MedGas Administration: High Flow Devices, Enclosure Systems, and Titration

High Flow Nasal Cannula (HFNC)

  • System Overview & Components:

    • Flow rates typically range from 30 L/min30\,\text{L/min} to 60 L/min60\,\text{L/min}.
    • Oxygen concentrations range independently from room air (21% FiO221\%\,F_i\text{O}_2) up to 100% FiO2100\%\,F_i\text{O}_2.
    • Powered by AC or DC electrical sources.
    • Requires three essential components: a patient interface (e.g., nasal prongs, nasal pillows), a gas delivery system/blender regulating FiO2F_i\text{O}_2, and a heated humidification system (active humidifiers or vapor transfer cartridges such as Vapotherm or AIRVO; interfaces include Fisher by Kell).

  • Mechanisms and Benefits:

    • Acts as a fixed oxygen delivery system by meeting or exceeding the patient's inspiratory minute volume.
    • Generates positive end-expiratory pressure (PEEP) at approximately 1 cmH2O1\,\text{cmH}_2\text{O} for every 10 L/min10\,\text{L/min} of gas flow, stenting small airways open throughout the respiratory cycle.
    • Flushes and washes out anatomical dead space, decreasing dead space ventilation and reducing PaCO2P_a\text{CO}_2.
    • Delivers warmed and humidified gas to preserve mucosal integrity, facilitate secretion clearance, and maximize patient comfort.
    • Accommodates coughing, eating, drinking, and talking without interruption of therapy.

  • Clinical Indications:

    • Acute hypoxemic respiratory failure (e.g., pneumonia).
    • Hypercapnic respiratory failure (e.g., COPD exacerbation).
    • Pre-intubation support to prevent invasive mechanical ventilation.
    • Palliative respiratory comfort in patients with do-not-intubate (DNI) status or in hospice care.
    • Intolerance of closed-mask interfaces (e.g., CPAP or non-rebreather masks) or when non-rebreather reservoir bags collapse at 15 L/min15\,\text{L/min}.

Gas Blending Systems

  • Manual Gas Blending:

    • Requires separate, dual high-flow flow meters (reaching up to 70 L/min70\,\text{L/min}) connected to medical air and oxygen supplies.
    • Calculations for a desired total flow and FiO2F_i\text{O}_2:     O2 Flow (L/min)=Total Flow×(%O2−21)79\text{O}_2\text{ Flow (L/min)} = \frac{\text{Total Flow} \times (\%\text{O}_2 - 21)}{79}Air Flow (L/min)=Total Flow−O2 Flow\text{Air Flow (L/min)} = \text{Total Flow} - \text{O}_2\text{ Flow}
    • Example: To deliver 50% FiO250\%\,F_i\text{O}_2 at a total flow of 60 L/min60\,\text{L/min}, set the oxygen flow meter to 22 L/min22\,\text{L/min} and the air flow meter to 38 L/min38\,\text{L/min}.

  • Mechanical Oxygen Blenders:

    • Connect to high-pressure (50 psi50\,\text{psi}) air and oxygen sources via dedicated yellow and green supply lines.
    • Feature integrated low-flow or high-flow meters, or bagging ports (often used in pediatric settings to prevent bag-mask ventilation with inadvertent 100% FiO2100\%\,F_i\text{O}_2).
    • Deliver continuous flows over 60 L/min60\,\text{L/min} compatible with heated humidifiers (not aerosol nebulizers) and artificial airways.
    • Must be verified using a calibrated oxygen analyzer placed at the patient delivery outlet upon initiation and at least once per shift due to risks of mechanical failure or inaccuracy.

Oxygen Enclosure Systems

  • Oxygen Tents:

    • Historical pediatric enclosure providing variable FiO2F_i\text{O}_2 between 40%40\% and 50%50\%.
    • Requires high flows between 12 L/min12\,\text{L/min} and 15 L/min15\,\text{L/min} to flush accumulated carbon dioxide.
    • Subject to significant oxygen dilution from frequent opening; requires frequent oxygen analysis near the patient's face.

  • Oxygen Hoods:

    • Rigid transparent plastic enclosures covering the infant's head only.
    • Requires gas flows exceeding 7 L/min7\,\text{L/min} to prevent carbon dioxide rebreathing.
    • Delivers precise, heated, and humidified FiO2F_i\text{O}_2 from 21%21\% to 100%100\% via blenders or entrainment nebulizers.
    • High gas flows can create excessive internal noise levels; oxygen concentration must be continuously monitored close to the infant's face.

