What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Non-Rebreather Mask: Setup, Flow Rates, and Use

Name: Non-Rebreather Mask: Setup, Flow Rates, and Use
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

A non-rebreather mask is a high-concentration oxygen delivery device. It uses a reservoir bag and one-way valves to separate fresh oxygen from the patient's exhaled gas, so the patient breathes from the bag rather than rebreathing their own carbon dioxide. In human medicine the device typically delivers roughly 60 to 80 percent inspired oxygen, compared with about 40 to 60 percent for a simple mask without a reservoir. That difference matters when a patient is severely hypoxemic and every percentage point of inspired oxygen affects tissue delivery.

In veterinary patients the non-rebreather is used less often than nasal cannulas, oxygen cages, or high-flow nasal oxygen, largely because most dogs and cats tolerate a tight-fitting mask poorly. When it is used, it is usually for a short period during stabilization of a patient who will accept the mask, or as a bridge to a better-tolerated modality. Setup takes two to three minutes. Achieving a stable, tolerated oxygen concentration takes five to ten minutes of observation, and the mask is rarely left in place for more than a brief stabilization window unless the patient is calm.

This guide covers the parts of the mask and what each does, a materials table, a numbered setup procedure, a weight-based flow-rate table for dogs and cats, a troubleshooting checklist for valve sticking, bag collapse, and carbon dioxide retention, and the practical limits of the device in veterinary practice.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

What the Non-Rebreather Mask Achieves

The non-rebreather mask sits in the middle of the oxygen delivery hierarchy. Below it are low-flow devices such as nasal cannulas and simple face masks, which deliver variable and often modest inspired oxygen. Above it are high-flow nasal oxygen therapy and invasive positive pressure ventilation, which deliver a controlled and predictable fraction of inspired oxygen [1].

The device's job is to raise the fraction of inspired oxygen, abbreviated FiO2, as high as a spontaneously breathing patient will allow without intubation. In people, a standard high-flow oxygen face mask with a reservoir bag has been shown to deliver a tracheal FiO2 of about 90 percent at high flow rates, comparable to high-flow nasal cannulae in that respect, while generating airway pressure close to zero [2]. That last point is important. The non-rebreather does not provide positive pressure support. It only enriches the gas the patient breathes.

A bench study comparing two non-rebreather reservoir masks found that mask fit changed delivered oxygen concentration substantially. The cumulative mean delivered oxygen concentration was 64 percent for one mask and 50 percent for another, and the difference was significant [3]. The lesson is direct. The seal between mask and face, and the ability of the patient to draw gas from the reservoir during inspiration, determine what the patient actually receives. A poorly fitted mask on a struggling patient can deliver far less than the label suggests.

How the Valves and Reservoir Bag Work

A non-rebreather mask has three functional parts beyond the mask shell itself.

The reservoir bag hangs below the mask and holds a reserve of oxygen. During inspiration the patient draws from this bag first, so the gas they inhale is nearly pure oxygen rather than room air. The bag refills from the oxygen source between breaths. For the bag to stay inflated, oxygen inflow must at least match the patient's peak inspiratory flow. If inflow is too low, the bag collapses and the patient pulls room air through the exhalation ports, which drops the delivered oxygen concentration.

The one-way valves control direction of flow. An inspiratory valve between the reservoir bag and the mask opens when the patient inhales, allowing oxygen from the bag into the mask. It closes when the patient exhales, preventing exhaled gas from entering the reservoir. Exhalation ports on the mask allow exhaled gas to escape to the room. Some masks have a valve on the exhalation ports that closes during inspiration to reduce room air entrainment.

The result is a system that delivers a high inspired oxygen fraction without rebreathing. Carbon dioxide retention is the specific failure mode when the system is set up incorrectly. A case report described a patient with severe chronic obstructive pulmonary disease who was given oxygen through a non-rebreather mask with the flow inappropriately limited to 4 liters per minute. The patient developed carbon dioxide narcosis and required intubation and mechanical ventilation [4]. The authors stated plainly that non-rebreather masks with reservoir bags must be fed with oxygen flow exceeding the patient's minute ventilation, and they gave a human threshold of greater than 6 to 10 liters per minute. Below that, the amount of oxygen delivered is too small to raise arterial oxygen saturation effectively, and the risk of carbon dioxide rebreathing rises sharply, especially in patients with low tidal volumes [4].

