Oxygen therapy for critically ill patients (Proceedings)

Oxygen is an ideal therapeutic agent because it is easy to administer, readily available and if used correctly relatively safe. There are very few contraindications to oxygen supplementation. Oxygen therapy is often used in emergency and critical care medicine. Optimizing oxygen supplied to tissues is imperative in any number of clinical conditions where hypoxia may play a role. During hypoxia, cellular metabolism is less efficient and may lead to organ dysfunction and death. The critical care clinician must understand the physiology of oxygen delivery and recognize cases that will benefit from oxygen therapy. The distinction between hypoxia and hypoxemia is an important one.

Hypoxemia indicates low concentrations of oxygen in arterial blood. This is identified by performing arterial blood gas analysis. Hypoxia is a decreased concentration of oxygen at the level of the tissue or cell. The reason this distinction is important in the clinical setting is there are a limited number of physiologic causes of hypoxemia and if a patient is hypoxemia, hypoxia must be present. However not all patients that are hypoxic are hypoxemic.

Indications

     • There are five pathophysiologic causes of hypoxemia:

     • Hypoventilation

     • Low % inspired oxygen

     • Diffusion barrier impairment

     • V/Q mismatch

     • Shunt

It is not difficult to identify these causes in the clinical setting with the use of arterial blood gas analysis. Hypoventilation will be characterized by hypercapnia and if that is not present, hypoventilation is not the cause. The possibility of reduced concentrations of inspired oxygen is remote in most patients and therefore the top 3 causes of hypoxemia are diffusion barrier impairment, V/Q mismatch and shunting. To further simplify this, a shunt is the physiologic extreme of a low V/Q mismatch, so in reality V/Q mismatch and diffusion barrier impairment are causes of hypoxemia. In the clinical setting the patients response to an oxygen challenge may help identify the cause of hypoxemia. Diseases leading to diffusion barrier impairment tend to be oxygen responsive. Diseases leading to V/Q mismatch are variably oxygen responsive with high V/Q mismatches being oxygen responsive and low V/Q mismatches being poorly oxygen responsive. This is the reason diseases leading to intrapulmonary shunting are not oxygen responsive.

In addition to the mentioned causes of hypoxemia, there are many clinical situations in which oxygen therapy may be of benefit including respiratory distress, sepsis, hyperthermia, pleural space disease, congestive heart failure, anemia, shock, pulmonary contusions, pulmonary hypertension, seizures and head trauma.

Clinical signs observed in hypoxemic patients include tachypnea, tachycardia, cyanosis, cardiac arrhythmias and anxiety. Clinical response to oxygen is important, but objective criteria such as blood gas analysis, pulse oximetry, and alveolar-arterial (A-a) gradients should be evaluated. Another useful ratio to assess response to oxygen therapy is the arterial partial pressure of oxygen/fraction of inspired oxygen (PaO2/FIO2).

A-a gradient = [(BP-47) 0.21) - (PaCO2/0.8)] - PaO2

Normal = < 10 mm Hg

BP = barometric pressure

47 = vapor pressure of water

0.21 = % oxygen in room air

0.8 = respiratory exchange ratio

Routes of administration

Face mask

The face mask is perhaps the easiest and least invasive technique for oxygen supplementation. Masks designed for people are not ideal and purpose made veterinary masks should be used. These masks come in an assortment of sizes that range from pediatric size to large dog sizes and are designed to fit over the muzzle. It is often awkward to administer oxygen via mask to some brachiocephalic breeds. The mask can be connected via tubing to a 100 % oxygen source and if used for long periods of time, the oxygen should be humidified. The advantages of a mask are related to ease of administration and that no specialized equipment is required. In addition, during oxygen administration the animal can be closely monitored. Disadvantages of this technique include lack of tolerance by some patients and that the mask may require constant manual application unless the patient is moribund. When using a face mask, it is estimated that 40% is the fraction of inspired oxygen.

