O2 Transport

Oxygen Equilibrium and Transport Overview

• Conceptual vs. Bedside Application:

  • While respiratory therapists rarely perform complex manual calculations at the bedside in modern practice, understanding the underlying concepts is critical for diagnosing why a patient is not oxygenating properly or why oxygen is not reaching the tissues.

  • Patient care involves analyzing how factors like low hemoglobin levels affect other bodily systems.

• Primary Functions of Oxygen Transport:

  • Oxygen transport between the lungs and the body's tissues is a shared function of the heart and the blood.

  • A failure in either system (e.g., poor cardiac output or substandard blood quality) results in impaired oxygen delivery.

Pressures of Oxygen ($P_{O_2}$)

• Partial Pressure Concepts:

  • $P_{O_2}$ represents the pressure of oxygen in a given system. In a gradient, gas moves from high concentration/pressure to lower concentration/pressure.

  • Subscripts are used to identify the specific location or type of pressure.

• Barometric Pressure of Oxygen ($P_{B_{O_2}}$):

  • Calculated by multiplying the barometric pressure ($P_B$) by the fraction of inspired oxygen ($F_IO_2$).

  • Example for Shreveport, Louisiana:

    • $P_B = 760\,mmHg$

    • $F_IO_2 = 0.21$

    • $760\,mmHg \times 0.21 = 159.6\,mmHg$

• Alveolar Oxygen Pressure ($PA_{O_2}$):

  • Calculated using the Alveolar Air Equation to account for water vapor and secondary gases (CO2) in the lungs.

  • Formula:         $PA_{O_2} = [(P_B - 47) \times F_IO_2] - (Pa_{CO_2} \times 1.25)$

  • Constants: Water vapor pressure is $47\,mmHg$. Respiratory quotient correction is typically $1.25$ (or dividing by $0.8$).

  • Normal Alveolar Pressure: Approximately $100\,mmHg$ on room air.

• Arterial Oxygen Pressure ($Pa_{O_2}$):

  • Represents the pressure of oxygen dissolved strictly in the plasma of arterial blood.

  • Normal Range: $80-100\,mmHg$.

  • Critical Value: $60\,mmHg$. Below this threshold, clinical intervention is usually required.

• Venous Oxygen Pressure ($Pv_{O_2}$):

  • Represents the pressure of oxygen in the venous system.

  • Normal Value: Approximately $40\,mmHg$.

Oxygen Saturation and Measurements

• Arterial Oxygen Saturation ($Sa_{O_2}$):

  • The percentage of hemoglobin in arterial blood that is saturated with oxygen ($O_2$).

  • Requires an invasive arterial blood sample and is measured by an Arterial Blood Gas (ABG) machine.

  • Normal Value: $95-100\%$ ($97.5\%$ at sea level).

• Pulse Oximetry Saturation ($Sp_{O_2}$):

  • A non-invasive measurement of saturated hemoglobin using a pulse oximeter.

  • Clinical Warning: Research indicates pulse oximeters may overestimate values in patients with darker skin tones, requiring careful monitoring.

• Mixed Venous Saturation ($Sv_{O_2}$):

  • Measured at the right atrium via a specialized line.

  • Normal Value: $75\%$. This indicates that tissues normally extract about $25\%$ of the oxygen available ($100\% - 25\% = 75\%$).

Hemoglobin ($Hb$) and Oxygen Binding

• Adult Hemoglobin (HbA):

  • A large, complex protein molecule found inside red blood cells (erythrocytes).

  • Capacity: Each red blood cell contains approximately $280\text{ million}$ hemoglobin molecules.

  • Binding: Each hemoglobin molecule can bind up to $4$ oxygen molecules.

  • Structure: Composed of a Heme portion (four iron-containing pigments) and a Globin portion (four polypeptide chains: two alpha and two beta).

  • Ferrous State: Iron must be in the ferrous state ($Fe^{2+}$) to bind oxygen.