That case report is human, and its numbers are human numbers. The principle transfers to veterinary patients. The specific flow rate does not. A 3 kg cat and a 40 kg dog have very different minute ventilations, and the flow that keeps a reservoir bag inflated for one will either starve or waste oxygen for the other.

Why Veterinary Use Differs from Human Use

Three features separate veterinary non-rebreather use from the human version.

Mask tolerance is the first and largest issue. Most dogs and cats will paw at, shake off, or panic against a tight-fitting face mask. A patient who fights the mask has erratic inspiratory flow, breaks the seal repeatedly, and may become more stressed, which increases oxygen demand at the moment oxygen delivery is least reliable. A stressed, struggling patient is a poor candidate for this device.

Dead space is the second issue. The mask shell, the valves, and the connection to the reservoir add volume that the patient rebreathes with each breath. In a large dog this added volume is trivial relative to tidal volume. In a small cat or a toy breed dog, mask dead space can be a meaningful fraction of each breath, which works against the goal of raising alveolar oxygen. This is one reason small patients are often better served by a nasal cannula or an oxygen cage.

Flow titration is the third issue. There is no single correct flow rate for a non-rebreather in veterinary medicine. Flow must be matched to patient size and minute volume. The reservoir bag must stay visibly inflated throughout the respiratory cycle, and the flow must be high enough that the patient never has to pull room air. That is the practical target, and it is judged by watching the bag, not by reading a fixed number off a device.

A related caution comes from work on self-inflating resuscitation bags used to deliver oxygen to spontaneously breathing patients. That study found that the flow delivered at the patient outlet was always less than the inlet flow and could be as little as about 20 percent of it, and the percentage delivered fell as inlet flow rose [5]. The devices tested were resuscitation bags, not non-rebreather masks, but the finding reinforces a general point. What arrives at the patient's face is not the same as what leaves the flowmeter, and the gap grows with the resistance and geometry of the delivery system.

Materials and Setup at a Glance

Item	Specification or working value	Purpose
Non-rebreather mask	Correct size for the patient, with reservoir bag and one-way valves	Delivers high-concentration oxygen
Oxygen source	Regulated flowmeter, 0 to 15 L/min or higher depending on patient size	Supplies gas to the reservoir
Oxygen tubing	Standard connecting tubing, no kinks	Carries gas from flowmeter to mask
Reservoir bag	Attached to mask, 1 L or larger depending on patient size	Holds oxygen reserve for inspiration
Pulse oximeter	Functioning, with probe sized to the patient	Tracks peripheral oxygen saturation
Emergency drugs and airway equipment	Drawn up and within reach	Available if the patient deteriorates
Restraint aid	Towel, muzzle, or handler as appropriate	Keeps the patient calm without occluding the mask

The working value for oxygen concentration is the FiO2 the device can deliver, roughly 60 to 80 percent in a well-fitted human patient, and the working flow range is whatever keeps the reservoir bag inflated for that specific patient. There is no single flow number that applies across species and sizes.

Step-by-Step Setup

Confirm the indication. The non-rebreather is for a spontaneously breathing patient with significant hypoxemia who will tolerate a mask. If the patient is apneic, agonal, or requires positive pressure support, this is not the right device. Move to bag-valve-mask ventilation or intubation.

Choose the correct mask size. The mask should cover the muzzle and nostrils without pressing on the eyes and without gaps larger than necessary at the edges. An oversized mask adds dead space and leaks. An undersized mask will not seal and will not cover the nares.

Inspect the valves before use. Look at the inspiratory valve between the reservoir and the mask, and at the exhalation ports. Each valve should sit flat and move freely. A valve that is stuck open, stuck closed, or curled at the edge will either allow rebreathing or block oxygen flow. This check takes seconds and prevents the two most common device failures.

Inspect the reservoir bag. Confirm it is attached securely, has no holes or tears, and is not twisted at the connection. A twisted connection acts like a closed valve and starves the patient of flow.

Connect the oxygen tubing to the flowmeter and to the mask inlet. Check the entire length for kinks, and check that the tubing is not pinched under the patient or a restraint device.

