Respiratory Care Procedures I: Need for Oxygen, Evaluation of Hypoxemia, and Hazards of Oxygen Therapy

Vocabulary flashcards covering key definitions, types of hypoxia, oxygen therapy goals and hazards, arterial blood gas values, pulse oximetry, and clinical oxygen calculations.

Hypoxemia

Decreased oxygen levels in the blood.

Hypoxia

Decreased oxygen levels at the tissue level.

$F_{IO_2}$ (Fraction of Inspired Oxygen Concentration)

The fraction of inspired oxygen concentration, ranging from $0.21$ ($21\%$) to $1.0$ ($100\%$).

Goals of $O_2$ Therapy

1. Correction of documented or suspected hypoxemia/hypoxia ($PaO_2 < 60\,\text{torr}$, $SpO_2 < 90\%$); 2. Decrease symptoms associated with chronic hypoxemia; 3. Decrease cardiopulmonary workload.

Hypoxemic Hypoxia

Decreased oxygen levels in the blood leading to decreased oxygen levels in the tissue; it is the most responsive type of hypoxia to oxygen therapy.

Hypoventilation

Decreased oxygen levels resulting from poor ventilation (e.g., CNS depression or post-anesthesia); it is the most common cause of hypoxemic hypoxia.

Alveolar Deadspace

An area of the lung that has ventilation without perfusion (blood flow), such as caused by a pulmonary embolism.

Shunting

An area of the lung that has perfusion (blood flow) without ventilation, preventing effective gas exchange.

Atelectasis

Collapsed alveoli that result in perfusion without ventilation (shunting) and poor gas exchange.

Diffusion Defect

A thickening of the alveolar-capillary ($A/C$) membrane that interferes with the ability of gas to cross the membrane (e.g., pulmonary fibrosis caused by pneumoconiosis).

Alveolar-Capillary Membrane ($A/C\text{ Membrane}$)

The multi-layered tissue interface between the alveolar air space and the pulmonary capillary through which oxygen and carbon dioxide diffuse.

Low Ambient Oxygen Concentration

A cause of hypoxemic hypoxia resulting from reduced atmospheric pressure, such as at high altitudes.

Circulatory (Stagnant) Hypoxia

Failure of the heart to pump an adequate amount of blood to transport oxygen to the tissues (e.g., shock, acute myocardial infarction, or cardiovascular instability); primarily managed by increasing cardiac output.

Anemic Hypoxia

Decreased oxygen-carrying capacity of the blood caused by insufficient hemoglobin ($< 10\,\text{g/dL}$) or conditions like carbon monoxide poisoning.

Histotoxic Hypoxia (Dysoxia)

An inability of the tissues to utilize oxygen due to decreased cellular uptake (e.g., sepsis or cyanide poisoning); rarely accompanied by hypoxemia.

Arterial Blood Gas (ABG) Normal Values

$pH$: $7.35\text{--}7.45$ (acid-base balance); $PaCO_2$: $35\text{--}45\,\text{torr}$ (ventilation status); $PaO_2$: $80\text{--}100\,\text{torr}$ (oxygenation status); $HCO_3^-$: $22\text{--}28\,\text{mEq/L}$ (renal compensation).

$PaO_2$ Severity Classification

At barometric pressure ($P_B$) of $760\,\text{torr}$: Normal = $80\text{--}100\,\text{mmHg}$; Mild hypoxemia = $60\text{--}79\,\text{mmHg}$; Moderate hypoxemia = $40\text{--}59\,\text{mmHg}$; Severe hypoxemia = $< 40\,\text{mmHg}$.

Cyanosis

A bluish color of the skin or mucous membranes caused by the presence of at least $5\,\text{g/dL}$ of unsaturated (reduced) hemoglobin in capillary blood.

Central Cyanosis

Cyanosis observed around the mucosal membranes, mouth, and lips, indicating systemic arterial desaturation.

Peripheral Cyanosis (Acrocyanosis)

Cyanosis localized to the extremities (hands and feet); normal in newborns immediately after birth and typically resolving within 10 minutes.

Pulse Oximetry

A non-invasive technique that measures the arterial oxygen saturation of hemoglobin ($SpO_2$) using two wavelengths of light (red and infrared) transmitted through the tissue.

$SpO_2$ Thresholds

Normal saturation is $95\text{--}97\%$, while the critical threshold indicating severe hypoxemia is $87\text{--}90\%$.

Hazards of Oxygen Therapy

1. Oxygen-induced hypoventilation; 2. Hyperoxic acute lung injury; 3. Retinopathy of prematurity (ROP); 4. Absorption atelectasis; 5. Fire hazard.

Hyperoxic Acute Lung Injury (ALI)

Cell damage and associated inflammatory response caused by an overproduction of oxygen free radicals during exposure to high partial pressures of oxygen ($PaO_2$).

Alveolar Air Equation ($PAO_2$)

$PAO_2 = (P_B - P_{H_2O}) \times F_{IO_2} - \frac{PaCO_2}{RQ}$, where $P_B = 760\,\text{mmHg}$, $P_{H_2O} = 47\,\text{mmHg}$, and $RQ = 0.8$. Normal value is $100\text{--}110\,\text{mmHg}$.

Alveolar-arterial Oxygen Gradient ($AaDO_2$ or $P(A-a)O_2$)

The difference between alveolar oxygen tension and arterial oxygen tension ($AaDO_2 = PAO_2 - PaO_2$); normal value is $10\text{--}20\,\text{mmHg}$ on room air.

Arterial Oxygen Content ($CaO_2$)

The total amount of oxygen bound to hemoglobin and dissolved in plasma: $CaO_2 = (\text{Hb} \times 1.34 \times SaO_2) + (PaO_2 \times 0.003)$. Normal value is approximately $20\,\text{vol\%}$ ($\text{mL/dL}$).