8.5 CO-Oximetry / Hemoximetry: Measurement, Dyshemoglobins, and Clinical Implications

8.5 CO-Oximetry / Hemoximetry: Measurement, Dyshemoglobins, and Clinical Implications

The Detailed Content Outline lists CO-oximetry / hemoximetry as its own procedure (Domain II item 7) and its own data topic (Domain III item 2), separate from blood gas analysis. It also caps it: the form specification allows a maximum of 2 CO-oximetry items on any examination form. That cap is useful information — it tells you to master the handful of high-yield concepts rather than memorize spectrophotometric minutiae.

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How a CO-Oximeter Works

A CO-oximeter (hemoximeter) is a multi-wavelength spectrophotometer. A whole-blood sample is drawn into a cuvette and hemolyzed, ultrasonically or chemically, so that intact red cells no longer scatter light. A light source then passes many discrete wavelengths — typically 6 to 128 depending on the instrument — through the hemolysate, and a detector array measures absorbance at each.

Each hemoglobin species has a distinct absorbance spectrum. Because absorbance is additive, the total absorbance at each wavelength is the sum of contributions from every species present, and the Beer-Lambert law relates each contribution to concentration:

$A_\lambda = \sum_i \varepsilon_{i,\lambda} \, c_i \, l$

where $\varepsilon_{i,\lambda}$ is the known extinction coefficient of species $i$ at wavelength $\lambda$, $c_i$ its concentration, and $l$ the path length. With measurements at more wavelengths than there are unknown species, the instrument solves the resulting system of simultaneous equations for each concentration. This is the entire reason a pulse oximeter cannot do the same job: two wavelengths can resolve only two species.

What Is Reported

• Total hemoglobin (tHb) in g/dL
• Oxyhemoglobin (O$_2$Hb) as a percentage of total hemoglobin
• Reduced/deoxyhemoglobin (HHb)
• Carboxyhemoglobin (COHb)
• Methemoglobin (MetHb)
• Fractional oxygen saturation, and on many analyzers oxygen content (CaO$_2$) and P50

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Measured Versus Calculated Saturation

Blood gas analyzers that lack a CO-oximetry module calculate an oxygen saturation from measured pH, PCO$_2$, and PO$_2$ using an assumed standard oxyhemoglobin dissociation curve. That calculation assumes a normal P50 and, critically, assumes no dyshemoglobins are present.

Consequences you must be able to state:

• In carbon monoxide poisoning, calculated SaO$_2$ is normal while measured fractional saturation is low. PaO$_2$ is also normal, because carbon monoxide does not affect dissolved oxygen — only bound oxygen. A "normal blood gas" in a fire victim is a measurement failure, not reassurance.
• In methemoglobinemia, calculated saturation is again normal while fractional saturation is low.
• A saturation gap — the difference between a pulse oximeter's SpO$_2$ or a calculated SaO$_2$ and a measured fractional SaO$_2$ — is the single most useful screening finding for a dyshemoglobin.
• Calculated values are also unreliable whenever the dissociation curve is shifted (fever, acidosis, abnormal 2,3-DPG, hemoglobin variants).

Rule for the laboratory: any oxygen prescription, disability determination, or carbon monoxide evaluation that hinges on saturation must rest on a measured value.

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Carboxyhemoglobin

Reference Ranges and Thresholds

Carbon monoxide binds hemoglobin with an affinity roughly 200–250 times that of oxygen, and it does two things simultaneously: it removes binding sites (an anemia-equivalent effect) and it shifts the oxyhemoglobin dissociation curve to the left, impairing offloading at the tissue. That combination is why symptoms exceed what the numeric saturation deficit alone would predict.

The elimination half-life of COHb is roughly 4–6 hours on room air, about 60–90 minutes on high-flow 100% oxygen, and 20–30 minutes with hyperbaric oxygen — the pharmacokinetic basis for treatment.

The Direct Pulmonary Function Connection

CO-oximetry is not an incidental topic for a PFT technologist; carboxyhemoglobin corrupts the DLCO measurement by two mechanisms:

1. Back-pressure: dissolved carbon monoxide in capillary blood reduces the alveolar-to-capillary partial pressure gradient that drives uptake.
2. Anemia effect: COHb-occupied binding sites are unavailable to the test gas.

The practical rule is that each 1% increase in COHb lowers measured DLCO by approximately 1%. This is why patients are asked to abstain from smoking before a DLCO study and why an unexplained low DLCO in a smoker should prompt a COHb measurement before the result is attributed to emphysema or interstitial disease.

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Methemoglobin

Methemoglobin is hemoglobin whose iron has been oxidized from the ferrous (Fe$^{2+}$) to the ferric (Fe$^{3+}$) state. Ferric heme cannot bind oxygen, and its presence shifts the remaining ferrous sites leftward, further impairing delivery.

• Normal: less than 1–2%.
• Common causes: topical benzocaine and lidocaine (a classic after bronchoscopy or transesophageal procedures), dapsone, nitrates and nitrites, aniline dyes, and inherited cytochrome-b5 reductase deficiency.
• Clinical signature: cyanosis that does not improve with oxygen, chocolate-brown arterial blood that does not turn red on exposure to air, an SpO$_2$ stuck near 85%, and a normal PaO$_2$.
• Treatment: methylene blue, which accelerates the NADPH-dependent reduction pathway — except in glucose-6-phosphate dehydrogenase (G6PD) deficiency, where methylene blue is ineffective and can precipitate hemolysis. Ascorbic acid or exchange transfusion is used instead.

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Sample Handling and Quality Control

• Anticoagulant: lyophilized (dry) balanced heparin, as for blood gases. Liquid heparin dilutes the sample and lowers measured total hemoglobin.
• Air bubbles: expel immediately; the same equilibration artifact that corrupts blood gas values corrupts co-oximetry.
• Clots: a micro-clot both obstructs the sample path and yields a spuriously low tHb. Mix thoroughly by inversion and rolling before aspiration; do not shake.
• Interference: lipemia, elevated bilirubin, sulfhemoglobin, and intravascular dyes such as methylene blue and indocyanine green all absorb light and can bias the spectral solution. Most modern analyzers flag interference rather than silently misreporting, but the flag must be read.
• Fetal hemoglobin absorbs differently from adult hemoglobin and can produce falsely elevated COHb on older analyzers — relevant to the 10 pediatric items on each form.
• Quality control: run at least the manufacturer-specified levels each shift, plot on Levey-Jennings charts, and apply Westgard multirules exactly as for the blood gas electrodes. Tonometry with certified gas mixtures is the reference verification method for the combined analyzer.