A COPD patient is admitted with an SpO2 of 84%. The respiratory therapist should initially start oxygen therapy with:

Nasal cannula at 1-2 LPM targeting SpO2 of 88-92%. COPD patients should be started on low-flow supplemental oxygen with a target SpO2 of 88-92%. Nasal cannula at 1-2 LPM is appropriate. Excessive oxygen (NRB at 15 LPM or high-concentration Venturi) may suppress the hypoxic ventilatory drive, causing hypoventilation and CO2 retention. However, never withhold oxygen from a hypoxemic patient — just titrate carefully.

A patient receiving oxygen by nasal cannula at 4 LPM has an approximate FiO2 of:

36%. Each liter of oxygen by nasal cannula increases FiO2 by approximately 4% above room air (21%). At 4 LPM: 21% + (4 x 4%) = 21% + 16% = approximately 37%, which is closest to 36%. This is an estimate — actual FiO2 varies with the patient's respiratory rate and tidal volume.

Which oxygen delivery device provides the MOST precise and consistent FiO2?

Venturi (air-entrainment) mask. The Venturi (air-entrainment) mask provides the most precise and consistent FiO2 because it is a high-flow device. Using color-coded jet orifices based on the Bernoulli principle, it delivers total flow exceeding the patient's inspiratory demand, ensuring a fixed FiO2 regardless of breathing pattern. This makes it ideal for COPD patients needing controlled oxygen therapy.

A patient has a PaO2 of 60 mmHg on an FiO2 of 0.50. What is the P/F ratio and what does it indicate?

P/F = 120, indicating ARDS. P/F ratio = PaO2 / FiO2 = 60 / 0.50 = 120. A P/F ratio between 100 and 200 meets the criteria for moderate-to-severe ARDS (Berlin Definition). A P/F ratio of 120 indicates significant oxygenation impairment and would typically require mechanical ventilation with PEEP.

What is the MINIMUM flow rate for a simple face mask to prevent CO2 rebreathing?

5 LPM. A simple face mask requires a minimum flow of 5 LPM to flush exhaled CO2 from the mask and prevent rebreathing. At flows below 5 LPM, the patient may rebreathe their own exhaled gas, leading to hypercapnia. Simple masks deliver approximately 35-50% FiO2 at flows of 5-10 LPM.

2.3 Oxygen Therapy Devices & Delivery | CRT/RRT Respiratory Therapist Exam | Free Exam Prep

Key Takeaways

Nasal cannula delivers 24-44% FiO2 at 1-6 LPM; each liter increases FiO2 by approximately 4%
Simple mask delivers 35-50% FiO2 at 5-10 LPM; minimum flow of 5 LPM to prevent CO2 rebreathing
Partial rebreather delivers 40-70% FiO2 at 6-10 LPM with a reservoir bag that should not fully deflate
Non-rebreather mask delivers 60-80% FiO2 at 10-15 LPM; has one-way valves and reservoir bag
Venturi mask (air-entrainment mask) delivers precise FiO2 (24-50%) using specific color-coded jet orifices
High-flow nasal cannula (HFNC) delivers 21-100% FiO2 at flows up to 60 LPM; provides PEEP effect and heated humidification
COPD patients: target SpO2 88-92% to avoid suppressing hypoxic ventilatory drive
The P/F ratio (PaO2/FiO2) assesses oxygenation severity: <300 = ALI, <200 = ARDS, <100 = severe ARDS

Oxygen Therapy Devices & Delivery

Oxygen therapy is one of the most common respiratory interventions. The TMC exam heavily tests your ability to select the appropriate oxygen delivery device, calculate FiO2, and adjust therapy based on patient response. Understanding the distinction between low-flow and high-flow systems is essential.

Low-Flow (Variable FiO2) Devices

Low-flow devices provide oxygen at flows that do not meet the patient's total inspiratory demand. Room air is entrained to make up the difference, resulting in a variable and somewhat unpredictable FiO2 that depends on the patient's respiratory pattern.

Device	Flow Rate	Approximate FiO2	Key Features
Nasal Cannula	1-6 LPM	24-44%	Most comfortable; each 1 LPM adds ~4% FiO2. Maximum 6 LPM (dries nasal mucosa)
Simple Mask	5-10 LPM	35-50%	Minimum 5 LPM to prevent CO2 rebreathing; covers nose and mouth
Partial Rebreather	6-10 LPM	40-70%	Reservoir bag with no one-way valves; bag should not fully deflate during inspiration
Non-Rebreather Mask (NRB)	10-15 LPM	60-80%	One-way valves on exhalation ports; reservoir bag with one-way valve; bag must stay inflated

Nasal Cannula FiO2 Estimation:

Flow (LPM)	Estimated FiO2
1 LPM	~24%
2 LPM	~28%
3 LPM	~32%
4 LPM	~36%
5 LPM	~40%
6 LPM	~44%

High-Flow (Fixed FiO2) Devices

High-flow devices deliver total flow that meets or exceeds the patient's inspiratory demand, providing a precise and consistent FiO2 regardless of the patient's breathing pattern.

Venturi Mask (Air-Entrainment Mask):

Delivers precise FiO2: 24%, 28%, 31%, 35%, 40%, or 50%
Uses specific color-coded jet orifices (adapters) for each FiO2
Based on the Bernoulli principle and air-entrainment (Venturi) effect
Total flow output exceeds the patient's peak inspiratory flow
Best for: COPD patients who need precise, low-concentration oxygen

High-Flow Nasal Cannula (HFNC):

Delivers heated, humidified oxygen at flows up to 60 LPM
FiO2 adjustable from 21% to 100% (blender-controlled)
Provides a positive airway pressure effect (~1 cmH2O per 10 LPM of flow)
Washes out anatomical dead space, improving ventilation efficiency
Better tolerated than CPAP masks by many patients
Indications: Acute hypoxemic respiratory failure, post-extubation support, pre-oxygenation before intubation

Oxygen Therapy in COPD

COPD patients require special consideration:

Target SpO2: 88-92% (not 95-100%)
Chronic CO2 retention may blunt the central chemoreceptor response to CO2
These patients rely partially on the hypoxic ventilatory drive (peripheral chemoreceptors responding to low O2)
Excessive oxygen can suppress this drive, leading to hypoventilation and CO2 narcosis
Use the lowest FiO2 necessary to maintain SpO2 88-92%
Venturi mask is ideal for precise FiO2 delivery in COPD
Monitor ABGs frequently when initiating or adjusting oxygen therapy in COPD

The P/F Ratio (PaO2/FiO2)

The P/F ratio is a quick assessment of oxygenation severity:

P/F Ratio	Interpretation
>400	Normal oxygenation
300-400	Mild impairment
200-300	Moderate impairment (ALI criteria)
100-200	Severe impairment (ARDS criteria)
<100	Very severe (severe ARDS)

Example: PaO2 80 mmHg on FiO2 0.40 = P/F ratio of 200 (ARDS threshold)

CRT/RRT Respiratory Therapist Exam

1Introduction: NBRC TMC Exam Overview

2Chapter 1: Patient Data Evaluation

3Chapter 2: Airway Management & Oxygen Therapy

4Chapter 3: Mechanical Ventilation

5Chapter 4: Pharmacology & Equipment

6Chapter 5: Infection Control, Ethics & Professional Practice