Residency · Residency · Anesthesiology
Oxygen Transport, the Oxyhemoglobin Dissociation Curve, and Hypoxemia
Introduction
Understanding oxygen transport physiology is foundational for every anesthesiologist. The delivery of oxygen from the atmosphere to the mitochondria involves a cascade of partial pressure gradients, carrier molecules, and regulatory mechanisms. Mastery of these concepts is essential for managing hypoxemia in the operating room and critical care settings.
Oxygen Transport Fundamentals
Oxygen Content of Blood (CaO2)
The oxygen content of arterial blood is calculated as CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2). The first term represents oxygen bound to hemoglobin, which accounts for approximately 98% of total oxygen content. The second term represents dissolved oxygen in plasma. Normal CaO2 is approximately 18 to 20 mL O2 per dL of blood. The Hufner constant of 1.34 mL represents the volume of oxygen carried per gram of fully saturated hemoglobin.
Oxygen Delivery (DO2)
Oxygen delivery is calculated as DO2 = CaO2 x cardiac output x 10. Normal DO2 is approximately 1000 mL O2 per minute. The determinants of DO2 are hemoglobin concentration, oxygen saturation, PaO2, and cardiac output. The critical DO2 is the threshold below which oxygen consumption becomes supply-dependent, approximately 300 to 330 mL per minute.
Oxygen Consumption (VO2)
Normal VO2 at rest is approximately 250 mL per minute. The oxygen extraction ratio (O2ER), calculated as VO2 divided by DO2, is normally approximately 25%. Mixed venous oxygen saturation (SvO2) reflects the balance between DO2 and VO2 and is normally 65 to 75%.
The Oxyhemoglobin Dissociation Curve
Sigmoid Shape and Clinical Significance
The sigmoid shape of the curve reflects cooperative binding of oxygen to hemoglobin's four heme groups. The plateau region (PaO2 60 to 100 mmHg) provides a safety buffer, as large changes in PaO2 cause minimal change in SaO2. The steep portion (PaO2 20 to 60 mmHg) means small drops in PaO2 cause rapid desaturation. The P50 is the PaO2 at which hemoglobin is 50% saturated, and the normal value is 26.7 mmHg.
Factors Shifting the Curve
A right shift (increased P50, decreased affinity, enhanced oxygen unloading) is caused by increased temperature (fever, malignant hyperthermia), increased 2,3-DPG (chronic hypoxia, anemia, altitude), acidosis (both metabolic and respiratory, known as the Bohr effect), and increased PaCO2.
A left shift (decreased P50, increased affinity, impaired oxygen unloading) is caused by hypothermia, decreased 2,3-DPG (stored bank blood, hypothermia), alkalosis, carbon monoxide poisoning (carboxyhemoglobin), fetal hemoglobin (HbF), and methemoglobinemia.
| Shift Direction | P50 | O2 Affinity | O2 Unloading | Causes | Clinical Example |
|---|---|---|---|---|---|
| Right shift | Increased (>26.7) | Decreased | Enhanced | ↑ Temperature, ↑ 2,3-DPG, acidosis, ↑ PaCO2 | Fever, chronic hypoxia, exercise |
| Left shift | Decreased (<26.7) | Increased | Impaired | ↓ Temperature, ↓ 2,3-DPG, alkalosis, CO, HbF, MetHb | Hypothermia, massive transfusion, CO poisoning |
| Mechanism of Hypoxemia | A-a Gradient | Response to 100% O2 | Example |
|---|---|---|---|
| Low FiO2 | Normal | Corrects | Altitude, gas supply failure |
| Hypoventilation | Normal | Corrects | Opioid overdose, residual NMB |
| V/Q mismatch | Elevated | Marked improvement | Atelectasis, pneumonia, PE (most common) |
| Shunt (R→L) | Elevated | Minimal improvement | Intracardiac shunt, ARDS, hepatopulmonary syndrome |
| Diffusion impairment | Elevated | Corrects | Interstitial lung disease (rarely sole cause) |
Hypoxemia: Definition, Causes, and Management
Definition
Hypoxemia is defined as PaO2 below 60 mmHg or SaO2 below 90% on room air. It is distinguished from hypoxia, which refers to inadequate tissue oxygenation.
Five Mechanisms of Hypoxemia
The first mechanism is low inspired oxygen (FiO2), caused by altitude, gas supply failure, or wrong gas administration. The second is hypoventilation, in which increased PaCO2 leads to decreased alveolar PO2 per the alveolar gas equation: PAO2 = FiO2 x (Patm - PH2O) - PaCO2/RQ. The third is ventilation-perfusion mismatch, which is the most common cause in clinical practice and responds to supplemental oxygen. The fourth is shunt (right-to-left), either intracardiac or intrapulmonary, which does not respond well to supplemental oxygen. The fifth is diffusion impairment, which is rarely a sole cause and is seen in interstitial lung disease.
Differentiating Mechanisms
The A-a gradient is calculated to help differentiate the cause of hypoxemia, with a normal value of approximately 2.5 + (0.21 x age). The A-a gradient is normal in hypoventilation and low FiO2 but elevated in V/Q mismatch, shunt, and diffusion impairment. The response to 100% oxygen distinguishes shunt (minimal improvement) from V/Q mismatch (marked improvement).
Intraoperative Causes of Hypoxemia
Common intraoperative causes include endobronchial intubation or bronchial obstruction, atelectasis and absorption atelectasis with high FiO2, bronchospasm, pneumothorax, pulmonary embolism, and circuit disconnection or ventilator malfunction.
Clinical Pearls
A PaO2 of 60 mmHg corresponds to an SaO2 of approximately 90%, which represents the "cliff" on the dissociation curve where desaturation accelerates rapidly. Massive transfusion with stored blood depletes 2,3-DPG, shifting the curve to the left and impairing tissue oxygen delivery despite adequate SaO2. The A-a gradient should always be calculated when evaluating hypoxemia because it is the single most useful tool for narrowing the differential diagnosis. In carbon monoxide poisoning, standard two-wavelength pulse oximetry is dangerously misleading because it cannot distinguish carboxyhemoglobin from oxyhemoglobin.
References
- Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail's Clinical Anesthesiology. 7th ed. McGraw-Hill; 2022. Chapter 23: Respiratory Physiology.
- West JB, Luks AM. West's Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2021.
- Collins JA, Rudenski A, Gibson J, Howard L, O'Driscoll R. Relating oxygen partial pressure, saturation and content: the haemoglobin-oxygen dissociation curve. Breathe. 2015;11(3):194-201.
- Yartsev A. Oxygen transport and the oxyhemoglobin dissociation curve. Deranged Physiology. 2023.