Blood gases are the dissolved gases in the bloodstream, namely oxygen and carbon dioxide. These gases are measured by taking a sample of blood from an artery, usually the wrist and measuring the partial pressure of the dissolved gases in the sample. Values that fall outside of the normal range, as well as shifts in blood pH that occur as a result are referred to as abnormal blood gases.
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The first step in evaluating a blood gas report is to look at pH. The normal range for arterial blood pH is 7.35 to 7.45. A pH greater than 7.45 indicates alkalosis or more alkaline blood and a pH below 7.35 indicates acidosis or more acidic blood. When the pH is greater than 7.0, the blood is alkaline and when it is less than 7.0, acidic. A pH of 7.0 is neutral. However, pulmonary and metabolic processes act to compensate for the shifts in pH to maintain a normal range of 7.35 - 7.45. Abnormal pH is a pH that is above 7.45 or below 7.35.
The amount of oxygen in the blood is measured in units of partial pressure. Normal partial pressure of oxygen in arterial blood is 80 to 95 mm (millimeters) of Hg (mercury). A partial pressure below 80 indicates respiratory insufficiency and resultant low oxygen level in the blood is referred to as hypoxemia. A partial pressure above 95 mm Hg usually indicates that an individual is on supplemental oxygen and is not usually as concerning.
With respiratory insufficiency often comes a high carbon dioxide levels because the lungs cannot effectively remove this gaseous bi-product of metabolism from the body. As the carbon dioxide level rises in the body, it causes a shift in the pH toward acidosis. The kidneys attempt to compensate for the downward shift in pH by causing an increase in bicarbonate, HCO3, which normally brings the pH back toward 7.0. If carbon dioxide decreases as a result of over-ventilation of the lungs, as a result of hyperventilation or mechanical ventilation, the pH will do the opposite and rise. In some cases, for example in people with brain injury on mechanical ventilation, a low pH is desired because a low carbon dioxide level causes constriction of the vessels in the head and a reduction in brain swelling.
In acidosis, the body converts carbon dioxide to bicarbonate to shift the pH back toward normal by making the blood more alkalotic. In alkalosis, bicarbonate is converted to carbon dioxide to make the blood more acidotic. Normal bicarbonate level is 22 to 26 mEQ (milliequivalents) per liter of blood. A bicarbonate level above 26 is considered abnormally high and a level below 22 is considered abnormally low.
Oxygen saturation is the percentage of hemoglobin that is saturated with oxygen. Oxygen is carried in the blood on the hemoglobin molecules of red blood cells as well as dissolved directly in the blood. The oxygen dissolved in the blood is measured by the partial pressure of oxygen and that which is bound to hemoglobin is measured as oxygen saturation. Normal arterial oxygen saturation is 95 to 99 percent. That is, normally between 95 and 99 percent of the available binding space on hemoglobin molecules is occupied by oxygen. An oxygen saturation below 95 percent usually indicates a need for supplemental oxygen. A value above 99 percent is not usually significant. This may be seen in a patient on supplemental oxygen.
Proper evaluation of a blood gas report to determine abnormalities requires looking at each of the components, not only in relation to each other, but in the context of the patient’s disease process. The body has a remarkable homeostatic mechanism for maintaining a normal pH necessary for physiological processes. The kidneys and lungs work together to maintain acid-base balance. Shifts in blood gas values can occur due to kidney or lung disease or disruption of normal metabolic processes which may occur due to trauma, cardiac arrest, or systemic infection. Only when the body is no longer able to compensate due to a serious or irreversible disease process do changes in blood gases result in an abnormal pH.