Set the initial flow based on patient size and minute volume. Use the table below as a starting point, not an endpoint. The correct flow is the one that keeps the reservoir bag at least two-thirds inflated throughout inspiration and exhalation.

Pre-fill the reservoir bag before placing the mask. Let oxygen run until the bag is fully inflated. Placing a collapsed bag on a hypoxemic patient means the first several breaths are room air.

Place the mask on the patient. Approach from the side or from below the muzzle rather than over the top of the head. Hold the mask in place with light pressure. Do not press it hard against the face. A gentle seal is enough, and force increases stress.

Watch the reservoir bag for the first minute. If it collapses with each breath, increase flow. If it stays fully inflated and the patient is comfortable, the flow is adequate. If it stays inflated but the patient is working hard to breathe, reassess whether this device is appropriate at all.

Attach the pulse oximeter and record a baseline saturation. Continue monitoring every 30 seconds during stabilization. A rising saturation with a calm patient is the goal.

Reassess tolerance continuously. If the patient paws at the mask, shakes the head, or becomes more distressed, remove the mask and switch to a nasal cannula, oxygen cage, or high-flow nasal oxygen. A mask that increases stress is working against the patient.

Document the flow rate, the mask size, the time started, and the serial saturations. This record guides the next clinician and shows whether the intervention helped.

The reason each step matters is the same throughout. The non-rebreather only works when the reservoir stays full and the seal stays intact. Every step in the sequence protects one of those two conditions.

Flow-Rate Table by Body Weight

The values below are starting points for a spontaneously breathing patient with an intact respiratory drive. Titrate up or down based on reservoir bag inflation, patient comfort, and pulse oximetry. Do not treat these as fixed prescriptions.

Patient	Approximate body weight	Starting oxygen flow	Target observation
Toy breed dog, small cat	2 to 5 kg	3 to 5 L/min	Reservoir bag stays two-thirds full or more
Small dog, average cat	5 to 10 kg	5 to 8 L/min	Bag stays inflated, patient calm
Medium dog	10 to 20 kg	8 to 12 L/min	Bag stays inflated through peak inspiration
Large dog	20 to 35 kg	12 to 15 L/min	Bag stays inflated, no room air entrainment
Giant breed dog	Over 35 kg	15 L/min or higher if the flowmeter allows	Bag stays inflated, reassess device choice

Two points about this table. First, the upper end of a standard flowmeter is often 15 L/min. A giant breed dog with high minute ventilation may need more than a standard flowmeter can deliver, and in that case the non-rebreather is the wrong device. Second, a small patient may need less flow than the lowest reliable setting on some flowmeters, and a flow that is too high for a tiny patient can be uncomfortable and wasteful. In that situation a nasal cannula or an oxygen cage is usually the better choice.

The human case report that described carbon dioxide narcosis used a threshold of greater than 6 to 10 L/min to exceed minute ventilation in an adult human [4]. That threshold is a human value and should not be applied directly to a dog or cat. It illustrates the principle that flow must exceed minute ventilation, and the table above translates that principle into size-based starting points.

A Decision Path for Oxygen Delivery

The following flow chart shows the main decision path for choosing and running a non-rebreather mask in a spontaneously breathing veterinary patient.

flowchart TD
    A[Patient is hypoxemic] --> B{Spontaneous breathing}
    B -->|No| C[Bag valve mask or intubate]
    B -->|Yes| D{Will tolerate a mask}
    D -->|No| E[Nasal cannula or oxygen cage]
    D -->|Yes| F[Select correct mask size]
    F --> G[Check valves and reservoir bag]
    G --> H[Pre fill reservoir bag]
    H --> I[Place mask gently on patient]
    I --> J{Reservoir bag stays inflated}
    J -->|No| K[Increase flow and recheck]
    K --> J
    J -->|Yes| L[Monitor saturation and tolerance]
    L --> M{Patient remains calm and saturation rises}
    M -->|No| E
    M -->|Yes| N[Continue and reassess frequently]

Reading the Results

Three signals tell you whether the non-rebreather is working.

The reservoir bag is the first and most immediate. A bag that stays at least two-thirds inflated throughout the respiratory cycle means inflow is matching or exceeding the patient's inspiratory demand. A bag that collapses with each breath means the patient is pulling harder than the flow can supply, and room air is entering through the exhalation ports. Increase flow, and if the bag still collapses at the maximum the flowmeter allows, the device cannot meet this patient's demand.

