blood gases, anion gap, delta gap, bicarbonate gap, serum electrolytes, arterial blood gases, pulmonary physiology, sodium, postassium, chloride, bicarbonate, CO2, serum CO2, metabolic acidosis, metabolic alkalosis, respiratory acidosis, respiratory alkalosis, acid-base balance, Lawrence Martin, M.D., physiology, renal disease, respiratory disease, blood gas analysis, serum electrolytes, venous blood gases, ketoacidosis, lactic acidosis

Lippincott Williams
& Wilkins
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What's New in Blood Gas Interpretation

Lawrence Martin, M.D.
Pulmonary Division, Mt. Sinai Medical Center
Cleveland, Ohio

"What's new in blood gas interpretation" is a better understanding of how to use lab values outside the traditional pH, PaO₂ and PaCO₂. These values include venous electrolytes, end-tidal CO₂, pulse oximetry and co-oximetry measurements. In most cases the tests themselves are not new, but the past decade has witnessed heightened understanding and implementation; as a result, in so far as their widespread use in clinical medicine goes, they are new. In many cases, with intelligent use of these tests, we no longer need arterial blood gases to assess OXYGENATION, VENTILATION and ACID-BASE BALANCE. These lab values will be discussed under 7 discrete topics. "References" (see link at bottom of this page) provides an alphabetical list of articles covering all 7 topics. (Much of this information is from All You Really Need to Know to Interpret Arterial Blood Gases, published 1999 by Lippincott Williams & Wilkins.)

1. Venous CO₂ as a screen for acid-base disorders.
Summary: Venous CO₂, part of the standard electrolyte panel, is ubiquitously measured in hospitalized patients. If venous CO₂ is abnormal, the patient always has some type of acid-base disorder.
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2. Anion and bicarbonate gaps for diagnosing mixed acid-base disorders.
Summary: Calculation of the anion gap (Na - [Cl + CO₂]) and the bicarbonate gap (Na - Cl - 39) can diagnose some mixed acid-base disorders (e.g., combined metabolic acidosis and metabolic alkalosis) without arterial blood gas measurements.
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3. Venous blood gases instead of ABGs for acid-base assessment.
Summary: Venous blood gases from a large vein, in a stable patient, can be used to assess acid-base status.
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4. Venous blood to check for CO or methemoglobin.
Summary: If you suspect CO or methemoglobin toxicity, a venous blood sample will suffice to make the diagnosis, as venous and arterial values are the same.
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5. PetCO₂ in lieu of PaCO₂ in intubated patients.
Summary: Measurement of end-tidal PCO₂, called capnography, has been shown to track PaCO₂ in stable patients. Because the measurement requires a closed system, PetCO₂ monitoring works best in intubated patients. Once a correlation is made between PetCO₂ and PaCO₂, the latter need no longer be measured (or measured as frequently) in intubated patients, including those being weaned from the ventilator.
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6. Simple lung function measurement to obviate need for checking PaCO₂.
Summary: PaCO₂ does not rise in patients with pulmonary impairment (e.g., asthma, COPD) until FEV-1 (or peak flow) is less than 35% of predicted. Below this value the PaCO₂ may be low, normal or high, but above this value the PaCO₂ should not be elevated.
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7. Pulse oximetry instead of PaO₂, SaO₂.
Summary: The pulse oximeter measures SpO₂, which in most situations closely correlates with SaO₂ as measured by the co-oximeter. However, in several situations the pulse oximeter can be dangerously misleading, and should not be used without blood gas confirmation.
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References
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Return to Table of Contents for All You Really Need to Know to Interpret Arterial Blood Gases
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Lippincott Williams & Wilkins Amazon.com

What's New in Blood Gas Interpretation

1. Venous CO2 as a screen for acid-base disorders.

2. Anion and bicarbonate gaps for diagnosing mixed acid-base disorders.

3. Venous blood gases instead of ABGs for acid-base assessment.

4. Venous blood to check for CO or methemoglobin.

5. PetCO2 in lieu of PaCO2 in intubated patients.

6. Simple lung function measurement to obviate need for checking PaCO2.

7. Pulse oximetry instead of PaO2, SaO2.

Lippincott Williams
& Wilkins
Amazon.com

1. Venous CO₂ as a screen for acid-base disorders.

5. PetCO₂ in lieu of PaCO₂ in intubated patients.

6. Simple lung function measurement to obviate need for checking PaCO₂.

7. Pulse oximetry instead of PaO₂, SaO₂.