Use of High-Flow Oxygen Therapy in Critical Canine Patients With Respiratory Distress

Respiratory distress is one of the most common presenting emergencies in veterinary medicine. Until recently, therapies were limited to medical management, conventional oxygen therapy, and mechanical ventilation. Conventional oxygen therapies are relatively simple, involving oxygen cages, nasal oxygen cannulas, or flow-by oxygen. Mechanical ventilation is less commonly available because it is expensive and requires extensive training.

Approximately 6 years ago, high-flow oxygen therapy (HFOT) became available as a bridge between conventional oxygen therapies and mechanical ventilation in the veterinary setting.¹ HFOT has been used in human medicine for at least 20 years, primarily for neonates with respiratory compromise. Applications of this therapy in veterinary medicine have proven promising.²

What is High-Flow Oxygen Therapy?

HFOT is the administration of warm, humidified oxygen via nasal prongs, using a commercially available unit (FIGURE 1). It allows the delivery of higher flow rates of oxygen (4 to 60 L/min with some devices). The flow rate is set to meet or exceed the inspiratory flow demand of the patient (the speed at which the patient inhales). In normal mesocephalic dogs, the average flow demand is approximately 500 to 1000 mL/sec.³ Brachycephalic dogs may have lower flow demands due to naturally occurring airway obstructions. When a critically ill patient is experiencing respiratory distress, the flow demands are considerably higher owing to increased work of breathing, as well as the metabolic demands of illness.

Figure 1. Patient receiving high-flow oxygen therapy.

How Does HFOT Compare to Conventional Oxygen Therapy?

Flow-by oxygen, oxygen cages, and nasal oxygen cannulas are often sufficient for initial support. One or more of these therapies is readily available in most clinical settings.

Flow-by Oxygen

Flow-by oxygen therapy is a temporary solution. It requires 1-on-1 nursing care and can often feel like the room is receiving more oxygen support than the patient. While easily accessible and very inexpensive, it is not feasible for long-term support.

Oxygen Cage

Oxygen cages take up a lot of space compared with other methods of support. They require frequent maintenance and routine soda lime changes, and the limited space can contribute to patient stress. While some oxygen cages come in larger sizes, patients often get overheated even with cooling support. Additionally, opening the cage door frequently to perform treatments may result in a decrease in oxygen levels inside the cage. Larger cages take longer to achieve adequate oxygen content, and flow-by is often needed to bridge the gap.

Nasal Cannulas

Oxygen delivered via conventional nasal cannulas is not heated and not sufficiently humidified, and the flow rates are limited (1 to 5 L/min). These flow rates are further diluted by room air (~21% oxygen) each time the patient inspires because mixing of gases leads to a less exact fraction of inspired oxygen (Fıo₂).⁴ Furthermore, the lack of temperature regulation and humidification may result in damage to the nasal mucosa, airway constriction, and inflammation.²

High-Flow Oxygen Therapy

Compared with conventional oxygen therapies, HFOT has been shown to improve oxygenation in moderate to severe cases of hypoxemia.^2,5 A patient is considered hypoxemic when the arterial partial pressure of oxygen (Pao₂) is less than 80 mm Hg or the oxygen saturation (Spo₂) is less than 95%.⁶ The heated and humidified oxygen is better tolerated by most patients and can be delivered at much higher flow rates. The patient also experiences less energy expenditure by not having to heat the oxygen gas on its own. The nasal prongs used in HFOT are usually well tolerated by most patients.

HFOT uses high rates of oxygen to flush out carbon dioxide from gas-conducting (versus gas-exchanging) areas of the upper airway and replacing it with oxygen-rich air.⁷ This allows for a more optimized Fıo₂ to reach the alveoli and eliminates the mixing of gases. The resulting maintenance of increased pressure throughout the breathing cycle aids in the recruitment of alveoli, effectively acting similarly to PEEP (positive end-expiratory pressure) in ventilated patients.^2,5 HFOT provides approximately 3 to 5 cm H₂O of simulated PEEP, which increases resistance to exhalation and promotes retention of oxygen.

How Does HFOT Compare to Mechanical Ventilation?

It is important to remember that HFOT is not mechanical ventilation. There are no inspiratory pressure settings and no ability to set PEEP, respiratory rate, or volume, and it offers no additional monitoring (e.g., pressure-volume loops). Mechanical ventilation allows for full control of the airway and breathing cycle; however, it is very expensive, requires full-time care and monitoring by specially trained doctors and veterinary nurses, as well as the use of heavy sedation or anesthetics, and is not commonly available in most veterinary practices.

