of 24 mEq/liter. The HCO3– value falls in the normal range, but the pH is low (acidic) and the pCO2 measurement is higher than the normal 40 mmHg level. This rise in PCO2 indicates respiratory failure and might indicate respiratory acidosis. The kidneys will most likely compensate for this imbalance, and subsequent HCO3– values would be higher (say 34 mEq/liter) and pH would return to normal levels.
Now let’s say we have a pH measurement of 7.7, a pCO2 measurement of 20 mmHg, and a HCO3– value of 24 mEq/liter. The pH is high out of range, the pCO2 is low out of range, and the HCO3– (bicarbonate) is normal. This condition may indicate respiratory alkalosis and may be caused by hyperventilation.
Read past ICC Prep and CCE Prep articles in the archives.
Metabolic acidosis would be indicated by a low-out-of-range pH (let’s say 7.2), a normal pCO2 level, and a low-out-of- range HCO3— value (let’s say 12.5 mEq/liter). Metabolic alkalosis would be indicated by a high-out-of-range pH reading (let’s use 7.55), a normal pCO2 reading, and a high-out-of-range HCO3– reading (let’s use 36 mEq/liter).
As you can see, when the kidneys are at fault, the imbalance is called metabolic acidosis or alkalosis; and when the respiratory system is at fault, the imbalance is called respiratory acidosis or alkalosis.
For the CBET exam, the specific diagnostic levels are not critical. What is important is the recognition of terms, knowing the normal levels of pH, pCO2, pO2, and HCO3– in arterial blood, and a basic understanding of how systems work together.
I am honored to write this column with my esteemed colleague, John Noblitt, BS, CBET, and I hope you find it useful in preparing for the CBET exam. As John mentioned, Dave Harrington, PhD, certainly left some big shoes to fill. I have enjoyed reading his column for many years.
Roger A. Bowles, EdD, CBET, is professor/department chair of biomedical equipment technology at Texas State Technical College, Waco, Tex; and a member of 24×7’s editorial advisory board. For more information, contact .
- The normal range for pH in arterial blood is____?
- 7.00 to 7.25
- 7.25 to 7.35
- 7.35 to 7.45
- 7.45 to 7.55
- Which of the following has the most effect in the regulation of blood pH?
- Renal system
- Respiratory system
- Endocrine system
- Chemical buffers
- The kidneys regulate the amount of____in the blood.
- None of the above
- Which of the following would be a normal pCO2 measurement?
- 20 mmHg
- 30 mmHg
- 40 mmHg
- 50 mmHg
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