Continuing Education

Decoding the oxyhemoglobin dissociation curve

Mrs. Glenn, a 72-year-old female on a medical-surgical floor, was hospitalized 3 days ago for pneumonia. Since her admission, she has been on continuous pulse oximetry and is receiving oxygen (2 L/minute by nasal cannula) and antibiotics. A chest X-ray taken earlier today showed little change in her pneumonia. She has a history of chronic lung disease.

At the beginning of the shift, the nurse hears the low alarm of Mrs. Glenn’s pulse oximeter sound, indicating a reading of 89% to 90%. On assessment, the nurse finds the patient alert, oriented, and in no apparent distress. Mrs. Glenn’s heart rate is 96 beats/minute; respiratory rate, 24 breaths/minute with diminished breath sounds; blood pressure, 124/80 mm Hg; and temperature, 38.1° C (100.6° F).

Because the nurse is unfamiliar with Mrs. Glenn, she consults the respiratory therapist (RT), who’s preparing to administer a breathing treatment. The RT assures her that Mrs. Glenn’s pulse oximetry values are always low, close to her baseline of 92%.

The nurse wonders how to interpret the patient’s pulse oximetry values in this context. She vaguely remembers something about the oxyhemoglobin dissociation curve and wonders if a better understanding of the curve would aid her assessment.

The oxyhemoglobin dissociation curve (OHDC) indicates the relationship between the oxygen saturation of hemoglobin (Sao2) and the partial pressure of arterial oxygen (Pao2). Neither linear nor static, the curve can change or shift depending on various factors. Yet understanding the curve and its implications for patient care can be challenging.

Pulse oximetry has become an essential tool in various settings for monitoring a patient’s oxygenation status. It indirectly indicates arterial hemoglobin saturation, measured as oxygen saturation by pulse oximetry (Spo2). How­ever, this technique is limited because oximetry measures just one component of oxygenation. For a more accurate picture of the patient’s overall oxygenation status, you need to assess pulse oximetry values in the context of the OHDC. This article decodes the curve to make it more understandable and discusses the benefits and limitations of pulse oximetry.

The curve: Just the basics

No doubt you remember learning about the OHDC as a nursing student. It’s discussed in nearly every nursing textbook. Nonetheless, it can be a somewhat puzzling concept to grasp and apply in clinical practice. To understand it, think about the oxygenation process occurring in the body. Staying alive hinges on adequate oxygen moving from the lungs to body tissues and cells. For this to occur, the lungs, blood, and environment within the body must be functioning properly:

  • The lungs must receive enough oxygen to be perfused and ventilated optimally.
  • Oxygen must be transported via the blood to the tissues.

Only 2% to 3% of the oxygen going to the tissues dissolves in plasma; the remainder travels in the plasma by attaching to hemoglobin molecules. The most important factor in the amount of oxygen that binds (attaches) to hemoglobin molecules is the partial pressure of arterial oxygen (Pao2); the higher the pressure, the more readily oxygen combines with hemoglobin in red blood cells. This hemoglobin-oxygen linkage is called oxyhemoglobin.

Hemoglobin is made up of four strands of amino acids. If oxygen is linked fully to all four strands, hemoglobin is 100% saturated with oxygen. Transport of sufficient oxygen to the tissues depends on an adequate number of hemoglobin molecules, as well as sufficient blood volume and circulation (cardiac output and blood pressure). Once hemoglobin transports oxygen to the tissues, the body’s environment determines how much (or how little) of the oxygen dissociates (unloads) from hemoglobin for use. Oxygen dissociation from hemoglobin is determined by tissue demand for oxygen. That’s where the OHDC comes in.

Relationship between Pao2 and Sao2

The OHDC represents the relationship between Pao2 and Sao2. Normal Pao2 ranges from 80 to 100 mm Hg. Normal Sao2 measures about 97% but may range from 93% to 97%. (See The curve: Relating Sao2 to Pao2.)