Pulse oximetry is the second signal. A rising saturation over the first several minutes supports the setup. A saturation that stays flat or falls despite a full reservoir bag suggests the patient needs a different modality, a higher flow than the device can provide, or has a problem the mask cannot address.

Patient comfort is the third signal, and in veterinary patients it is often the deciding one. A calm patient with a full reservoir bag and a rising saturation is a successful setup. A patient who is pawing, vocalizing, or trying to escape is not, regardless of what the numbers say. Stress increases oxygen consumption and makes the delivered oxygen less useful.

Troubleshooting Checklist

Symptom	Likely cause	Fix
Reservoir bag collapses with each breath	Flow too low for patient minute volume	Increase flow until bag stays two-thirds full
Reservoir bag stays collapsed even at high flow	Kinked or twisted tubing, blocked inlet, or faulty flowmeter	Trace tubing from flowmeter to mask, straighten kinks, check connections
Patient appears to rebreath or becomes drowsy	Exhalation valve stuck closed or inspiratory valve stuck open	Remove mask, inspect and replace valves or the mask
Delivered oxygen seems low despite high flow	Mask seal broken, mask oversized, or patient pulling room air	Resize mask, reduce leak, or switch to a better-tolerated device
Patient fights the mask	Poor tolerance, stress, or pain	Remove mask and switch to nasal cannula, oxygen cage, or high-flow nasal oxygen
Saturation does not improve	Device cannot meet demand, wrong device for the patient, or nonrespiratory cause	Reassess device choice and involve a veterinarian
Carbon dioxide retention suspected	Flow below minute ventilation, valve failure, or mask dead space	Increase flow above minute ventilation, inspect valves, consider a different modality
Bag inflates but patient breathes rapidly	Inadequate oxygen delivery, pain, or primary respiratory disease	Reassess the whole patient, not just the mask

The carbon dioxide retention row deserves emphasis. A human case report showed that limiting flow to 4 L/min on a non-rebreather mask led to carbon dioxide narcosis requiring intubation [4]. The mechanism is straightforward. When flow is below minute ventilation, the patient rebreaths exhaled gas, and carbon dioxide accumulates. In a veterinary patient with low tidal volumes or a compromised respiratory drive, the same risk applies. If a patient on a non-rebreather becomes progressively drowsy, the first response is to increase flow and reassess, and the second is to consider a different oxygen delivery method.

Variations and Alternatives

The non-rebreather is one option among several. Understanding where it sits helps you choose well.

A simple face mask without a reservoir delivers roughly 40 to 60 percent inspired oxygen in humans, lower than a non-rebreather because there is no reservoir to supply peak inspiratory flow. It is easier to tolerate in some patients and is a reasonable step down when high concentration is not needed.

A nasal cannula delivers low-flow oxygen and is generally better tolerated than a mask in dogs and cats. It does not achieve the high inspired oxygen fractions of a non-rebreather, but it can be left in place longer and does not require the patient to accept a mask over the face.

An oxygen cage delivers a controlled oxygen environment without a mask. It is well tolerated by most patients but limits access for examination and procedures.

High-flow nasal oxygen therapy delivers heated and humidified gas at adjustable flow rates up to 60 L/min and FiO2 up to 100 percent through nasal cannulae. It has been proposed to improve pulmonary mechanics and reduce respiratory fatigue by reducing anatomical dead space, providing low-level positive end-expiratory pressure, and delivering a constant FiO2 at rates matching patient requirements [1]. In a pilot study of dogs and cats undergoing bronchoscopy, high-flow oxygen therapy was feasible and safe, and no patient in that group experienced severe desaturation below 90 percent, whereas two patients in the conventional oxygen therapy group did [6]. High-flow nasal oxygen is not available in every practice, but where it is, it often replaces the non-rebreather for patients who need high FiO2 and tolerate nasal interfaces better than masks.

A self-inflating resuscitation bag with a reservoir is a manual ventilation device, not a spontaneous breathing device. It is used when the patient is not breathing adequately. It is mentioned here only to distinguish it from the non-rebreather. Work on these bags has shown that delivered oxygen concentration depends heavily on flow rate, ventilation rate, and peak inspiratory pressure, and that delivered FiO2 can range widely across these variables [7][8][9][10]. That variability is a property of manual resuscitation, not of the non-rebreather, but it reinforces the general principle that oxygen delivery devices must be matched to the patient and monitored.

