Hemoglobinopathies are inherited disorders that alter the structure or production of hemoglobin, leading to a range of clinical manifestations. The hemoglobinopathies most often encountered in clinical practice are sickle cell disease (HbS) and the thalassemias. Methemoglobin and carboxyhemoglobin are dyshemoglobins rather than inherited hemoglobinopathies (methemoglobinemia is congenital only in rare cytochrome b5 reductase deficiency or hemoglobin M), but they are considered here because they interfere with the oximeter directly. These variants can significantly distort the accuracy of pulse oximetry readings, making it essential for anesthesia providers to understand their effects and implement appropriate corrective measures during perioperative monitoring.
Sickle Cell Disease (HbS)
Sickle cell disease, caused by a mutation in the beta-globin gene, leads to the production of abnormal hemoglobin (HbS) that polymerizes under low-oxygen conditions, causing red blood cells to deform into a sickle shape. Patients with sickle cell disease often experience chronic hemolytic anemia, acute pain crises, and increased susceptibility to infections, all of which contribute to complications during anesthesia.
Hemoglobin S absorbs red and infrared light almost exactly as hemoglobin A does, so pulse oximetry is generally reliable in sickle cell disease: in steady state and in crisis, SpO2 agreed with co-oximetry to within about 1-2% in most patients, although the error widens at lower saturations and a minority of readings differed by more than 4% (Ortiz et al., 1999; Fitzgerald and Johnson, 2001). What is altered is the oxygen dissociation curve, which is shifted to the right (a higher P50) by hemoglobin S polymerization and raised 2,3-DPG. This matters when saturation is calculated from an arterial PaO2 rather than measured: the blood gas analyzer assumes a normal curve and reports a saturation higher than the patient actually has, so the SpO2 looks falsely low when it is the calculated value that is wrong. Chronic hemolysis also raises carboxyhemoglobin (often 2-4%) and sometimes methemoglobin, which make the SpO2 read slightly above the true oxyhemoglobin fraction.
For anesthesia providers, it is crucial to cross-check pulse oximetry readings against an arterial blood gas (ABG) analyzed with co-oximetry, so that saturation is measured rather than calculated, when there is clinical suspicion that the SpO2 readings may not be accurate. Additionally, ensuring optimal tissue perfusion and managing potential vaso-occlusive crises during anesthesia are paramount for patient safety.
Methemoglobinemia
Methemoglobinemia is characterized by an increased level of methemoglobin (MetHb), an oxidized form of hemoglobin that cannot bind oxygen. Normal methemoglobin is below 1-2%. Methemoglobinemia can be congenital or, far more often, acquired through exposure to oxidizing medications or chemicals, such as local anesthetics (benzocaine spray, prilocaine, including the prilocaine in EMLA cream applied to infants), dapsone, nitrites and nitrates (including high-dose nitroglycerin and nitroprusside), inhaled nitric oxide, and sulfonamides. When methemoglobin levels rise, pulse oximetry becomes unreliable due to its inability to differentiate between MetHb and oxyhemoglobin. Methemoglobin absorbs red and infrared light almost equally, which drives the ratio of ratios toward 1.0, a value the calibration curve reads as about 85%. As methemoglobin rises the display therefore drifts toward 85% and stays there once the fraction exceeds roughly 30-35%, regardless of the true oxygen saturation; it overestimates when the patient is truly hypoxemic and underestimates when the patient is not (Barker et al., 1989). The arterial PaO2 is normal, so a calculated saturation near 100% beside a low SpO2 (a "saturation gap") is a strong clue, as is chocolate-brown blood that does not redden on exposure to air.
This phenomenon can be dangerous in a perioperative setting, where reliance on an inaccurate SpO2 reading might delay necessary interventions such as oxygen therapy or administration of methylene blue. Methylene blue 1-2 mg/kg IV over 5 minutes, repeated once after 30-60 minutes if the level stays high, is indicated for symptomatic patients or a methemoglobin fraction above about 20-30%. It works through NADPH methemoglobin reductase, so it is ineffective and can cause hemolysis in G6PD deficiency (where ascorbic acid or exchange transfusion are the alternatives), it can precipitate serotonin syndrome in patients taking serotonergic drugs, and it itself produces a transient spurious fall in SpO2 for several minutes after injection. Anesthesia providers should be aware of risk factors for methemoglobinemia and be prepared to use co-oximetry, which can specifically measure MetHb levels, as part of their monitoring strategy. Identifying the clinical signs of methemoglobinemia (e.g., cyanosis unresponsive to oxygen) can prompt immediate corrective actions.
