Oxygenation & Oxygen Delivery
Master the principles of oxygen transport, hemoglobin binding, the oxyhemoglobin dissociation curve, cardiac output, tissue perfusion, and ABG interpretation, foundational concepts for all nursing practice.
Hemoglobin & Oxygen Binding
How oxygen travels in the blood
Oxygen is transported in the blood in two forms: dissolved in plasma (measured as PaO2, only about 1.5% of total oxygen) and bound to hemoglobin (measured as SaO2/SpO2, about 98.5% of total oxygen). Each hemoglobin molecule can carry up to 4 oxygen molecules. Understanding this distinction is critical because pulse oximetry measures oxygen saturation (how much hemoglobin is loaded), not the total oxygen content of the blood.
Hemoglobin Structure & Function
Structure: 4 globin chains (2 alpha, 2 beta in adult HgbA), each containing a heme group with an iron atom (Fe2+). Oxygen binds reversibly to the iron. Cooperative binding: Once the first O2 binds, the hemoglobin molecule changes shape, making it easier for subsequent O2 molecules to bind (this creates the S-shaped dissociation curve). Normal values: Hemoglobin 12-16 g/dL (female), 14-18 g/dL (male). Each gram of Hgb carries 1.34 mL O2 when fully saturated.
Oxygen Saturation (SpO2/SaO2)
SaO2: Arterial oxygen saturation measured from ABG (gold standard). SpO2: Peripheral oxygen saturation measured by pulse oximetry (non-invasive estimate). Normal: 95-100%. Critical insight: Due to the S-shaped curve, SpO2 stays high until PaO2 drops significantly. A SpO2 of 90% corresponds to a PaO2 of only ~60 mmHg, below the 'steep part' of the curve, small PaO2 drops cause large SpO2 drops. This is why SpO2 below 90% is considered critical.
Oxygen Content Equation
CaO2 = (Hgb × 1.34 × SaO2) + (0.003 × PaO2). The first term (hemoglobin-bound O2) contributes ~98.5% of total oxygen content. The second term (dissolved O2) is minimal. This equation explains why a patient can have a normal SpO2 but still be hypoxic if severely anemic, there isn't enough hemoglobin to carry adequate oxygen, even if what hemoglobin exists is fully saturated.
Oxyhemoglobin Dissociation Curve
Right and left shifts explained
The oxyhemoglobin dissociation curve is an S-shaped curve that shows the relationship between PaO2 (x-axis) and hemoglobin saturation (y-axis). The curve's position can shift right or left depending on physiologic conditions, affecting how readily hemoglobin binds and releases oxygen.
RIGHT Shift, O2 Released to Tissues
Hemoglobin affinity DECREASES, oxygen is unloaded more readily at the tissues. This makes physiologic sense: conditions that increase metabolic demand also shift the curve right to deliver more O2. Causes (mnemonic, CADET face Right): CO2 increased, Acidosis (decreased pH), 2,3-DPG increased, Exercise/Fever (increased temperature). Clinical relevance: A febrile, acidotic patient delivers oxygen to tissues more efficiently but may desaturate faster.
LEFT Shift, O2 Held by Hemoglobin
Hemoglobin affinity INCREASES, oxygen binds more tightly and is not released as easily at the tissues. Causes: Decreased CO2, Alkalosis (increased pH), Decreased 2,3-DPG, Hypothermia, Carbon monoxide (CO binds 200x tighter than O2), Fetal hemoglobin (HgbF has higher O2 affinity to extract O2 from maternal blood). Clinical relevance: SpO2 may look normal, but tissues may still be hypoxic because hemoglobin won't release its oxygen.
The Oxyhemoglobin Dissociation Curve
The oxyhemoglobin dissociation curve describes hemoglobin's affinity for oxygen at different partial pressures. A RIGHT shift (decreased affinity, hemoglobin releases oxygen more readily to tissues) is caused by increased temperature, increased CO2 (Bohr effect), increased 2,3-DPG, and decreased pH (acidosis). A LEFT shift (increased affinity, hemoglobin holds onto oxygen more tightly) is caused by decreased temperature, decreased CO2, decreased 2,3-DPG, increased pH (alkalosis), carbon monoxide, and fetal hemoglobin. Mnemonic for right shift: 'Right = Release', conditions that increase tissue metabolic demand shift the curve right to deliver more oxygen.
Cardiac Output & O2 Delivery
CO = HR × SV and the O2 delivery equation
Oxygen delivery to tissues depends on two factors: the oxygen content of the blood (CaO2) and the cardiac output (CO). Even if blood is well-oxygenated, tissues will become hypoxic if cardiac output is insufficient to deliver it. The oxygen delivery equation integrates both components: DO2 = CO × CaO2 × 10.
