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Adapted from
All You Really Need to Know to Interpret Arterial Blood Gases
Lawrence Martin, M.D.
Order book from
Amazon.com or
Lippincott W&W
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.
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.
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.
Click here for discussion.
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.
Click here for discussion.
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.
Click here for discussion.
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.
Click here for discussion.
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.Click here for discussion.
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.Click here for discussion.
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.Click here for discussion.
References
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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