The OHDC isn’t a straight line. Instead, it’s S-shaped. The flat upper portion where the curve is more horizontal depicts oxygen loading onto hemoglobin in the lungs. The pressure of oxygen entering the lungs exceeds the oxygen concentration in blood returning to the lungs. This enables oxygen to bind more easily to hemoglobin.

A significant Pao2 change in this relatively flat part of the curve produces only a small change in Sao2. Thus, a patient’s oxygenation status is better protected at this flat portion. For example, if Pao2drops from 96 to 70 mm Hg, hemoglobin saturation decreases from 97% to approximately 92%. Clinically, this means that if the patient receives supplemental oxygen, Pao2 will increase—but with little effect on Sao2. Hemo­globin can’t be saturated more than 100%, but Pao2 can rise significantly above 100 mm Hg if the patient receives high oxygen concentrations (as occurs with a hyperbaric oxygen chamber).

At the steep lower part of the curve (under the “knee”), where Pao2 measures between 40 and 60 mm Hg, oxygen is released from hemoglobin to the capillaries at the tissue level due to increased oxygen demand. At this part of the curve, an increase or decrease in Pao2 leads to a large Sao2 change. This means giving supplemental oxygen will significantly increase the patient’s Sao2.

A shift to the left or right

Now comes the more complicated part. The OHDC isn’t static or constant, because certain factors can alter hemoglobin’s affinity for oxygen. Depending on oxygen demand at the tissue level, oxygen will bind to hemoglobin more or less readily than normal. Various factors cause the curve to shift to the left or right of its normal position. (See Why the curve shifts and How 2,3-DPG affects the curve.)

Connecting the curve with pulse oximetry readings

Pao2 and Sao2 values can be obtained only from an arterial blood gas (ABG) sample. But although ABG studies are the gold standard for obtaining Pao2 and Sao2 values, frequent ABG sampling isn’t always feasible or cost effective. For ongoing monitoring, pulse oximetry provides a convenient, continuous, and noninvasive way to measure Sao2 indirectly and monitor trends in the patient’s oxygenation status.

Be sure to check for subtle or sudden changes in oximetry values. Changes in oxygenation status can precede clinical signs and symptoms. By detecting these changes early, clinicians can make timely modifications to the plan of care.

Generally speaking, a pulse oximetry value of 95% or higher is clinically acceptable, whereas a value of 90% or lower is a red flag. On the OHDC, a Sao2 value of 90% correlates to a Pao2 level of 60 mm Hg. Pao2 pushes or loads the oxygen onto hemoglobin. So if this level isn’t adequate, suspect the patient’s overall oxygenation is abnormally low.

What pulse oximetry values can’t tell you

Pulse oximetry can’t tell you the patient’s hemoglobin level or identify nonfunctional hemoglobin. In an anemic patient, hemoglobin may be fully saturated and Spo2 may be normal—yet the patient may be hypoxic due to lack of available hemoglobin to carry oxygen to the tissues.

Likewise, hemoglobin may be fully saturated but with dysfunctional strands, such as carboxyhemoglobin or methemoglobin strands. Hemoglobin binds much more readily to carbon monoxide than to oxygen. Hemoglobin may be fully saturated and the pulse oximetry value may be 98%, yet hemoglobin may be saturated with carbon monoxide instead of oxygen. Carboxyhemoglobin levels are elevated in heavy smokers. Methemoglobinemia may occur in patients receiving nitrate or lidocaine therapy.

Pulse oximetry also reveals nothing about the patient’s partial pressure of arterial carbon dioxide (Paco2) or ventilation status. Say, for example, a patient’s receiving a high percentage of supplemental oxygen by face mask for several hours after surgery. If the patient is too sedated to breathe effectively, Paco2 may rise to a dangerous level even though Sao2 may be near normal from the supplemental oxygen. So be sure to obtain baseline ABG values and recheck them periodically.

