Medical Gas Therapy - Egan's Chapter 42 Review

Clinical Foundations and Indications for Oxygen Therapy

• Acute Hypoxemia Definition and Diagnostic Thresholds:

  • Defined pathologically as an abnormally low level of oxygen in the arterial blood.
  • Laboratory threshold: Partial pressure of arterial oxygen ($PaO_2$) less than $60\,\text{mmHg}$.
  • Pulse oximetry / saturation threshold: Arterial oxygen saturation ($SaO_2$) less than $90\%$ while breathing ambient room air ($FiO_2 = 0.21$).

• Hypercapnia Definition:

  • Defined as an abnormally increased concentration of carbon dioxide ($CO_2$) within the blood.

• Cardiopulmonary Compensatory Mechanisms:

  • When blood oxygen levels decrease, both the heart and lungs increase their operational workload to compensate for the tissue oxygen deficit.
  • Properly titrated and applied oxygen ($O_2$) therapy alleviates cardiopulmonary stress and decreases overall ventilatory demand.

• Assessment Methods for Supplemental Oxygen Need:

  • Direct arterial blood gas analysis: Evaluation of $PaO_2$.
  • Noninvasive and co-oximetry saturation measurement: Assessment of $SaO_2$ or pulse oximetry saturation ($SpO_2$).
  • Assessment of the comprehensive clinical situation and underlying pathophysiology.

• Clinical Indications Beyond Diagnostic Oxygen Levels:

  • Severe physical trauma.
  • Acute myocardial infarction (MI).
  • Administration of anesthesia and monitoring during post-anesthesia recovery.
  • Acute chest pain suggestive of cardiac ischemia.
  • Documented or suspected hypoxemia.
  • Sickle cell anemia crises.

Hazards and Complications of Medical Gas Therapy

• Primary Hazards of Oxygen Administration:

  • Oxygen toxicity.
  • Retinopathy of prematurity (ROP).
  • Depression of ventilation (DOV).
  • Fire hazard.
  • Absorption atelectasis.

• Oxygen Toxicity Pathophysiology and Manifestations:

  • Target Organs Affected: Primarily damages the lungs (pulmonary system) and the central nervous system (CNS).
  • Determinant Risk Factors: Tissue damage is governed by two interacting variables:
  • High delivered concentration of oxygen ($FiO_2$).
  • Total duration and length of exposure to elevated oxygen concentrations.
  • Vicious Cycle of Oxygen Toxicity:
  • Step 1: Initial oxygen toxicity occurs due to high $FiO_2$ and prolonged exposure.
  • Step 2: Worsening alveolar and capillary damage leads to increased pulmonary shunting.
  • Step 3: Increased shunting causes refractory low arterial oxygen tension ($PaO_2$).
  • Step 4: Clinicians raise the supplemental oxygen concentration ($FiO_2$) in an attempt to correct the hypoxemia.
  • Step 5: The higher $FiO_2$ exacerbates oxygen toxicity, continuing the destructive cycle.

• Retinopathy of Prematurity (ROP):

  • Pathological consequence of excessive arterial oxygenation in premature infants.
  • May result in permanent visual impairment and irreversible blindness.

• Depression of Ventilation (DOV) Considerations:

  • Clinical occurrence in Chronic Obstructive Pulmonary Disease (COPD): Depression of ventilation is not typical in all COPD patients when exposed to elevated oxygen concentrations; hypoventilation is not an absolute or universal response across the entire COPD patient population.

• Mechanism of Absorption Atelectasis:

  • High concentrations of inspired oxygen ($FiO_2$) wash nitrogen ($N_2$) out of the alveoli.
  • Nitrogen normally exerts a "splinting" effect that keeps alveoli patent at low partial pressures; removing nitrogen allows rapid and complete alveolar collapse (absorption atelectasis).

• Combustion and Fire Hazard Requirements:

  • Oxygen vigorously accelerates combustion.
  • Three components required for a fire to start and continue burning (the fire triangle):
  • Oxygen ($O_2$).
  • Heat (ignition source).
  • Fuel.

Device Classifications and Low-Flow Delivery Systems

• Inspiratory Demand and Performance Classifications:

  • A delivery system must deliver an output flow that meets or exceeds the patient's peak inspiratory demand to guarantee a stable, precise fraction of inspired oxygen ($FiO_2$).
  • Variable-Performance Systems: Oxygen delivery devices that provide only a fraction of the patient's inspiratory flow demand; the remainder of the inspiratory volume is drawn from ambient room air, causing $FiO_2$ to fluctuate with ventilatory pattern changes.
  • High-flow criteria: A true high-flow oxygen delivery system must supply a minimum total output flow of at least $60\,\text{L/min}$.

• Nasal Cannula:

  • Operates as a low-flow, variable-performance device.
  • At an input flow rate of $4\,\text{L/min}$, delivered $FiO_2$ is approximately $0.36$ ($36\%$).

