Assessment of Oxygen Needs

Immediate needs - Post foaling resuscitation is discussed in (See Resuscitation Part 1). Hypoxia in the foal which becomes ill after a seemingly normal birth is not easily detected by mucous membrane color. Cyanosis is not readily apparent until the PaO2 is <40 mmHg. Hypoxia can be a component of septicemia, prematurity, neonatal maladjustment, pneumonia, atelectasis and foals that are in prolonged lateral recumbency due to any reason.

I. Methods of Evaluation Which May Reveal Need for Oxygen [1-3]

1. History and observation of asphyxia, apnea or airway obstruction.
2. Auscultation and percussion abnormalities of the chest.

1. Limited abnormalities may be heard with diffuse diseases.
2. There is a lack of correlation with the severity of pulmonary disease.
3. The down lung frequently has abnormal sounds due to position.

3. Labored respiration - Increased amount of abdominal contraction and asynchronous rib and abdominal contractions.
4. Pale or cyanotic mucous membranes.
5. Thoracic radiographs
6. Blood gases

1. Arterial

1. A hospital setting with ICU is required to effectively and safely provide long term respiratory support.
2. Valuable in determining severity of respiratory compromise, appropriate therapy and response to therapy.
3. PaO2 <80 mmHg indicates VQ mismatch, PaO2 <52 mmHg is life-threating and requires immediate therapy. Position of the foal and age affect the values.
4. SaO2 <90% requires immediate medical attention

2. Venous

1. Very limited value for respiratory O2 assessment.
2. Valuable for acid base assessment.
3. If PvO2 <28 mmHg, may indicate need for oxygen or improved circulatory status by IV fluids or nutritional support.
4. PvCO2 >70 mmHg - Arterial hypercapnia possible

7. Pulse Oximetry

1. Devices used to estimate the amount of oxygen saturation of arterial hemoglobin.

1. The Nellcor N-200 was evaluated in studies [4]
2. The type of transducer attached to foal and the anatomical site is critical for accurate reading [4]

2. Results from one study in foals [4]

1. Nellcor reflectance Transducer RS-10 attached to base of tail tended to underestimate the amount of arterial oxygen saturation but was very consistent and predictable.
2. Nellcor Durasensor DS-100A (fingertip) transducer attached to the ear of foals was reliable in determining if <90% saturation or if >90% saturation. However with foals with <80% saturation it tended to overestimate O2 saturation. When attached to lip had low sensitivity with a tendency to overestimate O2 saturation and was not recommended for use at that site.
3. Nellcor Oxisensor D-25 (adhesive) at the ear was judged to underestimate and be variable in readings.
4. Clinically useful sites and transducers were fingertip transducer at the ear and tongue and reflectance transducer at base of tail. Remember that at less than 80% saturation the fingertip on the ear and tongue overestimated O2 saturation somewhat.
   Editor's Comment - We place transducer on vulva, tongue and prepuce and seem to get reliable readings.

II. Method of Obtaining Arterial Samples

1. Blood Sampling Techniques (See Blood Sampling Techniques).

III. Circumstances of Sampling That Affect PaO2 - PaCO2 Values [5]

1. Position of foal - PaO2 values during lateral recumbency are 10-40 mmHg less than if the foal is sternal or standing. Consequently sternal position is recommended in convalescing foals to improve their ability to efficiently use their lungs. Foals should be restrained in lateral recumbency for arterial sampling except when using an indwelling arterial catheter.
2. Struggling and exertion may alter PaO2 and PaCO2 values.
3. Inspired O2 concentrations.
4. Degree of maturity and gestational age. Table of age related neonatal values in reference section
5. Obtaining venous blood and believing sample is arterial.
   Editor's Comment - With unexpectedly low PaO2, especially readings around 40 mmHg, always submit a venous blood gas for comparison.