HFOT bridges the gap to mechanical ventilation, is less invasive than ventilation, and is generally more cost effective. For example, the unit used in the authors’ institution (DRE Volumax; Avante Animal Health, dreveterinary.com) employs a disposable cartridge that may be used for up to 30 days.⁸ Disposable single-use nasal prongs are relatively inexpensive, and the sterile water bags necessary are often readily available.

Indications and Contraindications For the Use of HFOT

HFOT is indicated for any patient in which conventional oxygen therapy has failed or where mechanical ventilation is unavailable, too costly, or detrimental to patient safety. It is also a great alternative when owners are unwilling to consider mechanical ventilation. HFOT has also been utilized in brachycephalic dogs immediately after extubation to aid in recovery from anesthesia.⁹

Early indicators of a poor response to HFOT may include persistently increased respiratory rate and effort, increased heart rate, ventricular premature contractions, orthopneic posturing, head bobbing, restlessness, “guppy” breathing, true dyspnea or signs of respiratory fatigue, and cyanosis.¹⁰ HFOT is not a replacement for mechanical ventilation. If Spo₂ cannot be maintained above 93%, carbon dioxide cannot be maintained below 60 mm Hg, or the patient has clinical signs of respiratory fatigue, then intubation and mechanical ventilation are indicated.¹¹

Exclusion criteria for HFOT may include chronic lung disease or severe hypercapnic respiratory failure. Patients requiring a chest tube should have one placed prior to using HFOT or mechanical ventilation. Contraindications for HFOT may also include epistaxis, nasal masses or facial trauma, increased intracranial pressures and traumatic brain injury, esophageal foreign bodies or obstructions, or patients with increased sensitivity to facial manipulation.

Due to the potential for esophageal or gastric distention from aerophagia, patients with gastrointestinal bleeding may not be candidates for HFOT. Hemodynamically unstable patients or those unable to maintain a functional airway are also not always eligible.

Patients with chronic airway or pulmonary disease may have adapted to higher carbon dioxide levels; therefore, lowering carbon dioxide levels with the use of supplemental oxygen may result in severe hypoventilation. Lowering carbon dioxide levels even a small amount will decrease the respiratory drive and ultimately lead to respiratory failure.¹¹

Nasal prongs are not currently available for use with cats, neonates, micropets, or exotic pets. In these patients, the size of the prongs would completely occlude the nares and prevent proper gas exchange, resulting in hypercapnia or barotrauma. Cats cannot tolerate HFOT due to their inability to pant for prolonged periods of time. They are also obligate nasal breathers. Additionally, the increased pressures associated with HFOT may not be safe for patients with smaller tidal volumes (patients <4 kg).

The High-Flow Oxygen Unit

Most HFOT units consist of the high-flow unit itself, a roll stand for easy transport, the air compressor, detachable/disposable patient circuits, disposable nasal prongs, gas inlet filters, medical air/oxygen hoses, and a power cord. Most units have a backup battery system that allows for up to 15 minutes of power during an emergency outage. Systems are relatively quiet while in use, which is ideal in a critical care setting. See Using the DRE Volumax for an overview of unit setup in the authors’ institution as an example; however, details of setup and use vary by model, and the user manual should always be consulted for each specific unit.

Using the DRE Volumax

Supplies needed

• 1 patient circuit cartridge, chosen based on patient size
• 1 nasal prong oxygen cannula
• 1-L bags of sterile water
• Access to oxygen and medical air

Note: Each circuit cartridge can be used for multiple patients for up to 30 days after the initial setup. It is important to use the date sticker on the side of the cartridge to prevent the circuit from being overused (FIGURE A).

Figure A. Date sticker on circuit cartridge.

Unit setup

• After proper handwashing, open the new cartridge bag while wearing examination gloves and aseptically attach the new circuit to a 1-L bag of sterile water. Be certain a secondary liter bag of sterile water is available; do not let the bag attached to the unit run dry.
• Hang the sterile water bag from the pole and allow the cartridge to fill with approximately 200 mL of sterile water (FIGURE B).

Figure B. Setting up the DRE Volumax.

• Insert the cartridge.
• Attach the oxygen lines and the medical air lines to the outlets.