Flow-by

This technique is a modification of the face mask technique. The mask is disconnected and the oxygen line is held close to the patient's mouth and nose. The advantages and disadvantages are similar to the face mask. However there is no way to know the fraction of inspired oxygen and it will certainly be much less than that attained with a tight fitting face mask.

Nasal insufflation

This technique is often underutilized. There are various types of cannulas available for nasal catheterization including feeding tubes and urinary catheters. In our hospital we use small bore, flexible, feeding tubes with fenestrations to avoid mucosal jet lesions. The largest tube size possible should be passed. These sizes vary from 3.5 F-10 F depending on the size of the patient. These tubes can be placed in the conscious patient but local anesthesia is generally required. Topical 2 % lidocaine, lidocaine gel or proparacaine can be used on the nasal mucosa. The animals head should be tilted with the muzzle pointing toward the ceiling. A few drops of local anesthetic can be placed in the external naris to desensitize the nasal mucosa. The tube should be pre-measured to the medial canthus of the eye and advanced to this level. Once placed the tube can be secured as close to the nostril as possible with suture and or cyanoacrylate adhesive (Superglue). The tube should also be attached over the nasal bones and frontal sinus. These animals should be fitted with an Elizabethan collar to prevent removal. The nasal catheter can be adapted using a catheter (Christmas tree) adapter and syringe barrel to fit into a humidified oxygen line. The advantages of this technique include ease of application, no special equipment required, versatility in different sized animals and animals can be closely monitored. The disadvantages include lack of patient tolerance, variable flow rates required to provide oxygen, nasal mucosal irritation and gastric distension at high flow rates. The estimated fraction of inspired oxygen with this technique is 40%.

Other nasal cannula designed for use in people can also be used in large dogs. These devices have short nasal prongs and can be fitted over the dog's ears. Higher flower rates are generally required as compared with nasal catheters.

A recent publication has described the benefit of utilizing bilateral nasal cannulae.

Transtracheal administration

Transtracheal oxygen administration can provided using a nasal catheter which is advanced via the nasopharynx to the proximal trachea or a catheter placed percutaneously through the cervical trachea. A long flexible intravenous (jugular) catheter can be placed percutaneously into the trachea through the cricothyroid membrane or between tracheal rings. These catheters can be over the needle or through the needle type. The entry area should be aseptically prepared and infiltrated with local anesthetic prior to catheter introduction. In an emergency situation, a hypodermic needle attached to a fluid extension set can be used to administer intratracheal oxygen. Oxygen lines used with this technique should be humidified. The advantages of this technique include ease of administration and patient tolerance. Disadvantages include the invasive nature of this procedure. The estimated fraction of inspired oxygen with this technique is 40%.

Oxygen canopy

An oxygen canopy can be created by using an Elizabethan collar and a humidified oxygen line. The patient is fitted with an Elizabethan collar and 50%-75% of the collar diameter is covered with cellophane or plastic material. The oxygen line is run inside the E-collar and provides oxygen into the cellophane covered area. Basically the patients head is within an oxygen tent. The advantages of this technique include ease of application and no special equipment requirements. The major disadvantages are patient tolerance, increased humidity within the canopy, increased temperature within the canopy and no carbon dioxide scavenging ability. If flow rates are maintained appropriately or at higher levels this may reduce heat, humidity and carbon dioxide. The estimated fraction of inspired oxygen is 40%.

Oxygen cage

This equipment usually involves a chamber that can be flooded with oxygen to provide an oxygen enriched environment. The cages can be programmed for temperature and humidity control. A carbon dioxide scavenging system is also incorporated. There are currently cages available for most sized animals. They have working ports for fluid line and patient access. The advantages of this equipment are the controlled environment provided. The disadvantages include substantial financial investment, poor access to patient, difficulty in monitoring the patient and inadvertent uncontrolled environmental conditions.

Ventilator

Mechanical ventilation can be a considered option for supplying oxygen to selected patients. Ventilator therapy has some very specific indications which are beyond the scope of this discussion. For the most part ventilator therapy is reserved for patients that demonstrate signs of hypoventilation (hypercapnia), are refractory to oxygen supplementation by the previously described routes or have respiratory muscle fatigue. Ventilator therapy requires specialized equipment as well as heavy sedation or general anesthesia in most patients. Nursing care and close monitoring are the cornerstones of patient survival.

