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Blood Gas Interpretation + Acid/Base Balance


Normal metabolism is associated with continuous production of H+ and CO2 that both tend to decrease pH. Despite this normal tendency towards acidosis, pH remains tightly controlled with very narrow limits (pH 7.35-7.45). Even a small deviation outside this normal range can have numerous detrimental effects on cellular metabolism that translates to tissue/organ dysfunction. Keep in mind that pH < 6.8 and > 7.8 is usually incompatible with life.

Detection of abnormal pH and its underlying causes are essential. The maintenance of normal pH (acid-base homeostasis) is a complex synergy of actions involving the lungs, kidneys, and brain. In addition to pH, two other parameters CO2 and bicarbonate (HCO3-) are equally necessary to accurately identify an acid-base disturbance.

Several global medical technology companies produce, manufacture, and distribute point of care devices for the most critical patients. Measuring of the following analytes pH, PCO2, bicarb, electrolytes, glucose, and hemoglobin are commonplace in those environments. These devices may be present in an ICU setting or even in the back of a critical care ambulance. A few examples include but are not limited to Abbott and their i-STAT cartridges and Siemens Healthineers and their EPOC solution.

This blog is not a formal endorsement of either product, their information is simply being shared in the spirit of education.

Source: Abbott

Source: Siemens Healthineers

Returning to the acid-base topic: A non-exhaustive list of diseases or conditions in which acid-base may be disturbed includes: (COPD, pneumonia, pulmonary edema, pulmonary embolism, asthma, ARDS, renal failure, DKA, liver failure, circulatory shock, drug overdoses, fetal distress and traumatic chest injury)

ABG vs. VBG -

Arterial and venous blood gases provide crucial information about the acid-base status of critically ill ED patients. Arterial blood gases (ABGs) are considered the gold-standard, but they come at a cost. ABGs can be more difficult to obtain, are more painful and require arterial puncture that risks complications. Arterial specimens should be reserved for your severely hypoxemic patients or those with ARDS because VBG's cannot be used to determine oxygenation status.

A peripheral venous blood gas (VBG) can be obtained as the nurse obtains IV access upon patient arrival, requiring no additional sticks or risk of arterial injury. The good news is that for the most part the BIG 3 analytes (pH, PCO2, HCO3-) all correlate pretty well regardless of specimen type. pH is +/- 0.05, PCO2 is +/- 5-8 mm Hg. The PO2 from a venous specimen is usually < 40 mmHg - VBG's are also a much deeper red/purple color, due to the poor oxygen content.

Determination of acid-base disorders require a disciplined approach.

There are four (4) primary acid-base disorders: respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis. For respiratory think lung, for metabolic think kidney. (Respiratory disorders can be subcategorized as acute or chronic disorders. Because one cannot simultaneously hypo- and hyperventilate, two primary respiratory disorders cannot occur simultaneously. In healthy individuals, a primary acid-base disturbance will invoke countervailing compensation by the “other” organ. For example, in metabolic acidosis, lowered [HCO3–] is compensated by increased “lung” ventilation (i.e., ↓PCO2). However, the respiratory compensation is incomplete, and arterial pH/[H+] is not returned to the baseline.

Simple or pure acid-base disturbances follow this rule: PCO2 and [HCO3–] are displaced in the same direction. When the limitations of compensation formulas are exceeded, a second primary disturbance is present.

This quick reference chart (amongst others) can be downloaded for FREE at our website:

February 12, 2024

Author: Joshua Ishmael, MBA, MLS(ASCP)CM, NRP

Pass with PASS, LLC.

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