Oxygen Therapy - Chasing the Blues Away

Overview

Hypoxemia is defined as inadequate oxygenation of arterial blood (PaO2 < 80 mm Hg or SPO2 <~ 95%). Hypoxemia is a result of either low fractional inspired oxygen (FIO2), hypoventilation, or venous admixture (ventilation/ perfusion mismatch (most types of lung disease), right to left anatomic shunts, and diffusion defects). Oxygen therapy may be beneficial to those patients with impaired gas exchange. The goal of oxygen therapy is to provide adequate oxygen to the tissues, using the lowest possible inspired oxygen concentrations. In the absences of arterial blood gases or pulse oximetry one will need to rely on clinical signs. Clinical signs of hypoxemia include cyanosis, increased respiratory rate and effort, tachycardia, orthopnea, use of accessory respiratory muscles, and anxiety. Individually, the clinical signs do not prove hypoxemia, but together they are suggestive of hypoxemia and indicate the need for more definitive testing.

Methods of Oxygen Administration

There are a variety of methods of oxygen delivery. The method selected depends on the expected duration of therapy, patient size, demeanor, and equipment availability. Available methods include flow-by, face mask, oxygen bag (hood), oxygen collar / tent, oxygen cage, transtracheal, nasal insufflation and high flow nasal canula.

Face Mask

Masks are readily available and easy to use. Masks are only good for short-term use. High-inspired oxygen concentrations can be obtained if a properly fitted facemask is used. Unfortunately, patients often fight the face mask (unless obtunded) thereby increasing oxygen consumption and canceling the effects of the oxygen therapy. The patients face and nose should fill the mask as much as possible to reduce the amount of dead space in the mask. Increased dead space will increase the work of breathing. Sometimes it is helpful to remove the rubber dam from the face mask to achieve a better fit.

Oxygen bag (hood)

An alternative to the face mask is the oxygen bag. A clear plastic bag is placed over the head of a patient and a hose from an oxygen source is placed near the animal’s nose. The bag remains open along the animal’s neck to allow the gas to escape. A flow rate of five to eight liters per minute is used. It has been reported that animals tolerate this bag/hood method when they resist the oxygen mask . Care should be taken to ensure that the bag does not collapse around the nose and mouth. It is very easy for animals (especially large dogs) to overheat with this technique and should be avoided if dogs become hot.

Oxygen Collar (tent)

Another alternative to the mask and hood is the oxygen tent. Take an Elizabethan collar (e-collar) placing plastic wrap over the front, leaving a one – two-inch (5.1 Cm) opening at the top. The opening allows expired carbon dioxide and heat to escape. An oxygen hose is inserted into the e-collar.

Oxygen Cage

A good oxygen cage should have the following features: it must have a system for eliminating carbon dioxide; deliver a known amount of oxygen in a concentration beneficial to the patient (40 - 50%); and a mechanism for controlling temperature (70ºF) and humidity (50%). The disadvantages to this system are, it's expensive to operate, the nursing staff have minimal access to the patient, and it is difficult to accommodate large patients.

Transtracheal

The placement of a "through-the-needle" catheter into the trachea can facilitate the administration of oxygen. The transtracheal route may be indicated when the nasal route is contraindicated. The procedure is carried out much like a transtracheal wash. An area over the trachea is clipped and prepared. Using aseptic technique, the catheter is inserted two - three tracheal rings below the cricoid cartilage or through the cricothyroid membrane. The tip of the catheter should lie in the region of the carina. A light bandage is placed around the patient's neck. The catheter is attached to a humidified oxygen source. Humidified oxygen is used to prevent irritation of the mucosal lining of the airway. Excessive oxygen flow rates are avoided, to prevent trauma from catheter “whip” to the tracheal wall. In one study it was determined that transtracheal oxygen administration permitted lower oxygen flow rates than nasal oxygen administration. The flow rates used for transtracheal O2 administration produced significantly higher inspired O2 concentrations and PaO2 than corresponding nasal O2 flow rates . The flow rates evaluated in this study ranged from 10 ml/kg/min - 250 ml/kg/min.

Nasal Insufflation

Nasal insufflation is an excellent method for administering oxygen. The technique is inexpensive; minimal restraint is required, the nursing staff has good access to the patient, and most of the supplies needed may be found in most practices. A nasal oxygen catheter is easily placed and well tolerated by most patients. Nasal insufflation is contraindicated in patients with significant nasal masses, rhinitis, nasal fractures and nasal hemorrhage.

After treatment with a topical anesthetic and the premeasurement of the nasal catheter (3.5 – 10 Fr patient size dependent) from the tip of the nose to the medial canthus of the eye, the catheter is inserted. A lubricated catheter can be placed in the ventral nasal meatus (angling ventromedially) to the predetermined distance. Multiple catheter fenestrations minimize the risk of jet lesions of the mucous membranes. The catheter is sutured or stapled as it exits the alar notch and along side the face or brought up and over the forehead between the eyes and is secured again. The catheter is attached to a humidified oxygen source. A flow rate of 50 - 200 ml/kg/min should be effective in increasing tracheal O2 concentration in most patients to 40% or greater2, . Human nasal prongs are an alternative that can be effective and less invasive but may deliver a lower fraction of inspired oxygen than a nasal catheter. The ability to use nasal prongs will depend on patient anatomy and behavior.

Several complications may be observed: gastric distension, epistaxis, sneezing and serous nasal discharge. High flow rates can cause catheter "whip" trauma to the nasopharyngeal mucosa. To prevent mucosal drying humidify the oxygen through an in-line bubble humidifier.

High Flow Nasal Oxygen (HFNO)

HFNO has more recently been utilized in dogs. This method is advantageous because it can achieve flow rates up to 40-60 L/min and can more reliably deliver high FiO2. HFNO systems can deliver an FiO2 of 21-100%. The air is heated and humidified, then delivered to the patient through a heated breathing circuit and specialized nasal prongs sized to occlude about 50% of the nares. The system allows 100% humidification and control of temperature, thereby enhancing patient comfort and tolerance of these high flow rates.

Summary

A variety of options for oxygen therapy have been discussed. The lowest flow rate that improves the patient’s condition is the desired flow rate. The patient's response to oxygen therapy should be evaluated at periodic intervals. The goal is to see an improvement in mucous membrane color, decreased anxiety, decreased breathing and or heart rate, decrease in the magnitude of respiratory distress, and an improvement in PaO2 or SPO2 to an acceptable level.