Factors that can reduce pulse oximetry accuracy

Certain technical and patient variables can reduce the accuracy of pulse oximetry.

  • Technical variables: Motion artifact, ambient light, dark nail polish, improperly placed sensors, and patient movement can cause inaccurate readings. Clinicians should try to control these variables to the extent possible.
  • Patient variables: Pulse oximetry is less accurate when Spo2 values are below 70%, limiting its effectiveness in severely hypoxic patients. Values also may vary in patients with poor perfusion (as from arrhythmias, hypotension, or heart failure) or vasoconstrictive conditions (such as sickle cell anemia, hypothermia, smoking, or certain medications). To determine if low perfusion is interfering with oximetry readings, compare the pulse rate displayed on the oximeter to a good electrocardiographic waveform that correlates to a palpated pulse.

Pulse oximetry values in the context of the curve

Understanding how to use pulse oximetry in the context of your patient’s OHDC can improve outcomes. Used correctly, pulse ox­imetry gives an overall indication of a patient’s oxygenation status and promotes early intervention for high-risk patients. It also allows early recognition of conditions that increase tissue demand for oxygen, helping to ensure that the patient’s oxygen supply (hemoglobin saturation) meets demands.

Keep the following key principles in mind when caring for patients like Mrs. Glenn—those with underlying lung disease who’ve suffered an acute respiratory insult that puts them at risk for impaired gas exchange.

  • After assessing the patient’s respiratory status and determining that the pulse oximeter is functioning properly, visualize the spot on the OHDC where the Spo2 value would line up with the Pao2 value. Is this spot at the flat part or the steep part of the curve?
  • When the pulse oximeter’s low alarm goes off, don’t assume you need to start giving oxygen or increase the oxygen flow. Assess the patient, not the machine: Is the patient in respiratory distress? Check the oxygen supply: Is the oxygen tubing kinked? Is the oximeter applied properly? Does the patient have a disease or condition that increases oxygen demand, such as fever, acidosis, or infection? If so, decreasing Spo2 values may indicate the need to contact the physician for further orders, in addition to increasing the oxygen flow.
  • If the pulse oximetry value is within a normal range, don’t assume the patient is adequately oxygenated. Instead, assess respiratory status, especially if the patient’s receiving supplemental oxygen. Is the patient breathing adequately? Because of compensatory mechanisms, good Spo2 values may give false reassurance despite deterioration in the patient’ respiratory status. For example, patients in near respiratory failure may be hyperventilating, resulting in respiratory alkalosis. This causes the OHDC to shift to the left, with more hemoglobin hanging on to oxygen instead of releasing it at the tissue level where it’s needed.
  • Patients with similar Spo2 values don’t necessarily have the same total oxygen content in their blood. Suppose, for instance, Mr. M and Mr. R both have Spo2 values of 97%, but Mr. M’s hemoglobin value is 15 g/dL, whereas Mr. R’s hemoglobin value is 8 g/dL. In this case, oxygen-carrying capacity is greater in Mr M than in Mr. R, who may be showing signs of hypoxia.
  • Interpret values in light of the patient’s overall condition. Patients with chronic disease, such as chronic obstructive pulmo­nary disease (COPD), may function adequately despite lower Spo2 values. Be sure to check the patient’s baseline ABG and pulse oximetry values, watching for trends. Also remember that Pao2 values normally decrease with age. Elderly patients typically try to compensate for a low Pao2 value with a rightward shift of the curve. But this shift doesn’t completely compensate for the hypoxic changes and hypercapnia that come with aging. As a result, many older adults have decreased activity tolerance.
  • Collaborate with other professionals involved in the patient’s care. Review the physician’s orders to determine the type of monitoring needed and specific protocols to follow. Consult with the respiratory therapist about proper pulse oximetry alarm settings and correct use of the device. Make sure you’re familiar with evidence-based practice guidelines for using pulse oximetry, such as those from the American Association of Critical-Care Nurses and the American Association for Respiratory Care.

