Decoding the oxyhemoglobin dissociation curve Understanding the curve helps you put pulse oximetry in context. By Julia Hooley, MSN, RN a 72-year-old fe - The oxyhemoglobin dissociation ture of the pa - MRS . G LENN , male on a medical-surgical curve (OHDC) indicates the rela - tient’s overall oxy - floor, was hospitalized 3 tionship between the oxygen satu - genation status, you need to assess days ago for pneumonia. ration of hemoglobin (Sa O2) and pulse oximetry values in the con - Since her admission, she the partial pressure of arterial oxy - text of the OHDC. This article de - has been on continuous gen (Pa O2). Neither linear nor static, codes the curve to make it more pulse oximetry and is re - the curve can change or shift de - understandable and discusses the ceiving oxygen (2 L /minute pending on various factors. Yet un - benefits and limitations of pulse by nasal cannula) and an - derstanding the curve and its impli - oximetry. tibiotics. A chest X-ray taken earli - cations for patient care can be er today showed little change in challenging. The curve: Just the basics her pneumonia. She has a history Pulse oximetry has become an No doubt you remember learning of chronic lung disease. essential tool in various settings for about the OHDC as a nursing stu - At the beginning of the shift, the monitoring a patient’s oxygenation dent. It’s discussed in nearly every nurse hears the low alarm of Mrs. status. It indirectly indicates arterial nursing textbook. Nonethe less, it Glenn’s pulse oximeter sound, indi - hemoglobin saturation, measured as can be a somewhat puzzling con - cating a reading of 89% to 90%. oxygen saturation by pulse oxime - cept to grasp and apply in clinical On assessment, the nurse finds the try (Sp O2). How ever, this technique practice. To understand it, think patient alert, oriented, and in no is limited because oximetry meas - about the oxygenation process oc - apparent distress. Mrs. Glenn’s ures just one component of oxy - curring in the body. Staying alive heart rate is 96 beats/minute; respi - genation. For a more accurate pic - hinges on adequate oxygen mov - ratory rate, 24 breaths/minute with ing from the lungs to body tissues diminished breath sounds; blood and cells. For this to occur, the pressure, 124/80 mm Hg; and tem - lungs, blood, and environment perature, 38.1° C (100.6° F). CNE within the body must be function - Because the nurse is unfamiliar 1.42 contact ing properly: hours with Mrs. Glenn, she consults the • The lungs must receive enough respiratory therapist (RT), who’s LEARNING OBJECTIVES oxygen to be perfused and ven - preparing to administer a breath - 1. Recall the physiologic basis for the tilated optimally. ing treatment. The RT assures her oxyhemoglobin dissociation curve • Oxygen must be transported via that Mrs. Glenn’s pulse oximetry (OHDC). the blood to the tissues. values are always low, close to her 2. Describe the shifts that can occur Only 2% to 3% of the oxygen baseline of 92%. in the OHDC. going to the tissues dissolves in The nurse wonders how to inter - 3. Discuss the relationship between plasma; the remainder travels in pret the patient’s pulse oximetry the OHDC and pulse oximetry. the plasma by attaching to hemo - values in this context. She vaguely globin molecules. The most impor - The author and planners of this CNE activity have remembers something about the disclosed no relevant financial relationships with tant factor in the amount of oxygen oxyhemoglobin dissociation curve any commercial companies pertaining to this that binds (attaches) to hemoglobin activity. See the last page of the article to learn and wonders if a better under - how to earn CNE credit. molecules is the partial pressure of standing of the curve would aid arterial oxygen (Pa O ); the higher Expiration: 1/1/18 2 her assessment. the pressure, the more readily oxy - American Nurse Today Volume 10, Number 1 www.AmericanNurseToday.com 18 The curve: Relating Sa O2 to Pa O2 The oxyhemoglobin dissociation curve graphically represents the affinity between oxy - but with little effect on Sa O . gen and hemoglobin—specifically, how the oxygen saturation of hemoglobin (Sa O ) 2 2 Hemo globin can’t be saturated relates to the partial pressure of arterial oxygen (Pa O2). The curve’s position and overall shape (shown in purple below) depend on various factors, including the partial pres - more than 100%, but Pa O2 can rise sure of carbon dioxide (Pa CO 2), body temperature, and blood pH. significantly above 100 mm Hg if the patient receives high oxygen 100 - concentrations (as occurs with a hyperbaric oxygen chamber). 