Non-Invasive Blood Gas Interpretation (without arterial blood)

Adapted from

All You Really Need to Know to Interpret Arterial Blood Gases
Lawrence Martin, M.D.

Order book from or Lippincott W&W

It's quite possible to manage many acid-base and oxygen-related problems without drawing arterial blood for pH PaO2 and PaCO2. Non-arterial values for venous electrolytes, end-tidal CO2, pulse oximetry and co-oximetry measurements can office suffice to diagnose and manage some patients with disorders of OXYGENATION, VENTILATION and ACID-BASE BALANCE.

These non-arterial blood 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, and much of this information is also discussed in the above book.

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 and 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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Table of Contents: All You Really Need to Know to Interpret Arterial Blood Gases
Alphabetical Index to all web sites / Subject Index to all web sites