4.1 Oxygen Therapy and Noninvasive Support
Key Takeaways
- Initiation and Modification of Interventions is the largest TMC section: 70 of 140 scored items, exactly 50% of the 2026 Therapist Multiple-Choice content outline.
- Oxygen device selection depends on the size of the oxygen need, ventilation status, work of breathing, and how precisely the fraction of inspired oxygen (FiO2) must be controlled.
- A Venturi (air-entrainment) mask delivers a fixed FiO2 and is the classic exam choice for a chronic obstructive pulmonary disease (COPD) patient at risk for carbon dioxide retention.
- Noninvasive ventilation requires an awake, cooperative patient who can protect the airway; it is contraindicated in arrest, shock, vomiting, and severe facial trauma.
- On bilevel support, pressure support equals inspiratory positive airway pressure minus expiratory positive airway pressure (IPAP minus EPAP); raise IPAP for carbon dioxide, raise EPAP or FiO2 for oxygenation.
Why This Section Is Half the Exam
Initiation and Modification of Interventions is the largest of the three Therapist Multiple-Choice (TMC) sections: 70 of the 140 scored items, exactly 50% of the content outline in effect through December 31, 2026. The TMC delivers 160 items total (140 scored plus 20 unscored pretest) in a three-hour window; the low cut earns the Certified Respiratory Therapist (CRT) credential and the high cut qualifies you for the Clinical Simulation Examination. Oxygen and noninvasive items recur because most clinical stems open with hypoxemia, dyspnea, a shifting arterial blood gas (ABG), or a patient failing current therapy.
The exam rarely wants one memorized device for one diagnosis. It asks whether the patient needs more oxygen, more flow, more positive pressure, better ventilation, or escalation to an artificial airway.
Oxygen Device Decision Table
| Clinical problem | First question to ask | Likely device |
|---|---|---|
| Mild hypoxemia, stable breathing | Is precise FiO2 needed? | Nasal cannula 1-6 L/min |
| Moderate hypoxemia, variable breathing | Will low flow be diluted? | Simple mask 5-10 L/min |
| Severe hypoxemia, preparing definitive care | Is the reservoir inflated? | Non-rebreather 10-15 L/min |
| COPD with carbon dioxide retention risk | What is the SpO2 target? | Venturi mask, controlled FiO2 |
| Cardiogenic pulmonary edema, alert | Need alveolar recruitment? | CPAP |
| Hypercapnia, awake, spontaneous | Airway protected? | Bilevel (BiPAP) |
Device Logic and Exact Flows
A nasal cannula is a low-flow device; the rough rule is each liter adds about 4% FiO2 above room air (1 L/min ≈ 24%, 4 L/min ≈ 36%), but actual FiO2 falls with high inspiratory demand or mouth breathing. A simple mask must run at least 5 L/min to flush exhaled carbon dioxide from the mask. A non-rebreather delivers roughly 0.60-0.80 FiO2 only if the bag stays inflated through inspiration. A Venturi (air-entrainment) mask is the answer when the stem stresses a fixed, predictable FiO2 — the jet entrains a set room-air ratio, so the delivered concentration does not drift with breathing pattern.
High-flow nasal cannula delivers up to 60 L/min of heated, humidified gas, washes out nasopharyngeal dead space, and creates a small distending pressure, but it does not fix rising carbon dioxide from pump failure.
Noninvasive Support: CPAP vs Bilevel
Continuous positive airway pressure (CPAP) delivers one constant pressure. It raises functional residual capacity, recruits flooded alveoli, and is the recruiting choice for cardiogenic pulmonary edema in an awake patient. It does not actively assist a breath, so it does not directly lower carbon dioxide.
Bilevel positive airway pressure (BiPAP) sets a higher inspiratory positive airway pressure (IPAP) and a lower expiratory positive airway pressure (EPAP). Pressure support = IPAP minus EPAP. Worked example: BiPAP 12/6 means 6 cmH2O of pressure support; if the ABG shows pH 7.29 with PaCO2 62 mmHg, raise IPAP (to 15/6 = 9 cmH2O support) to increase tidal volume and blow off carbon dioxide. Raising EPAP mainly improves oxygenation and splints airways but can drop venous return if set too high.