90 - At the steep lower part of the 80 - curve (under the “knee”), where Pa O measures between 40 and 60 70 - 2 mm Hg, oxygen is released from 60 - hemoglobin to the capillaries at the 2 tissue level due to increased oxy - O 50 - a S gen demand. At this part of the 40 - curve, an increase or decrease in Pa O leads to a large Sa O change. 30 - 2 2 This means giving supplemental 20 - oxygen will significantly increase the patient’s Sa O . 10 - 2 0 |||. || || || ||| || | A shift to the left or right 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Now comes the more complicated Pa O2 part. The OHDC isn’t static or con - stant, because certain factors can alter hemoglobin’s affinity for oxy - gen combines with hemoglobin in about 97% but may range from gen. Depending on oxygen de - red blood cells. This hemoglobin- 93% to 97%. (See The curve: Relat - mand at the tissue level, oxygen oxygen linkage is called oxyhemo - ing Sa O2 to Pa O2.) will bind to hemoglobin more or globin. The OHDC isn’t a straight line. less readily than normal. Various Hemoglobin is made up of four Instead, it’s S-shaped. The flat up - factors cause the curve to shift to strands of amino acids. If oxygen is per portion where the curve is the left or right of its normal posi - linked fully to all four strands, he - more horizontal depicts oxygen tion. (See Why the curve shifts and moglobin is 100% saturated with loading onto hemoglobin in the How 2,3-DPG affects the curve .) oxygen. Transport of sufficient oxy - lungs. The pressure of oxygen en - gen to the tissues depends on an tering the lungs exceeds the oxy - Connecting the curve with pulse adequate number of hemoglobin gen concentration in blood oximetry readings molecules, as well as sufficient returning to the lungs. Pa O2 and Sa O2 values can blood volume and circulation (car - This enables oxygen be obtained only from diac output and blood pressure). to bind more easily an arterial blood gas Once hemoglobin transports oxy - to hemoglobin. (ABG) sample. But al - gen to the tissues, the body’s envi - A significant though ABG studies ronment determines how much (or Pa O2 change in are the gold standard how little) of the oxygen dissoci - this relatively flat for obtaining Pa O2 ates (unloads) from hemoglobin for part of the curve and Sa O2 values, fre - use. Oxygen dissociation from he - produces only a quent ABG sampling moglobin is determined by tissue small change in Sa O2. isn’t always feasible or demand for oxygen. That’s where Thus, a patient’s oxy - cost effective. For ongoing the OHDC comes in. genation status is better monitoring, pulse oximetry protected at this flat portion. For provides a convenient, continuous, example, if Pa O2 drops from 96 to and noninvasive way to measure Relationship between Pa O2 70 mm Hg, hemoglobin saturation Sa O indirectly and monitor trends and Sa O 2 The OH2DC represents the relation - decreases from 97% to approxi - in the patient’s oxygenation status. ship between Pa O2 and Sa O2. Nor - mately 92%. Clinically, this means Be sure to check for subtle or mal Pa O2 ranges from 80 to 100 that if the patient receives supple - sudden changes in oximetry values. mm Hg. Normal Sa O2 measures mental oxygen, Pa O2 will increase— Changes in oxygenation status can www.AmericanNurseToday.com January 2015 American Nurse Today 19 Why the curve shifts The normal oxyhemoglobin dissociation curve (OHDC), shown here by the solid blue precede clinical signs and symp - line, indicates that when the partial pressure of arterial oxygen (Pa O ) is 40 mm Hg, oxy - toms. By detecting these changes 2 gen saturation of hemoglobin (Sa O2) is 75%. At the tissue or capillary level, a Pa O2 of 40 early, clinicians can make timely mm Hg is normal. At this point, about 25% of the oxygen carried on hemoglobin from modifications to the plan of care. the tissues to the lungs has been unloaded and used at the tissue level. Much of the Generally speaking, a pulse oxygen still remains on the hemoglobin molecule, indicating an oxygen reserve isn’t oximetry value of 95% or higher is normally used at the tissue level. However, this reserve is available when extra oxygen clinically acceptable, whereas a val - is needed, as during strenuous exercise or high metabolic demand. ue of 90% or lower is a red flag. Conditions that alter hemoglobin’s affinity for oxygen can shift the OHDC to the On the OHDC, a Sa O value of right or left. 2 • A shift to the right (dotted purple line) decreases hemoglobin’s affinity for oxygen for 90% correlates to a Pa O level of 60 2 a given Pa O value, and the Sa O value decreases below normal.