11.6 Beyond CPAP: BiPAP, High-Flow Nasal Cannula Therapy & Choosing a Ventilation Strategy

11.6 Beyond CPAP: BiPAP, High-Flow Nasal Cannula Therapy & Choosing a Ventilation Strategy

CPCF Appendix A skill #25 lists, as minimum entry-to-practice content for the Primary Care Paramedic, not only manual positive pressure ventilation and CPAP but also bilevel positive airway pressure (BiPAP), high flow nasal cannula therapy (HFNCT), PEEP and manometry, pulse oximetry, and capnography. Section 11.4 covered bag-valve-mask mechanics, PEEP valves, and CPAP in depth. This section completes the skill by covering the two modalities PCPs increasingly encounter — either as transport therapy or on patients already receiving them — and by giving a decision framework for choosing between them.

The Three Things Non-Invasive Support Can Do

Every non-invasive modality works through some combination of three mechanisms. Knowing which mechanism a patient needs is how you select the device.

CPAP Versus BiPAP: One Pressure or Two

CPAP delivers a single, constant pressure throughout the respiratory cycle. The patient does all the work of inspiration; the device holds the alveoli open.

BiPAP delivers two pressures: a higher IPAP during inspiration and a lower EPAP during expiration. The difference between them (the pressure support) actively assists each breath.

A useful shorthand: CPAP fixes oxygenation; BiPAP fixes oxygenation and ventilation. A patient who is hypoxaemic but ventilating adequately needs CPAP. A patient who is tiring, retaining CO2, and has a rising end-tidal with a falling respiratory rate needs inspiratory assistance.

High-Flow Nasal Cannula Therapy (HFNCT)

HFNCT delivers heated, humidified gas through wide-bore nasal prongs at flows far above a patient's inspiratory demand — typically 30 to 60 L/min in adults, with an independently set FiO2.

Its four mechanisms:

1. Dead-space washout. Continuous high flow flushes CO2-rich gas from the nasopharynx between breaths, so each breath starts with fresh gas. This is why HFNCT improves CO2 clearance without any inspiratory pressure support.
2. Reliable FiO2. Because flow exceeds inspiratory demand, entrainment of room air is minimal and the delivered FiO2 approaches the set value — unlike a standard mask, where a distressed patient's high inspiratory flow dilutes the oxygen.
3. Modest positive airway pressure. A small distending pressure (a few cm H2O, greater with the mouth closed) provides some recruitment.
4. Conditioning of inspired gas. Heating and humidifying to near body conditions preserves mucociliary function and is markedly better tolerated than dry high-flow oxygen.

Practical implications for a PCP:

• HFNCT is far better tolerated than a tight mask. Patients can speak, be suctioned, and take oral medication without interrupting therapy — a real advantage in delirium, claustrophobia, or facial trauma where a mask seal is impossible.
• It is flow- and oxygen-hungry. Verify cylinder capacity before departure; a 60 L/min transport can exhaust portable oxygen supplies rapidly.
• It provides no guaranteed pressure and no inspiratory assistance. A patient who is failing on HFNCT needs escalation, not a higher flow.

Choosing a Strategy

Inadequate oxygenation and/or ventilation despite basic oxygen therapy
        |
        +-- Apnoeic / agonal / unprotected airway / arrest
        |        -> Manual positive pressure ventilation (BVM), airway adjuncts, consider SGA
        |
        +-- Awake, cooperative, protecting airway, haemodynamically tolerant
        |        |
        |        +-- Hypoxaemic, adequate ventilation (e.g. cardiogenic pulmonary edema)
        |        |        -> CPAP
        |        |
        |        +-- Hypercapnic / tiring / rising EtCO2 with falling effort (e.g. COPD)
        |        |        -> BiPAP where available; otherwise assisted BVM ventilation
        |        |
        |        +-- Cannot tolerate a mask, needs to talk/eat/be suctioned, or needs
        |                 reliable high FiO2 without pressure
        |                 -> HFNCT where available
        |
        +-- Deteriorating on any non-invasive modality
                 -> Escalate: assisted ventilation, advanced airway resource, rapid transport

Shared Contraindications and Cautions

Non-invasive support of any kind is inappropriate or hazardous when the patient:

• Is apnoeic, in arrest, or agonal — these need positive pressure ventilation, not a non-invasive device
• Cannot protect the airway or has active vomiting or heavy secretions
• Has a decreased level of consciousness such that they cannot cooperate or remove the interface
• Has facial trauma, burns, or anatomy preventing a seal (HFNCT may still be possible)
• Has a suspected pneumothorax, and particularly a tension pneumothorax — positive pressure can enlarge it
• Is hypotensive, because raised intrathoracic pressure reduces venous return and can precipitate collapse
• Has had recent upper gastrointestinal surgery or a known oesophageal or gastric problem (gastric insufflation risk)

Monitoring and Recognizing Failure

Regardless of modality, the monitoring set is the same and is named explicitly in skill #25: pulse oximetry, capnography (EtCO2), and manometry where available, plus continuous clinical reassessment (H2.2).

Signs the patient is failing and needs escalation rather than adjustment:

• Respiratory rate that is falling while EtCO2 rises — this is exhaustion, not improvement
• Deteriorating level of consciousness
• Persistent or worsening hypoxaemia despite an adequate seal and flow
• New haemodynamic instability after starting therapy
• Inability to tolerate the interface despite coaching and reassurance

[!IMPORTANT] The most dangerous moment in non-invasive support is the patient who becomes calm and quiet. Distinguish genuine improvement — respiratory rate down, work of breathing down, saturation up, EtCO2 stable or improving, patient more alert — from exhaustion, where the rate falls, the EtCO2 climbs, and the patient becomes drowsy. They look similar from the doorway and mean opposite things.

Transporting a Patient Already on a Device

PCPs frequently transfer patients who arrive on home or facility equipment — home CPAP for sleep apnoea, home BiPAP for neuromuscular or chronic respiratory disease, hospital HFNCT. Entry-level expectations:

1. Record the settings before you move anything, and photograph or write them down: IPAP/EPAP or CPAP pressure, FiO2, flow, and backup rate.
2. Confirm the device's power and oxygen requirements for the full journey plus a margin, and know what the device does when it loses power.
3. Carry a bag-valve-mask and be ready to ventilate manually the moment the device fails. Every non-invasive device is one interruption away from needing manual support.
4. Do not alter prescribed settings outside your authorization; escalate to medical direction instead.