RTH104 Unit 3 Study Guide: Oxygenation and Arterial Blood Gas Interpretation

Comprehensive vocabulary flashcards covering the oxyhemoglobin dissociation curve, hypoxia and hypoxemia classifications, pulmonary shunt equations, oxygen content formulas, and respiratory clinical math from the RTH104 Unit 3 Study Guide.

Oxyhemoglobin ($\text{HbO}_2$) Dissociation Curve

A graphical representation that correlates the relationship between arterial oxygen partial pressure ($PaO_2$) and hemoglobin saturation ($SaO_2$).

Dissociation Curve Benchmarks

Six key normal coordinate points on the oxyhemoglobin curve: $27\,\text{mmHg}$ ($50\%$), $30\,\text{mmHg}$ ($60\%$), $40\,\text{mmHg}$ ($75\%$), $60\,\text{mmHg}$ ($90\%$), $95\,\text{mmHg}$ ($98\%$), and $150\,\text{mmHg}$ ($100\%$).

Left Shift

An increase in hemoglobin affinity for oxygen causing it to hold onto $O_2$ and decreasing $P_{50}$; caused by decreased $CO_2$, decreased $H^+$, decreased temperature, decreased 2,3-DPG, or increased pH. Occurs normally at the lungs to facilitate $O_2$ loading.

Right Shift

A decrease in hemoglobin affinity for oxygen causing it to release $O_2$ to tissues and increasing $P_{50}$; caused by increased $CO_2$, increased $H^+$, increased temperature, increased 2,3-DPG, or decreased pH. Occurs normally at the tissues to facilitate $O_2$ unloading.

Bohr Effect

The physiological mechanism where increased $CO_2$ and $H^+$ ions decrease hemoglobin affinity for oxygen, causing hemoglobin to release $O_2$ at the tissues.

Haldane Effect

The physiological mechanism where oxygen binding to hemoglobin in the lungs promotes the release of carbon dioxide ($CO_2$).

Carboxyhemoglobin (COHb)

A hemoglobin compound formed when carbon monoxide binds hemoglobin over $200$ times more strongly than $O_2$, reducing oxygen-carrying capacity and shifting the dissociation curve to the left, which impairs tissue oxygen delivery.

Methemoglobin

A form of hemoglobin in which iron is oxidized from $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$, rendering it incapable of normal oxygen transport; normal concentration is less than $1\%$.

Sickle-Cell Anemia

A genetic condition in which red blood cells become sickle-shaped and fragile, predisposing them to rupture or occlude small blood vessels and reducing oxygen delivery.

Total Oxygen Content Equation ($C_xO_2$)

The mathematical formula $C_xO_2 = (1.34 \times \text{hgb} \times S_xO_2) + (0.003 \times P_xO_2)$, where the first term represents oxygen bound to hemoglobin and the second term represents oxygen dissolved in plasma.

Normal Oxygen Content Values

Standard physiologic content levels: normal arterial content ($C_aO_2$) is $20\,\text{mL/dL}$, normal venous content ($C_{\bar{v}}O_2$) is $15\,\text{mL/dL}$, and normal arterial-venous difference ($C(a-\bar{v})O_2$) is $5\,\text{mL/dL}$.

Deoxygenated Hemoglobin Formula

Calculated as $(1 - \text{saturation}) \times \text{Hb}$; with a standard hemoglobin of $15\,\text{g/dL}$, normal arterial deoxygenated hemoglobin is $0.3\,\text{g/dL}$ and normal mixed venous is $3.75\,\text{g/dL}$.

Cyanosis

A bluish discoloration of the skin and mucous membranes caused by approximately $5\,\text{g/dL}$ or more of deoxygenated hemoglobin in the blood.

Acrocyanosis

Bluish discoloration confined to the hands and feet (extremities), caused by reduced peripheral blood flow, cold-induced vasoconstriction, low blood pressure, or arterial blockage.

Central Cyanosis

Bluish discoloration of the lips, tongue, oral mucosa, or trunk, typically caused by decreased arterial oxygen tension ($PaO_2$) or decreased saturation ($SaO_2$).

Polycythemic Cyanosis

The clinical appearance of cyanosis without underlying tissue hypoxia, caused by an increased total red blood cell mass and hemoglobin concentration that allows $5\,\text{g/dL}$ of deoxygenated hemoglobin to accumulate despite adequate tissue delivery.

