Oxygen is a frequently recommended therapy—for respiratory disease, shock, heart failure and severe anemia. It should be a readily available therapy in any veterinary practice. Oxygen is used most frequently, in the non-anesthetized patient, for treating hypoxemia. Hypoxemia is defined as a partial pressure of oxygen (PaO₂) <80 mm Hg and corresponds to pulse oximetry (SpO₂) of ∼<95%; a PaO₂<60 mm Hg denotes severe hypoxemia and corresponds to an SpO₂ <90%.

The fraction of inspired oxygen (FIO₂) in room air is 21%. During oxygen supplementation, FIO₂ is increased. Clinically, the FIO₂ is frequently difficult to measure in the conscious, non-intubated patient. Patients, on a variety of methods of oxygen supplementation, breathe an admixture of 100% oxygen and room air combined. Excessive oxygen has deleterious effects.¹ Monitoring for animals receiving supplemental oxygen includes patient parameters (respiratory rate and effort, heart rate, mucus membrane colour), blood gases and pulse oximetry. Oxygen therapy aims to maintain an adequate PaO₂ (>65, optimally ∼80 mm Hg) with the lowest FIO₂ possible. Oxygen supplementation adjusted to SpO₂ readings between 94–96% ensure adequate oxygenation without excessive supplementation. In the face of severe respiratory distress normalization of oxygenation is not always possible, however maintaining PaO₂>60 or SpO₂>90% is critical.¹

Multiple means of oxygen delivery are available to the practicing clinician. Traditional methods include flow-by-oxygen, oxygen by face mask, oxygen hoods, or nasal prongs and cannulas. The latter methods only support low oxygen flows 50–100 ml/kg/min unilateral cannula, up to 200 ml/kg/min for bilateral supplementation. Oxygen supplementation in veterinary medicine has routinely been provided with cold air bubble humidifiers despite lack of evidence for their efficacy.²

For patients who fail traditional oxygen supplementation, options have been limited to the need for endotracheal intubation ± positive pressure ventilation, or euthanasia. An augmented intermediate non-invasive respiratory support modality that successfully achieves continuous positive airway pressure (CPAP) with predictable, controlled FIO₂ delivery has been sorely needed for veterinary patients.

High Flow Nasal Cannula Oxygen Supplementation

Two high-flow nasal cannula (HFNC) oxygen therapy systems are currently developed for human use in North America—Fisher Paykels’ Optiflow™ system, and Vapotherm High Velocity Nasal Insufflation (High-VNI®). Heated, humidified, HFNC oxygen therapy is a non-invasive intermediate method for providing oxygen supplementation as well as promoting improved respiratory function.^2-4,9,10 The HFNC system delivers oxygen flow rates up to 10x higher than traditional oxygen supplementation methods. The high flows achieve pressure to the airways known as continuous positive airway pressure (CPAP).

The HFNC system involves specifically designed nasal prongs (neonatal to adult sizing). These prongs are adapted for high gas flows (up 60 L/min using adult prongs). Oxygen (21–100%) is delivered heated and humidified (100% relative humidity) using a medical air/oxygen admixer, with an in-system humidifier, and wire-heated tubing (set to 37°C).² The high gas flows are well tolerated by patients because of the heated, humidified air.² The high gas flows meet the patients peak inspiratory flows, minimize entrainment of room air, and alleviate work of breathing. When compared to traditional methods of oxygen supplementation, HFNC therapy increases FIO₂ values with increasing flow rates and successfully provides CPAP in people and dogs.^2,4 Recommended flow rates in dogs range from 0.5 to 2 L/kg/min up to 60 L.⁴ CPAP is observed at flow rates of 1–2 L/kg/min, and most consistently achieved using 2 L/kg/min.⁴ Even in the awake open-mouth breathing healthy dog, CPAP is achieved, though to a lesser extent. Flows should be gradually increased for improved tolerance. Flows above 2 L/kg/min are not tolerated.⁴

The HFNC facial interface developed for human patients, is reasonably adaptable and generally well tolerated by dogs, sedation may be necessitated.^3,4 Due to the high gas flows, risk of airway overdistension is possible, without ensuring a leak in the system. Nasal prongs should be selected to occupy only 50% of the internal diameter of the nares. Commercial pediatric devices (providing 1–8 L/min) have a pressure relief valve built into the circuit or are designed to sense excessive circuit pressure and reduce gas flow accordingly. This is not included in adult commercial circuits.