Note: Medical air is only necessary when providing an Fıo₂ (fraction of inspired oxygen) less than 100% to the patient.

Unit operation

• Plug in and turn on the unit. If the air compressor will be used, it must be turned on before the Volumax machine. Medical air must flow through the unit before humidifying so that moisture does not build up inside the machine.
• Set parameters (gas, flow rate, Fıo₂, temperature) (FIGURE C). Flow rates are chosen based on the severity of disease and size of the animal (TABLE 1).
• Activate the machine.

Figure C. Setting parameters on the DRE Volumax.

Begin therapy
Attach the circuit to the nasal cannula. Make sure you do not connect the unit to the patient until the system has had a chance to warm up and purge the excess moisture. Connection immediately after setup can cause the patient discomfort due to improperly heated air or excess moisture in the line. Proper warmup takes about 5 minutes.

Controls

Parameters that can be set and adjusted include temperature, Fıo₂, and flow rate. Air delivered to the patient may be heated and humidified.

Compressor

The purpose of the compressor is to blend medical air with 100% oxygen to reach Fıo₂ levels lower than full concentration.

Oxygen toxicity (hyperoxia) can occur in patients maintained at 100% Fıo₂ in as little as 24 hours.¹² Oxygen toxicity is an iatrogenic condition in which supplemental oxygen becomes toxic to the lungs over a prolonged period. Signs range from coughing, nausea, and muscle twitching to disorientation, seizures, and even death.^10,12

Choosing and Using Nasal Prongs

Once the HFOT machine is set up, the nasal prongs should be chosen and fitted to the patient. A new set of nasal prongs should be used with every patient.

Measuring the Prong Diameter

The cannula diameter should be carefully measured to be sure it does not exceed approximately 50% of the diameter of the nares (FIGURE 2). Too large a diameter can obstruct the patient’s ability to ventilate, while too small a diameter will provide inadequate flow and offer poor respiratory support. Prong size can be assessed by placing a hand or cotton ball in front of the animal’s nose. If air flow can be felt, or if the cotton ball moves, the prongs are likely not occluding more than 50% of the nares. Additionally, if the prongs are the right size, there should be an audible “break” in the flow each time the patient exhales. The HFOT unit manufacturer may also provide recommendations for cannula diameter related to patient size and compatible flow rate.

Figure 2. Patient measured for proper nasal prong diameter. Courtesy Yu Ueda, DVM, PhD, DACVECC, North Carolina State University College of Veterinary Medicine

Securing the Cannula

The nasal prong cannula should be secured to the patient in a way to prevent inadvertent disconnection. A variety of techniques can be used; however, the authors have found suturing the nasal prongs to each side of the patient’s muzzle using a finger-trap method to be the most successful (FIGURE 3).

Figure 3. Nasal prongs being secured to the patient with sutures. Courtesy Michael Kato, DVM, DACVECC, North Carolina State University College of Veterinary Medicine

Some nasal prongs may be better secured by creating a small tape “bridge” over the muzzle to help the prong tubing hold the proper shape as well as decrease suture tension on the patient’s face (FIGURES 4 AND 5).

Figure 4. Proper tape bridge in use on a patient receiving high-flow oxygen therapy.

Figure 5. Top view of a patient illustrating a tape bridge over the nose.

The prongs can also be secured using a tape butterfly method. When using the taped butterfly method or when using a tape bridge, waterproof surgical tape is usually more effective than porous tape. The tubing should be completely dried with a piece of gauze prior to applying the tape to the prongs, as waterproof tape may slip if the tube is wet.

Placing the Nasal Prongs

The direction of the prongs is very important. The curvature of the nasal prongs should always point downward to prevent trauma to the mucous membranes and properly direct the flow without obstruction (FIGURE 6).

Figure 6. Nasal prongs held in the proper direction for placement.

When placing nasal prongs, nasoesophageal and nasogastric tubes may be left in place; however, it is recommended to remove all red rubber nasal cannulas prior to initiating HFOT. Red rubber nasal cannulas often become occluded with nasal secretions and are ideally replaced every 2 days. A new red rubber cannula may be placed after HFOT is discontinued.

Initiating HFOT

HFOT is a fairly new treatment in veterinary medicine. Hospital staff should be properly educated on the use and equipment management prior to launching the use of HFOT in any hospital.