Carboxyhemoglobinemia
Carboxyhemoglobinemia occurs due to the binding of carbon monoxide (CO) to hemoglobin, forming carboxyhemoglobin (COHb), which reduces oxygen-carrying capacity. COHb also shifts the oxygen dissociation curve to the left, making it harder for oxygen to be released to tissues. Nonsmokers carry 1-3% carboxyhemoglobin, smokers 5-10%, and heavy smokers up to 15%. Patients exposed to CO from environmental sources (house fires, faulty heaters) and patients exposed in the operating room itself, from electrocautery smoke absorbed across the peritoneum during prolonged laparoscopy or from degradation of volatile anesthetics (desflurane most, then isoflurane) by a desiccated carbon dioxide absorbent, are at risk for elevated COHb levels. Carboxyhemoglobin absorbs about as much red light as oxyhemoglobin and almost no infrared light, so the oximeter counts it as oxyhemoglobin: the displayed SpO2 approximates the oxyhemoglobin fraction plus about 90% of the carboxyhemoglobin fraction (Barker and Tremper, 1987), and a patient with 30% carboxyhemoglobin and 68% oxyhemoglobin reads in the mid-90s. This is particularly hazardous in anesthesia, where hypoxemia may go unnoticed due to deceptive pulse oximetry values. As with methemoglobinemia, co-oximetry is necessary to detect and quantify carboxyhemoglobin levels. Early recognition of carboxyhemoglobinemia and the prompt initiation of 100% oxygen therapy or hyperbaric oxygen can prevent tissue hypoxia and subsequent complications: the half-life of carboxyhemoglobin is about 4-5 hours breathing room air, 60-90 minutes on 100% oxygen, and about 20-30 minutes with hyperbaric oxygen at 2.5-3 atmospheres.
Thalassemias
Thalassemias are a group of genetic disorders characterized by reduced or absent production of one of the globin chains of hemoglobin, leading to microcytic anemia and varying degrees of hemolysis. The globin chains that are produced are structurally normal, so the absorbance spectrum, and with it the SpO2, is usually accurate. The trap is different: saturation says nothing about oxygen content. Arterial oxygen content is about 1.34 x hemoglobin x saturation (plus a little dissolved oxygen), so a patient with a hemoglobin of 6 g/dL and an SpO2 of 99% carries barely half the oxygen of a patient with a hemoglobin of 12 g/dL at the same reading. Pulse oximeter accuracy itself degrades only with very profound anemia (hematocrit below about 10-15%), and then mainly at low saturations, where the device reads falsely low (Severinghaus and Koh, 1990). Chronic hemolysis adds small amounts of carboxyhemoglobin, and patients with transfusion-dependent thalassemia bring iron-overload cardiomyopathy, pulmonary hypertension, and post-splenectomy thrombotic risk to the operating room.
Anesthesia providers caring for patients with thalassemia must ensure that oxygenation status is assessed with methods beyond pulse oximetry, especially during episodes of acute anemia or hemodynamic instability. Regularly assessing hemoglobin levels and ensuring adequate transfusion therapy preoperatively can help mitigate some of these issues.
Rare Variants With Altered Light Absorbance
A small group of structural variants (among them hemoglobin Köln, Bonn, Lansing, Cheverly, Bibba, Rothschild, Santa Ana, Denver, Grifton, Louisville, and the hemoglobin M family) absorb light differently from hemoglobin A and produce a persistently low SpO2, commonly 80-90%, in a patient who is pink, asymptomatic, and has a normal PaO2 and a normal measured saturation on co-oximetry. The finding is lifelong, often runs in families, and is frequently discovered for the first time in the operating room or recovery room. Recognizing it spares the patient a cardiopulmonary workup and spares the anesthetic a cascade of unnecessary interventions; the diagnosis is made by hemoglobin electrophoresis or high-performance liquid chromatography with gene sequencing, and the patient should be told to carry the information to future anesthetics.
| Variant | Effect on pulse oximetry | What to do |
|---|---|---|
| Sickle cell disease (HbS) | HbS absorbs light like HbA, so SpO2 is generally reliable (within about 1-2%, wider at low saturations); the right-shifted curve makes a saturation calculated from PaO2 read too high, and hemolysis adds a little carboxyhemoglobin. | Cross-check against a measured co-oximetry saturation, not a calculated one, whenever the reading is clinically suspect; maintain tissue perfusion and manage vaso-occlusive crisis. |
| Methemoglobinemia | Methemoglobin is oxidized and cannot bind oxygen, and the oximeter cannot distinguish it from oxyhemoglobin; the display typically sits near 85% regardless of true saturation. | Use co-oximetry, which measures methemoglobin specifically; watch for cyanosis unresponsive to oxygen, a normal PaO2 beside a low SpO2, and chocolate-brown blood; methylene blue 1-2 mg/kg IV over 5 minutes (not in G6PD deficiency). |
| Carboxyhemoglobinemia | Carboxyhemoglobin is read as oxyhemoglobin (SpO2 is roughly the oxyhemoglobin fraction plus 90% of the carboxyhemoglobin fraction), so SpO2 is falsely elevated and hypoxemia can go unnoticed. | Co-oximetry to detect and quantify carboxyhemoglobin; early 100% oxygen (half-life 60-90 min) or hyperbaric oxygen (20-30 min) prevents tissue hypoxia. |
| Thalassemias | Hemoglobin structure is normal, so SpO2 is usually accurate, but a normal SpO2 hides a low oxygen content when hemoglobin is low; accuracy falls only in profound anemia (hematocrit below about 10-15%) at low saturations, where the device reads low. | Assess oxygen content and delivery, not saturation alone, during acute anemia or instability; check hemoglobin regularly and transfuse preoperatively where indicated. |
| Rare variants with altered absorbance (Hb Köln, Bonn, Lansing, Cheverly, Bibba, Rothschild, Santa Ana, Denver, Grifton, Louisville, Hb M) | Persistently low SpO2, often 80-90%, in a pink, asymptomatic patient with a normal PaO2 and a normal measured saturation. | Confirm with co-oximetry; hemoglobin electrophoresis or HPLC and gene sequencing; document so future anesthetics are not derailed. |