The HFNC system is used in neonatal, pediatric and adult human patients, its use is currently adapted to the emergency department, during bronchoscopy, and in the post-extubated patient, as well as in the palliative do-not-intubate setting.^2,5 Indications of HFNC are broad, encompassing most if not all causes of acute hypoxemic respiratory failure.^5,6 This new oxygen support system has been successfully adapted to deliver augmented oxygenation support to dogs.^3,4

A case series of six dogs reports successful delivery of HFNC in dogs failing traditional oxygen therapy.³ Another case series of HFNC in 22 dogs with hypoxemic respiratory failure of varying etiologies found that 60% of dogs responded to HFNC use (improved clinical respiratory parameters) by 30 minutes, and 45% ultimately responded to HFNC use and survived.⁹ Similarly, HFNC has been trialed in 5 brachycephalic dogs that developed increased work of breathing ± hypoxemia in the recovery phase of anesthesia.¹⁰ Respiratory rate and dyspnea scores were decreased in 3/5 dogs following HFNC application.¹⁰

Safety Implications

HFNC is not a ventilatory support system. Changes in CO₂ (increasing or decreasing) following HFNC application may be noted. Washout of CO₂ may allow a decrease in CO₂, alternatively the high flow rates provided by the HFNC system can impede exhalation in some patients. Hypercapnia is infrequently observed with HFNC therapy in people and dogs.^1,9,10 HFNC did not negatively impact CO₂ elimination in healthy dogs,^4,7 or dogs with acute hypoxemic respiratory distress,^3,9 but hypercapnia (pCO₂>50 mm Hg) was noted in 3/5 brachycephalic dogs when HFNC was applied.¹⁰ Regardless, the effects of high flows on CO₂ elimination may become deleterious in the compromised respiratory patient.⁴ CO₂ monitoring is advised. Abdominal distention is also possible, and was noted in normal dogs trialed with HFNC; all developed radiographic signs of aerophagia, with no apparent clinical signs.⁴ One brachycephalic dog treated with HFNC developed clinically significant aerophagia that required clinical intervention.¹⁰ Air-leak syndromes are also potential complications, with a low rate of ≤1% for pneumothorax secondary to HFNC.⁸ Radiographic assessment of the thorax of healthy study dogs found that no dogs experienced pneumothorax, pneumomediastinum or other ill effects when flow rates were trialed up to 2.5 L/kg/min.⁴ However, a persistent pneumothorax while on HFNC has been reported in veterinary medicine.³

Some important issues remain to be resolved, such as definitive indications for HFNC and criteria for timing of its initiation, discontinuation and need for escalating treatment. Despite these issues, HFNC is an effective modality for early treatment of dogs with respiratory failure with diverse underlying diseases.⁴

Selecting HFNC

Patients meeting criteria for emergency intubation should be treated accordingly (unless owners decline endotracheal intubation). For patients who are not responding favourably to traditional oxygen supplementation a trial with HFNC is advised. The potential for early application of HFNC is also to be considered (in lieu of traditional methods). Failure to respond to HFNC within 30–60 minutes or active deterioration should prompt immediate endotracheal intubation ± ventilation.

Indications for mechanical ventilation include:

• PaO₂<60 mm Hg despite oxygen supplementation
• PaCO₂>50 mm Hg
• Increased work of breathing

References

1.  Waddell LS. Oxygen therapy. Clinician’s Brief. 2016;43–48.

2.  Ward JJ. High-flow oxygen administration by nasal cannula for adult and perinatal patients. Respir Care. 2013;58:98–122.

3.  Keir I, Daly J, Haggerty J, Guenther C. Retrospective evaluation of the effect of high flow oxygen therapy delivered by nasal cannula on PaO₂ in dogs with moderate-severe hypoxemia. J Vet Emerg Crit Care. 2016;26(4):598–602.

4.  Jagodich TA, Bersenas AME, Bateman SW, Kerr CL. Comparison of high flow nasal cannula oxygen administration to traditional nasal cannula oxygen therapy in healthy dogs. J Vet Emerg Crit Care. 2019;29(3):246–255.

5.  Doshi P, Whittle JS, Bublewicz M, et al. High-velocity nasal insufflation in the treatment of respiratory failure: a randomized clinical trial. Annals of Emergency Medicine. 2018;72:73–83.

6.  Frat JP, Thille AW, Mercat A, et al. High-flow oxygen through nasal cannula in acute hypoxemic respiratory failure. N Engl J Med. 2015;372:2185–2196.

7.  Daly JL, Guenther CL, Haggerty JM, Keir I. Evaluation of oxygen administration with a high-flow nasal cannula to clinically normal dogs. Am J Vet Res. 2017;78(5):624–630.

8.  Baudin F, Gagnon S, Crulli B, et al. Modalities and complications associated with use of high-flow nasal cannula: experience in a pediatric ICU. Respir Care. 2016;61(10):1305–1310.

9.  Jagodich TA, Bersenas AM, Bateman SW, Kerr CL. Preliminary evaluation of the use of high flow nasal cannula oxygen therapy during recovery from general anesthesia in brachycephalic dogs with obstructive upper airway breathing. J Vet Emerg Crit Care. In Press.

10.  Jagodich TA, Bersenas AM, Bateman SW, Kerr CL. High-flow nasal cannula oxygen therapy in acute hypoxemic respiratory failure in 22 dogs requiring oxygen support escalation. J Vet Emerg Crit Care. In press.