Cardiac Output Components
Heart Rate (HR): Normal 60-100 bpm. Too fast (tachycardia) reduces ventricular filling time → decreased stroke volume. Too slow (bradycardia) may not provide adequate output. Stroke Volume (SV): Volume ejected per beat, normally ~70 mL. Determined by: Preload (venous return/end-diastolic volume, Frank-Starling: more stretch = more force up to a point), Afterload (resistance to ejection, primarily SVR; high afterload = decreased SV), Contractility (intrinsic muscle force independent of preload/afterload).
O2 Delivery Equation
DO2 = CO × CaO2 × 10 (normal ~1000 mL O2/min). Tissues extract about 250 mL O2/min at rest (25% extraction ratio). This reserve means the body can compensate for moderate reductions in delivery. Clinical application: Improving O2 delivery can target any component, give O2 (increases PaO2/SaO2), transfuse blood (increases Hgb), give fluids (increases preload → increases SV → increases CO), give inotropes (increases contractility → increases SV).
Cardiac Output = HR × SV
Cardiac output (CO) = Heart Rate (HR) × Stroke Volume (SV). Normal CO is approximately 4-8 L/min. Stroke volume is determined by three factors: Preload (volume of blood filling the ventricle, Frank-Starling mechanism), Afterload (resistance the ventricle must pump against, primarily systemic vascular resistance), and Contractility (strength of ventricular contraction, inotropic state). Increasing preload or contractility increases CO; increasing afterload decreases CO. Medications target these factors: fluids increase preload, vasodilators decrease afterload, and inotropes increase contractility.
Perfusion vs Oxygenation & Organ Hypoxia
Why tissues fail without adequate oxygen
Oxygenation and perfusion are related but distinct concepts. A patient can have excellent oxygenation (high SpO2) but poor perfusion (low cardiac output, shock). Conversely, a patient can have adequate perfusion but poor oxygenation (respiratory failure). Both must be adequate for tissue survival.
Oxygenation
Refers to how well oxygen gets into the blood from the lungs. Assessed by PaO2 and SpO2. Problems include: ventilation failure (COPD, pneumonia), diffusion impairment (pulmonary fibrosis, ARDS), V/Q mismatch (PE, atelectasis), shunt (blood bypassing ventilated alveoli). Treated with supplemental O2, mechanical ventilation, treating the underlying lung pathology.
Perfusion
Refers to how well oxygenated blood is delivered to tissues. Assessed by blood pressure, cardiac output, capillary refill, urine output, mental status, lactate levels. Problems include: cardiogenic shock (pump failure), hypovolemic shock (volume loss), distributive shock (vasodilation in sepsis/anaphylaxis). Treated with fluids, vasopressors, inotropes, treating the underlying cause.
Organ-Specific Hypoxia Effects
Pulse Oximetry & ABG Basics
Monitoring oxygenation at the bedside
Pulse oximetry and arterial blood gases are the two primary tools for assessing oxygenation. Understanding their principles, normal values, and limitations is essential for safe nursing practice.
Pulse Oximetry (SpO2)
Principle: Uses two wavelengths of light (red and infrared) passed through a pulsatile vascular bed. Oxyhemoglobin and deoxyhemoglobin absorb these wavelengths differently, allowing calculation of saturation percentage. Normal: 95-100%. Limitations: Inaccurate with poor perfusion (shock, cold extremities, vasoconstriction), nail polish (especially dark colors), carbon monoxide poisoning (reads falsely high, CO-Hgb absorbs like O2-Hgb), severe anemia (can show normal SpO2 with critically low O2 content), methemoglobinemia (reads ~85% regardless of true saturation), motion artifact, ambient light.
ABG Normal Values
ABG Interpretation Steps (ROME Method)
Respiratory = Opposite: When pH and PaCO2 move in opposite directions, the primary disorder is respiratory. Metabolic = Equal: When pH and HCO3 move in the same direction, the primary disorder is metabolic. Example: pH 7.30 (acidosis), PaCO2 55 (high = acidic) → pH down, CO2 up = opposite directions → Respiratory acidosis. Example: pH 7.50 (alkalosis), HCO3 32 (high = alkaline) → pH up, HCO3 up = same direction → Metabolic alkalosis.
ABG Interpretation Framework
Arterial blood gas (ABG) interpretation is a critical nursing skill. Normal values: pH 7.35-7.45, PaCO2 35-45 mmHg, HCO3 22-26 mEq/L, PaO2 80-100 mmHg. Step 1: Look at pH, acidosis (<7.35) or alkalosis (>7.45). Step 2: Check PaCO2, if it explains the pH change, the primary disorder is respiratory. Step 3: Check HCO3, if it explains the pH change, the primary disorder is metabolic. Step 4: Check for compensation, the body tries to normalize pH using the opposite system (respiratory compensates for metabolic and vice versa). Step 5: Check PaO2, is the patient hypoxemic?
Match the Oxygenation Concept
Terms
Definitions
Oxygenation & O2 Delivery Quiz
1/20Approximately what percentage of oxygen in the blood is carried by hemoglobin?