Titration, Targets, and the COPD Trap
Follow every oxygen change with an objective check — respiratory rate, accessory-muscle use, mental status, pulse-oximetry waveform quality, and an ABG when carbon dioxide retention is a concern. The most-tested trap: in chronic hypercapnic COPD do not chase a normal saturation. The exam rewards a target near SpO2 88-92% with controlled FiO2 and reassessment, because aggressive oxygen can blunt respiratory drive and worsen V/Q matching, raising PaCO2.
- Contraindications to noninvasive ventilation: cardiac or respiratory arrest, shock, vomiting, copious secretions, inability to protect the airway, uncooperative patient, severe facial trauma.
- Failure cues that demand escalation: worsening mental status, rising PaCO2 with acidemia, persistent hypoxemia despite the device, mask intolerance, or fatigue.
Always confirm the equipment before escalating a stable patient: verify source and flow, check for mask leak entraining room air, and confirm the non-rebreather reservoir is full. Correct setup first, then change the support that addresses the physiology.
Reading the Stem: A Worked Sequence
Most oxygen items can be solved with a fixed five-step sequence. First, name the gas-exchange problem — is it pure hypoxemia, or hypoxemia plus a ventilation (carbon dioxide) failure? Second, decide whether the delivered FiO2 must be precise; if the answer is yes, a Venturi mask or blended high-flow system beats any low-flow device whose FiO2 drifts with the breathing pattern. Third, estimate the size of the oxygen need: a SpO2 of 84% on room air in a non-COPD patient usually justifies a reservoir device, while the same number in a COPD patient with an 88-92% order justifies a controlled low FiO2.
Fourth, ask whether positive pressure would help — recruitment for hypoxemia (CPAP) or assisted ventilation for hypercapnia (BiPAP). Fifth, set an objective reassessment endpoint, not just a saturation number.
A common distractor is a device that can deliver high oxygen but does not match the physiology. A non-rebreather titrated to 100% in a hypercapnic COPD patient is the wrong answer precisely because it overshoots the ordered target and can suppress drive. Conversely, choosing CPAP for a hypercapnic, awake patient who needs carbon dioxide removal is wrong because CPAP provides a single pressure and no inspiratory assist; BiPAP is the ventilating choice. Memorize the pairing: CPAP recruits, BiPAP ventilates.
Monitoring and Documentation Endpoints
After every oxygen or noninvasive change, document the device, flow or pressure settings, FiO2, the SpO2 response, respiratory rate, and a tolerance note. For noninvasive ventilation, also record the exhaled tidal volume, mask-leak fraction, and synchrony. A delivered tidal volume that stays below roughly 5-6 mL/kg despite adequate IPAP, or a leak large enough to prevent pressure cycling, predicts noninvasive failure and should prompt early escalation rather than repeated mask adjustments.
Trend the arterial blood gas: a falling pH with a rising PaCO2 one to two hours into noninvasive ventilation is the strongest single predictor that intubation is coming, and the exam expects you to escalate rather than wait.
An alert patient with chronic hypercapnic COPD arrives with SpO2 84% on room air, respiratory rate 24/min, and no severe distress. The order is to maintain SpO2 88-92%. Which initial device best matches the goal?
A patient on BiPAP 12/6 cmH2O has SpO2 94% on FiO2 0.35, but the ABG shows pH 7.29 and PaCO2 62 mmHg. The patient is awake and tolerating the mask. Which adjustment best targets the acid-base problem?
A patient in acute cardiogenic pulmonary edema is awake, hypertensive, and using accessory muscles on a non-rebreather mask. SpO2 is 89%, crackles are diffuse, and blood pressure is stable. Which support most directly recruits alveoli while the team treats the cause?