Storage and Stability Notes

Store non-rebreather masks in their original packaging, flat and away from sharp objects. The valves are the fragile part. A valve that has been crushed in a drawer or exposed to heat may curl or stick, and a stuck valve is the most common cause of device failure.

Inspect each mask before use. Check that the reservoir bag holds air, that the valves sit flat, and that the mask shell is not cracked. Discard any mask with a damaged valve or a leaking bag. These devices are inexpensive relative to the cost of a failed oxygen delivery attempt.

Oxygen tubing should be stored coiled without tight bends, because a permanent kink in the tubing restricts flow even after it is straightened. Replace tubing that has been crushed or that shows visible damage.

Keep a range of mask sizes on hand. A single size will not fit a Chihuahua and a Great Dane, and using the wrong size either adds dead space or fails to seal.

Limitations and When to Contact a Veterinarian

The non-rebreather mask has real limits in veterinary medicine. It requires a patient who tolerates a mask, which excludes many dogs and cats in respiratory distress. It adds dead space, which matters most in small patients. It does not provide positive pressure support. It cannot deliver more oxygen than the flowmeter and reservoir allow, and a patient with very high minute ventilation may exceed what a standard flowmeter can supply.

Escalation criteria are straightforward. Contact a veterinarian immediately if the patient's breathing becomes more labored, if the gums or tongue change color, if the patient becomes progressively drowsy or unresponsive, if the reservoir bag cannot be kept inflated despite increasing flow, if the pulse oximeter reading falls or fails to rise, or if the patient fights the mask to the point of distress. Any of these signs means the current setup is not working and the patient needs a different approach.

Individual patients vary, and the right oxygen delivery method for a specific dog or cat is a clinical decision. A veterinarian who can examine the patient, measure oxygenation, and reassess continuously should make that call.

Frequently Asked Questions

What is a non-rebreather mask?

A non-rebreather mask is a face mask with a reservoir bag and one-way valves that delivers high-concentration oxygen, typically around 60 to 80 percent inspired oxygen in a well-fitted human patient, by preventing the patient from rebreathing exhaled gas.

How is a non-rebreather mask different from a simple mask?

A simple mask has no reservoir bag, so it delivers a lower inspired oxygen fraction, roughly 40 to 60 percent in humans. The reservoir bag on a non-rebreather supplies the patient's peak inspiratory flow with fresh oxygen, which raises the delivered concentration.

What flow rate should I use for a dog or cat?

There is no fixed flow rate. Start with a size-based estimate, then adjust until the reservoir bag stays at least two-thirds inflated throughout the respiratory cycle. Flow must match the patient's minute volume, not a number on a chart.

Why does the reservoir bag collapse?

The bag collapses when oxygen inflow is lower than the patient's peak inspiratory flow. Increase the flow rate. If the bag still collapses at the maximum the flowmeter allows, the device cannot meet the patient's demand and a different modality is needed.

Can a non-rebreather mask cause carbon dioxide problems?

Yes, if the oxygen flow is below the patient's minute ventilation. A human case report described carbon dioxide narcosis when flow was limited to 4 liters per minute, and the authors stated that flow must exceed minute ventilation to avoid rebreathing.

Why do some pets refuse the mask?

Most dogs and cats dislike having their face covered. A patient who fights the mask has erratic breathing and a broken seal, which reduces oxygen delivery and increases stress. A nasal cannula, oxygen cage, or high-flow nasal oxygen is often better tolerated.

Is a non-rebreather mask better than high-flow nasal oxygen?

They serve different purposes. The non-rebreather delivers high inspired oxygen without positive pressure. High-flow nasal oxygen delivers heated, humidified gas at adjustable flow and FiO2 and may improve tolerance and reduce dead space, but it is not available in every practice.

When should I stop using the mask and call a veterinarian?

Stop and call if the patient's breathing worsens, the gums change color, the patient becomes drowsy, the reservoir bag cannot stay inflated, the saturation falls or fails to rise, or the patient becomes distressed by the mask.