Mixed Venous Oxygen Tension ($P_{\bar{v}}O_2$)

A parameter indicating tissue extraction and delivery; values below $35\,\text{mmHg}$ suggest significant tissue hypoxia, while values below approximately $27\,\text{mmHg}$ indicate anaerobic metabolism and lactic acidosis.

Non-Invasive Cardiac Assessment

Indirect methods for monitoring cardiovascular function without invasive lines (such as Swan-Ganz catheters or transesophageal echocardiography), including heart rate, blood pressure, urine output, peripheral pulses, skin color and temperature, and capillary refill time.

Hypoxemia Severity Ranges

Arterial blood gas thresholds based on $PaO_2$: Mild is $60\text{--}79\,\text{mmHg}$, Moderate is $40\text{--}59\,\text{mmHg}$, and Severe is $<40\,\text{mmHg}$.

Signs and Symptoms of Hypoxemia

Clinical manifestations including dyspnea, tachypnea, hyperpnea, tachycardia, hypertension, cyanosis, arrhythmias, restlessness, confusion, lethargy, and coma in severe cases.

Hypoxia vs. Hypoxemia

Hypoxia is a state of abnormally low oxygen availability at the tissue level, whereas hypoxemia is an abnormally low partial pressure or content of oxygen in the blood.

Hypoxic Hypoxemia

A form of hypoxemia resulting from a low ventilation/perfusion ($V/Q$) ratio, pulmonary shunt, diffusion defect, or low inspired oxygen tension ($P_IO_2$); low $V/Q$ is the most common cause and usually responds to supplemental $FiO_2$.

Shunt Fraction ($Q_s/Q_t$)

The ratio of shunted blood flow to total cardiac output; increases when blood bypasses ventilated alveoli or perfuses non-ventilated lung units and does not fully correct with supplemental $FiO_2$.

Classic Shunt Equation

The equation defined as $\frac{Q_s}{Q_t} = \frac{C_cO_2 - C_aO_2}{C_cO_2 - C_{\bar{v}}O_2}$, where $C_cO_2$ is ideal end-capillary oxygen content, $C_aO_2$ is arterial oxygen content, and $C_{\bar{v}}O_2$ is mixed venous oxygen content.

Modified Shunt Equation

The estimated formula $\frac{P(A-a)O_2 \times 0.003}{[P(A-a)O_2 \times 0.003] + 3.5}$, indicated when the patient's $PaO_2$ is greater than $100\,\text{mmHg}$ and hemodynamic status is stable.

Estimated Shunt Formula on $100\%$ $FiO_2$

A bedside formula used during pure oxygen breathing to calculate percentage shunt: $\text{Shunt } \% = \frac{PAO_2 - PaO_2}{20} \times 100$.

Shunt Severity Classifications

Physiologic ranges for pulmonary shunt fraction: $5\text{--}10\%$ is normal, $10\text{--}19\%$ is abnormal, $20\text{--}29\%$ is significant (often requiring ventilatory support), and above $30\%$ is severe and potentially life-threatening.

Diffusion Defect

Impaired alveolar-capillary gas exchange caused by conditions like pulmonary edema (commonly secondary to left-sided CHF), emphysema, or interstitial lung disease; may improve with supplemental $FiO_2$ but is not fully corrected.

Absolute Anemic Hypoxia

Tissue hypoxia resulting from an absolute quantitative deficit of hemoglobin, caused by acute blood loss, iron deficiency, reduced erythropoietin production in renal failure, or red blood cell hemolysis.

Functional (Relative) Anemic Hypoxia

Tissue hypoxia occurring when hemoglobin is quantitatively present but structurally or chemically incapable of normal oxygen binding and transport, as seen in carbon monoxide poisoning, methemoglobinemia, and sickle-cell crisis.

Circulatory Hypoxia

A form of hypoxia without hypoxemia resulting from inadequate systemic blood flow delivering insufficient oxygen to tissues; common causes include low cardiac output, left-sided CHF, shock, and peripheral vascular disease.

Histotoxic Hypoxia

A condition in which normal amounts of oxygen are delivered to the peripheral tissues, but cellular poisoning prevents cells from utilizing it; classically caused by cyanide poisoning.

Temperature Effect on Oxygen Consumption

The physiological relationship where a patient's oxygen consumption increases by approximately $10\%$ for every $1\,^\circ\text{C}$ increase in body temperature.

Alveolar Air Equation ($PAO_2$)

The mathematical formula for ideal alveolar oxygen tension: $PAO_2 = [(P_B - P_{H_2O}) \times FiO_2] - \frac{PaCO_2}{RQ}$.