Clinical scenario revisited

Mrs. Glenn’s pulse oximetry values continue to remain low, in the upper 80% range. Her vital signs are unchanged. The physician calls with orders to obtain a urine culture and start another I.V. antibiotic. The nurse clamps the catheter to obtain the culture, but when she returns to collect the culture, she sees that only scant urine has been collected.

Mrs. Glenn remains alert but seems a bit restless. The nurse helps her to the chair to eat dinner. Twenty minutes later, she walks by and sees Mrs. Glenn slumped over in her chair and unresponsive. She calls for help to get her back to bed. Although the nurse attempts oral suctioning, the patient remains unresponsive.

Because Mrs. Glenn has “do not resuscitate” orders, no further interventions are taken. The nurse calls the patient’s husband. When he arrives 30 minutes later, he tells the nurse, “I know you gave her good care and probably didn’t know she was going to die, but I would have liked to have been here when it happened.”

Later, the nurse reflects on her experience with Mrs. Glenn. She realizes she missed or ignored clues of quickly developing hypoxia. Although Mrs. Glenn was a COPD patient and thus her pulse oximetry values were lower than normal, the nurse didn’t carefully review her ABG values and previous pulse oximetry values. If she had reviewed these in light of the OHDC, she might have realized the Spo2 decrease from 91% to 88% placed Mrs. Glenn at the steep part of the curve. Her estimatedPao2 would have been lower than 60 mm Hg. As her Pao2 continued to drop, her Spo2 value would have fallen rapidly. Getting her up to eat increased her oxygen demands and contributed to further Spo2 lowering. Her increased pulse and respiratory rates and decreased blood pressure and urinary output also indicated worsening hypoxia.

Pulse oximetry is used in a wide range of care settings to assess oxygenation status. But it must be correlated with the OHDC to get a full picture of the patient’s condition. Correlating Spo2 with Pao2 values provides valuable clues about the balance between oxygen supply and demand. When combined with astute assessment, understanding this relationship
can lead to earlier detection of oxygenation problems and allow prompt intervention. Ignoring or misinterpreting the relationship between Spo2 and Pao2 can lead to disastrous consequences for vulnerable patients, such as Mrs. Glenn.

Selected references

Casey G. Pulse oximetry—what are we really measuring? Nurs N Z. 2011;17(3):24-9.

Elliott M, Coventry A. Critical care: the eight vital signs of patient monitoring. Br J Nurs. 2012;21(10):621-5.

Korkushko OV, Ivanov LA, Pisaruk AV, Chebotarev ND. The respiratory function of blood in elderly and old age and the factors that determine it. (Translated.) Human Physiol. 2009;35(2):40-6.

McCance KL, Huether SE. Pathophysiology: The Biologic Basis for Disease in Adults and Children. 7th ed. St. Louis: Mosby; 2014.

Morton PG, Fontaine DK. Critical Care Nursing: A Holistic Approach. 10th ed. Philadelphia: Lippincott Williams & Wilkins; 2012.

Pruitt B. Interpreting ABGs: an inside look at your patient’s status. Nursing. 2010;40(7):31-36.

Tortora GJ, Derrickson, BH. Principles of Anatomy and Physiology. 12th ed. Danvers, MA: Wiley; 2012.

Valdez-Lowe C, Ghareeb SA, Artinian NT. Pulse oximetry in adults. Am J Nurs. 2009;109(6):52-60.

Wagner KD, Hardin-Pierce MG. High-Acuity Nursing. 6th ed. Boston: Prentice-Hall; 2013.

World Health Organization. Pulse Oximetry Training Manual. 2011. www.who.int/patientsafety/safesurgery/pulse_oximetry/who_ps_pulse_oxymetry_training_manual_en.pdf. Accessed October 30, 2014.

Julia Hooley is the director of the Center for Study and Testing at Malone University School of Nursing and Health Sciences in Canton, Ohio.

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