• Cooperative Binding:

  • When the first molecule of oxygen binds to deoxygenated hemoglobin (also called reduced hemoglobin), the protein changes shape, allowing the next three molecules to bind in rapid succession.

  • Hemoglobin is typically either fully saturated (all 4 spots filled) or completely deoxygenated (0 spots filled).

• Formulas for Oxygen Carrying Capacity:

  • Each gram of fully saturated hemoglobin can carry $1.34\,mL$ of oxygen.

  • Normal Hemoglobin Range: $12-16\,g/dL$ (grams per deciliter).

Mechanisms of Oxygen Transport

• 1. Dissolved in Plasma:

  • Governed by Henry's Law: The amount of gas dissolved in a liquid is proportional to the partial pressure of that gas.

  • Calculation: $Pa_{O_2} \times 0.003$

  • Normal Value ($Pa_{O_2} = 100$): $0.3\,mL/dL$. This represents a very small fraction of total transport.

• 2. Bound to Hemoglobin:

  • This is the primary method of oxygen transport.

  • Calculation: $Hb \times 1.34 \times Sa_{O_2}$

  • Normal Value ($Hb = 15, Sa_{O_2} = 0.97$): $15 \times 1.34 \times 0.97 \approx 19.5\,mL/dL$.

  • Comparison: Hemoglobin transports roughly $67\text{ times}$ more oxygen than plasma.

Total Oxygen Content calculation ($CaO_2$)

• Formula for Arterial Oxygen Content ($CaO_2$):

  • $CaO_2 = (Hb \times 1.34 \times Sa_{O_2}) + (Pa_{O_2} \times 0.003)$

  • Normal Adult Value: Approximately $20\,vol\%$ (or $20\,mL/dL$).

• Formula for Mixed Venous Oxygen Content ($Cv_{O_2}$):

  • $Cv_{O_2} = (Hb \times 1.34 \times Sv_{O_2}) + (Pv_{O_2} \times 0.003)$

  • Normal Value: Approximately $15\,vol\%$.

• Case Study Example:

  • Patient: 27-year-old female, history of anemia.

  • Data: $Hb = 6\,g/dL$, $Pa_{O_2} = 80\,mmHg$, $Sa_{O_2} = 0.90$.

  • Step 1 (Hb Bound): $6 \times 1.34 \times 0.90 = 7.236$

  • Step 2 (Dissolved): $80 \times 0.003 = 0.24$

  • Step 3 (Total): $7.236 + 0.24 = 7.476\,vol\%$

  • Clinical Finding: The patient has less than half the normal oxygen-carrying capacity due to severe anemia.

The Oxyhemoglobin Dissociation (Equilibrium) Curve

• Shape: Sigmoidal (S-shape) due to cooperative binding.

• Relationship: Illustrates how hemoglobin saturation ($Sa_{O_2}$) relates to the partial pressure of oxygen ($Pa_{O_2}$).

• Flat Upper Portion (Association):

  • Occurs in the Lungs ($Pa_{O_2}$ from $60$ to $100\,mmHg$).

  • Hemoglobin has a high affinity for oxygen to facilitate loading.

  • Act as a Safety Net: Large drops in $Pa_{O_2}$ result in only minor changes in saturation (e.g., dropping from 100 to 60 mmHg only drops saturation from approx. 97% to 90%).

• Steep Lower Portion (Dissociation):

  • Occurs at the Tissues ($Pa_{O_2}$ from $20$ to $60\,mmHg$).

  • Hemoglobin has a lower affinity, facilitating the rapid unloading/release of oxygen to tissues.

• The P50 Value:

  • The partial pressure at which hemoglobin is $50\%$ saturated.

  • Normal P50: $27\,mmHg$ (at standard conditions: $pH = 7.4, Temp = 37^{\circ}C$).

Factors Affecting Curve Shifts

• Right Shift (Decreased Affinity - Releasing):

  • Occurs at Tissues (RT - Right at Tissues).

  • Hemoglobin releases oxygen more easily.