Miscellaneous

Other techniques are available for oxygen supplementation. Human neonatal incubators have been used to supplement oxygen in small patients and kits to modify normal clinic cages into oxygen cages are available. Pet carriers can be modified by covering some of the carrier holes with cellophane and providing a humidified oxygen source. Elevated temperatures, humidity and carbon dioxide levels may be a problem with this technique.

Monitoring

Most patients requiring oxygen supplementation will be hospitalized in an intensive care unit and observed closely due to the nature of the underlying disease. In addition it is important to closely monitor the typical vital parameters including temperature, pulse and respiratory rate. Specific evaluation for animals on oxygen supplementation includes thoracic auscultation, evaluation of mucous membrane color, capillary refill time, blood gas analysis and or pulse oximetry.

Complications

All of the techniques described other than the oxygen cage have the disadvantage of unknown fraction of inspired oxygen (FIO2) provided. This can potentially be a problem if animals are exposed to excessive levels of oxygen for long periods of time. Administration of 100% oxygen for > 12 hours or 80-90% for > 18 hours can lead to alterations in pulmonary function and signs of oxygen toxicity. Oxygen toxicity is in part related to production of free radicals and induced membrane damage. Oxygen toxicity is often difficult to recognize because the clinical signs can be similar to those seen with hypoxemia. However, oxygen toxicity is relatively uncommon, it takes at least 12 hours to occur, it is reversible in the early stages if oxygen is discontinued and supplementation with 50% oxygen appears safe in the dog.

Another possible complication can be seen in animals with chronic pulmonary disease. Due to desensitization of chemoreceptors to carbon dioxide, hypoxemia becomes the stimulus for respiratory drive. In these cases oxygen supplementation can lead to hypoventilation.

Discontinuation of therapy

Knowing when and how to discontinue oxygen therapy can provide the clinician with a difficult dilemma. As mentioned previously this therapy is not innocuous. Monitoring patients as with other therapeutics is very important. When patients are responsive to oxygen, small changes in oxygen supplementation may have dramatic effects. It is with this in mind that it is suggested to slowly wean patients off of oxygen. The rate of tapering therapy should be based on patient condition and response but is generally done over a 24-48 hour period. It is also important to remember that oxygen is often an adjunctive therapy and treatment directed toward the underlying disease process is paramount.

Conclusions

Oxygen therapy can be an important adjunctive therapy in emergency and critical care medicine. It can be adapted to be used in most veterinary hospitals. The techniques for administration differ depending on the case and the patient. This type of therapy is not without complications and must be utilized carefully by the clinician. Clinical evaluation of these cases is the best diagnostic tool to assess response to therapy and cessation of therapy.

Suggested reading

Camps-Palau MA, Marks SL, Cornick JL. Small Animal Oxygen Therapy. Compendium for Contin Educ 1999; 21(7):587-599.

Drobatz KJ, Hackner S, Powell S. Oxygen supplementation. In: Bonagura JD(ed.) Kirk's Current Veterinary Therapy XII, Small Animal Practice. Philadelphia: WB Saunders, 1995; 175-179.

Dunphy ED, Mann FA, et al. Comparison of unilateral versus bilateral nasal catheters for oxygen administration in dogs. Jour of Vet Emer Crit Care, 2002 12: 4, 245-251.

Fitzpatrick RK, Crowe DT. Nasal oxygen administration in dogs and cats: Experimental and clinical investigations. J Am Anim Hosp Assoc 1986;22:293-300.

Mann FA, Wagner-Mann C, Allert JA, Smith J. Comparison of intranasal and intratracheal oxygen administration in healthy awake dogs. Am J Vet Res 1992; 53(5):856-860.

West JB. Pulmonary Pathophysiology. Baltimore: Lippincott, Williams & Wilkins, 2008.