At the authors’ institution, an Fıo₂ of 100% is chosen initially unless the clinician requests otherwise (compressor use is required to reduce Fıo₂). The temperature can be set between 34 °C and 38 °C; however, the authors suggest starting at 37 °C for nearly all patients.

The flow rate may be calculated at 0.5 to 2 L/kg.² The initial flow rate should be set lower and titrated up as needed based on patient response. The authors suggest taping a printout of flow rate settings to the HFOT machine for quick reference (FIGURE 7). 

Figure 7. At-a-glance flow rates.

TABLE 1 lists suggested initial HFOT flow rates; however, it is important to remember that each patient’s need and response are individual, and flow rates and Fıo₂ should be titrated based on need, comfort, and tolerance.

Patient Monitoring and Nursing Care Requirements

Once HFOT is initiated, patients must be carefully monitored for response to therapy or worsening of clinical signs. A patient on HFOT will generally exhibit a positive response within 30 to 60 minutes after initiation of therapy.

Serial venous blood gas or arterial blood gas measurements should be assessed frequently to gauge response to HFOT. Blood gas results, in conjunction with continuous pulse oximetry (i.e., Spo₂) evaluation, will help determine if HFOT is effective for the patient. Nasal prongs should be checked frequently for proper fit/location and connection to the unit, as patients may inadvertently become disconnected.

Most patients seem to tolerate HFOT and the nasal prongs fairly well. Elizabethan collars are occasionally used to deter a patient from pawing or rubbing at the nasal cannula and accidentally becoming disconnected (FIGURE 8). Once the nasal prongs are secured, if the patient seems irritated or uncomfortable, unit settings should be adjusted and inspected to ensure that the irritation is not a sign of continued hypoxia.

Figure 8. Patient receiving high-flow oxygen therapy with Elizabethan collar.

Respiratory rate and effort, temperature, and heart rate monitoring are invaluable. Setting the temperature on the HFOT unit too high may result in iatrogenic hyperthermia. Temperature probes are available to limit patient handling and stress. Electrocardiography (ECG) monitors allow continuous evaluation of heart rate and rhythm and provide early indications of distress and hypoxemia. Eye lubrication is important to prevent corneal ulceration and irritation of ocular mucosa.

Patients on HFOT are occasionally sick enough to be recumbent. Turn orders to prevent atelectasis and aid in potential alveolar recruitment are necessary for these patients. Additionally, passive range of motion exercises may assist in venous return and increased circulation of oxygenated blood.

Patients should also be monitored for stomach distention, prong-related side effects such as nose sores or skin lesions over the bridge of the nose, and difficulty synchronizing breathing. If the patient is not responding adequately to HFOT, mechanical ventilation should be considered.

Sedation Protocols

Anxiolytics such as gabapentin and trazodone can be given as needed to keep the patient calm. On rare occasions a patient may require sedation to tolerate either the high-flow nasal prongs or their current respiratory distress. Boluses of butorphanol at 0.2 mg/kg IV are preferred for these circumstances. Butorphanol boluses can be escalated to a continuous rate infusion or dexmedetomidine can be added (when not contraindicated due to comorbidities) only when absolutely necessary.

If continued anxiety and activity centered on removing the nasal prongs persist, this may be an indication the patient is feeling well enough to discontinue HFOT.

De-escalation of HFOT

Much like weaning a patient from conventional oxygen therapies, discontinuing the use of HFOT is initiated by the gradual decrease of oxygen concentration and flow rate. Continued patient monitoring is crucial, and other sources of oxygen should be available should a need arise. If the patient is not responding well to weaning, therapy should be reinstituted and maintained for an additional 12 to 24 hours. In the authors’ experience, patients routinely require conventional oxygen therapy for some time following the use of HFOT. Similar de-escalation techniques are used to transition the patient to room air oxygen concentrations.

Once the patient no longer requires high-flow support, the unit should be properly shut down and cleaned to prevent delays in treating the next patient.

Conclusion

For patients that do not respond adequately to conventional oxygen therapy, HFOT is an effective alternative method of delivering high volumes and pressures of oxygen without the use of mechanical ventilation. Cost-effective equipment, easily obtained supplies, and patient tolerance are advantages of HFOT in any practice. If the patient is not responding to traditional therapy or HFOT, referral to a specialty practice for mechanical ventilation is indicated.

Editor’s Note: A previous version of this article incorrectly stated that a patient is considered hypoxemic when the the oxygen saturation (Spo2) is greater than 95%. This has been corrected in the text.