• Simple Oxygen Mask:

  • Minimum operational flow setting for adult application: $5\,\text{L/min}$.
  • Flow rates below $5\,\text{L/min}$ permit the accumulation and rebreathing of expired carbon dioxide ($CO_2$) inside the mask reservoir.

• General Complications and Disadvantages of Oxygen Masks:

  • Localized skin irritation and breakdown over contact surfaces.
  • Risk of pulmonary aspiration if emesis occurs while wearing the appliance.
  • Potential for patient suffocation if gas flow is inadvertently occluded or disconnected.

Reservoir and Emergency Oxygen Delivery Devices

• Partial-Rebreather Mask:

  • Reservoir device equipped with an attached gas reservoir bag.
  • Valve configuration: Contains $0$ valves (no one-way valves between the bag and mask or on the exhalation ports).
  • Delivered oxygen concentration range: $FiO_2 = 0.40-0.70$ ($40\%-70\%$).

• Non-Rebreather Mask:

  • Utilized for the administration of high concentrations of supplemental oxygen.
  • Delivered oxygen concentration range: Approximately $FiO_2 = 0.60-0.80$.
  • Valve configuration: Incorporates one-way valves (flapper valves) that prevent exhaled gases from entering the reservoir bag and prevent ambient room air entrainment through the mask exhalation ports.
  • Distinguishing feature: Unlike the partial-rebreather mask (which has zero valves), the non-rebreather mask utilizes one-way valves to prevent rebreathing and room air dilution.

• High-Flow Nasal Cannula Classification:

  • A high-flow nasal cannula is categorized as an active high-flow delivery device and is NOT classified as a reservoir device.

• Emergency and Resuscitation Delivery Systems:

  • Standard appliance used in resuscitation or emergency situations to administer close to $100\%$ oxygen ($FiO_2 \approx 1.0$): Bag-mask device (BMD / BVM - Bag-Valve-Mask).
  • Initial $FiO_2$ indicated during acute emergency resuscitation: $FiO_2 = 1.0$ ($100\%$).

Air-Entrainment Systems and Flow Dynamics

• Operating Principles of Air-Entrainment Masks (AEM):

  • Deliver a fixed $FiO_2$ by utilizing high-velocity oxygen gas passing through a fixed jet nozzle to entrain atmospheric room air through adjustable air-entrainment ports.
  • Adjusting Flowmeter Settings: Increasing the delivered oxygen flow from the flowmeter does not alter the delivered $FiO_2$; it proportionally increases the total output flow delivered to the patient.

• Effects of Downstream Resistance and Port Obstruction:

  • Back pressure generated by downstream resistance or obstructions in the tubing causes: Delivered oxygen concentration ($FiO_2$) to increase, and total delivered output flow to decrease.
  • Direct obstruction of the air-entrainment ports or pathway causes: Delivered oxygen concentration to increase, total system output flow to decrease, and the delivered $FiO_2$ to become unstable or variable.

• Step-by-Step Flow Calculations for Air-Entrainment Systems:

  • Calculation Parameters for an AEM delivering $35\%$ oxygen at an input flow of $7\,\text{L/min}$:
  • Air-to-Oxygen Entrainment Ratio: An $FiO_2$ of $0.35$ ($35\%$) utilizes an approximate $5:1$ ratio ($5\,\text{parts air}$ to $1\,\text{part } O_2$).
  • Total Flow Multiplier: Total ratio parts equal $5 + 1 = 6$.
  • Total System Output Flow:       $\text{Total Flow} = (5 + 1) \times 7\,\text{L/min} = 6 \times 7\,\text{L/min} = 42\,\text{L/min}$
  • Patient Inspiratory Demand Evaluation:
  • Patient Parameters: Respiratory Rate ($RR$) = $14/\text{min}$; Tidal Volume ($V_T$) = $500\,\text{mL}$ ($0.5\,\text{L}$).
  • Minute Ventilation ($MV$) Calculation:       $MV = RR \times V_T = 14/\text{min} \times 0.5\,\text{L} = 7\,\text{L/min}$
  • Estimated Peak Inspiratory Demand:       $\text{Peak Inspiratory Demand} = MV \times 3 = 7\,\text{L/min} \times 3 = 21\,\text{L/min}$
  • Performance Verification: Because the total system output flow ($42\,\text{L/min}$) exceeds the patient's estimated peak inspiratory demand ($21\,\text{L/min}$), the system functions as a true fixed-performance device and maintains an accurate $35\%$ $FiO_2$.

Oxygen Weaning and Titration Protocols

• Weaning Criteria:
  • Oxygen weaning protocols begin when clinical indicators demonstrate that oxygenation has stabilized or improved, indicating the patient no longer requires the current level of supplemental therapy.
  • Weaning is executed by systematically reducing the delivered $FiO_2$ while monitoring target saturation parameters.