  • Isolettes:

    • Temperature-controlled enclosures maintaining a neutral thermal environment for stable newborns.
    • Oxygen is supplied via an internal 50 psi50\,\text{psi} connection, tubing bleeds from an external flow meter, or free-flowing anesthesia bags.
    • Less confining than hoods, but internal FiO2F_i\text{O}_2 fluctuates significantly during routine access; hoods provide superior oxygen stability if tight control is needed.

Device Selection and Clinical Scenarios

  • The Three Ps Selection Framework:

    • Purpose: Correct arterial hypoxemia, relieve hypoxic symptoms, and reduce cardiopulmonary workload.
    • Patient: Consider etiology, severity, age group, consciousness/aspiration risk, airway appliances, minute ventilation stability, and route of breathing (oral vs. nasal).
    • Performance: Critically ill patients require precise, high FiO2F_i\text{O}_2 devices; stable patients tolerate variable or lower FiO2F_i\text{O}_2 delivery.

  • Patient Clinical Categories:

    • Emergent / Severe Trauma: Suspected tissue hypoxia or acute resuscitation warrants immediate initiation of the highest possible FiO2F_i\text{O}_2 (100%100\%) using a non-rebreather mask, CPAP, or HFNC.
    • Critically Ill Adult: Moderate to severe hypoxemia warrants fixed high-flow or reservoir devices delivering at least 60% FiO260\%\,F_i\text{O}_2, titrated to blood gases and clinical work of breathing.
    • Stable Adult: Mild to moderate acute hypoxemia (e.g., uncomplicated postoperative recovery, post-myocardial infarction) requires low to moderate FiO2F_i\text{O}_2 via standard nasal cannula or simple mask.
    • Chronic Lung Disease: Acute-on-chronic hypoxemia requires controlled, low FiO2F_i\text{O}_2 delivery (1 to 2 L/min1\text{ to }2\,\text{L/min} nasal cannula or 24% to 28%24\%\text{ to }28\% air-entrainment mask) to prevent blunting hypoxic ventilatory drive, targeting a PaO2P_a\text{O}_2 between 50 mmHg50\,\text{mmHg} and 70 mmHg70\,\text{mmHg} and an SpO2S_p\text{O}_2 between 85%85\% and 92%92\%.

  • Desired FiO2F_i\text{O}_2 Formula:Known PaO2Known FiO2=Desired PaO2Desired FiO2\frac{\text{Known } P_a\text{O}_2}{\text{Known } F_i\text{O}_2} = \frac{\text{Desired } P_a\text{O}_2}{\text{Desired } F_i\text{O}_2}

    • Example: A patient exhibiting a PaO2P_a\text{O}_2 of 50 mmHg50\,\text{mmHg} on room air (0.21 FiO20.21\,F_i\text{O}_2) seeking a normal target PaO2P_a\text{O}_2 of 80 mmHg80\,\text{mmHg} requires an FiO2F_i\text{O}_2 of approximately 34%34\% (0.340.34).

Oxygen Titration and Protocol-Based Therapy

  • Protocol-Based Oxygen Therapy:

    • Standardized clinical pathways (e.g., Cleveland Clinic protocol models) authorizing clinicians to initiate, titrate, and discontinue oxygen therapy within defined parameters without requiring individual physician orders for every setting change.
    • Ensures systematic initial assessment, objective guideline-directed titration, and routine re-evaluation to avoid unmonitored, prolonged oxygen dependency.

  • Titration Guidelines:

    • Increase or decrease oxygen flow in increments of 1 L/min1\,\text{L/min} to 2 L/min2\,\text{L/min} or small fractional percentages.
    • Standard target oxygen saturation: SpO2≥92%S_p\text{O}_2 \ge 92\%.
    • Discontinuation threshold: Consider discontinuing oxygen when the measured arterial PaO2P_a\text{O}_2 exceeds 55 times the fractional delivered FiO2F_i\text{O}_2.
    • Home oxygen evaluation: Discharge qualification requires documentation meeting established Medicare or third-party insurer clinical criteria.
    • Reassessment windows: Re-evaluate blood gas and saturation values 10 min10\,\text{min} post-adjustment for patients without underlying pulmonary disease, and 20 min20\,\text{min} post-adjustment for patients with chronic pulmonary disease.
    • Formal clinical reference recommendations suggest weaning concentrations no more than once per clinical shift.