  • Causes:

    • Increased $Pa_{CO_2}$

    • Decreased $pH$ (Acidosis)

    • Increased Temperature (e.g., fever, exercise)

    • Increased $2,3\text{-DPG}$ (BPG)

• Left Shift (Increased Affinity - Loading):

  • Occurs at Lungs (LL - Left at Lungs).

  • Hemoglobin holds onto oxygen more tightly.

  • Causes:

    • Decreased $Pa_{CO_2}$

    • Increased $pH$ (Alkalosis)

    • Decreased Temperature (Hypothermia)

    • Decreased $2,3\text{-DPG}$

Oxygen Delivery and Consumption

• Oxygen Delivery ($DO_2$):

  • The total amount of oxygen delivered to tissues per minute.

  • Calculation: $DO_2 = CaO_2 \times Q \text{ (Cardiac Output)} \times 10$

  • Normal Value: Approximately $1000\,mL/min$.

  • Compensatory mechanism for low oxygen: Increased heart rate to boost cardiac output.

• Oxygen Consumption ($VO_2$):

  • The amount of oxygen extracted by tissues per minute.

  • Calculation: $VO_2 = Q \times C(a-v)O_2 \times 10$

  • Normal Value: Approximately $250\,mL/min$.

  • Increases with exercise, seizures, and fever. Decreases with hypothermia, sedation, and certain poisons (e.g., Cyanide).

• Arterial-Venous Oxygen Content Difference ($C(a-v)O_2$):

  • The difference between arterial and venous oxygen content.

  • Normal Value: Approximately $5\,vol\%$

• Oxygen Extraction Ratio ($O_2ER$):

  • The percentage of delivered oxygen the tissues actually use.

  • Calculation: $\frac{CaO_2 - Cv_{O_2}}{CaO_2}$

  • Normal Value: Approximately $25\%$.

Cyanosis and Clinical Implications

• Definition: A bluish discoloration of the skin or mucous membranes caused by desaturated hemoglobin.

• Threshold: Requires at least $5\,g/dL$ of deoxygenated (reduced) hemoglobin in the capillary bed.

• Types:

  • Peripheral Cyanosis: Blueness in extremities (fingers/toes). Caused by low blood flow or increased tissue demand.

  • Central Cyanosis: Blueness in lips, tongue, and oral mucosa. Indicates low arterial saturation (Sa_{O_2} < 83\%) and is often a medical emergency.

• Polycythemia vs. Anemia:

  • Patients with Polycythemia (high RBC count) show cyanosis sooner because they reach the $5\,g/dL$ reduced hemoglobin threshold while still having adequate oxygenated blood.

  • Patients with Anemia show cyanosis much later (or not at all) because they have so little total hemoglobin that they might reach fatal hypoxia before reaching $5\,g/dL$ of reduced hemoglobin.

Pulmonary Shunting

• Definition: Perfusion without ventilation ($V/Q$ of zero). Blood passes from the right side of the heart to the left without picking up oxygen.

• True (Absolute) Shunt:

  • Anatomic: Blood bypasses the lungs entirely (e.g., Ventricular Septal Defect - VSD).

  • Capillary: Alveoli are completely collapsed (atelectasis) or filled with fluid.

  • Refractory Hypoxia: True shunts do not respond well to supplemental oxygen alone; they require pressure (PEEP/CPAP) to open the unit.

• Shunt-like Effect (Relative):

  • Alveoli are partially ventilated, but oxygenation is incomplete (e.g., hypoventilation or diffusion defects like pulmonary fibrosis).

• Shunt Significance:

  • Normal: < 10\%.

  • Moderate: $10-20\%$

  • Severe/Life-threatening: $20-30\%$ or higher.

Questions & Discussion

• Q: How does skin color affect pulse oximetry?

• A: Darker skin can lead to overestimation of oxygen levels on pulse oximeters, which may hide falling levels in sick patients.

• Q: Is cyanosis always present in hypoxia?

• A: No. Anemic patients may be severely hypoxic without ever appearing blue because they don't have enough hemoglobin to reach the $5\,g/dL$ reduced threshold.