IV. Conditions Producing Hypoxemia

1. Newborns

1. Birth asphyxia
2. Aspiration pneumonia - Meconium.
3. Persistent fetal circulation.
4. Congenital heart disease.
5. Hypoventilation -neurologic or fatigue related.
6. Atelectasis.
7. Surfactant deficiency.
8. Intra-pulmonary shunts.
9. Respiratory distress syndrome.
10. Rib fracture

2. Acquired after 24 hours

1. Septicemia.
2. Conditions producing prolonged recumbency.
3. Respiratory distress syndrome.
4. Reversion to fetal circulation.

V. Methods of Oxygen Therapy [2]

1. If hypoxemia cannot be rapidly alleviated and the patient is ventilating adequately, oxygen therapy is indicated.
2. Face mask - high inspired oxygen concentrations can be attained with a face mask. The patient's nose and face should fill the mask as much as possible to reduce the dead space within the mask. Leaks allow room air to be drawn into the mask during inspiration and may be satisfactory as long as the net inspired oxygen concentration is sufficient to alleviate the hypoxemia. If not, the fresh oxygen flow could be increased or the mask could be made to form an airtight seal around the muzzle, provided expired gases are eliminated via exhaust or CO2 absorber.
3. An oxygen insufflation catheter may be placed in the nasopharynx, through the cricothyroid membrane or intratracheally, if circumstances warrant.

1. A soft, flexible catheter should be used for the nasopharynx.
   Editor's Comment - Caudal end of nasopharyngeal catheter corresponds to level of medial canthus of eye. Don't put tube far back in pharynx; you may enter esophagus.
2. Several holes should be present near the catheter tip to facilitate the diffusion of the oxygen into the airway and minimize its jetting against one spot on the epithelium.
3. Oxygen flow rates of 4-6 L/min should provide a 30-40% inspired oxygen concentration. Specific flow rates should be adjusted to the needs of the individual patient.
4. The oxygen should be bubbled through warm water so that it can be humidified prior to reaching the patient, if providing oxygen for more than 1 hour.

1. Humidification chambers that are sterile are available to accept an O2 line in and out.

4. Place foal in sternal position.
5. If oxygen therapy does not alleviate the hypoxemia, endotracheal intubation and positive pressure ventilation is indicated.
6. See Respiratory Therapeutics (See Respiratory Therapeutics).

VI. Blood Gas Response to O2

1. Aids assessment of degree of shunting and degree of maturation.
2. Administer nasal or face mask 100% O2 at 10 L/min for 5 minutes.

1. PaO2 >200 mgHg - Normal degree of response.
2. PaO2 <160 mmHg - Indicates severe shunting - Requires increased monitoring and indicates more protracted illness.

VII. Hypercapnia (Increased Arterial CO2)

1. PaCO2 >60 mmHg causes:

1. Overwhelming pneumonia.
2. Atelectasis and edema.
3. Hypoventilation due to brain damage, rib fracture or toxicosis (drugs, etc.).
4. Progressive respiratory disease that leads to muscle fatigue.
5. Apparatus dead space or insufficient fresh oxygen flow.

2. PaCO2 >60 mmHg. (Treatment)

1. Oxygen therapy will not correct - Will only temporarily raise O2 levels.
2. Need to improve animal's ventilation.
3. PaCO2 >65 mmHg and rising with a pH <7.2 may be an indication for mechanical ventilation, or continuous positive airway pressure, or chemical stimulation of ventilation.
   Editor's Comment - We have many foals with PaCO2 >70 mmHg that have survived without mechanical ventilation using chemical stimulation of ventilation.

1. Intermittent intubation and ambu bag positive pressure ventilation 2-4 times daily to expand collapsed alveoli.
2. Doxapram hydrochloride (Dopram®) respiratory stimulant. This will increase the ventilation drive and lower PaCO2 even when foal is not "drug depressed". Repeat administration may be required. (See Drug Formulary-Equine Neonate)
   Editor's Comment - This works for most of our hypoventilation cases. Very effective in our hands; also used to help level of alertness in dummy foals. 0.5 mg/kg iv., or 0.01-0.02mg/kg/min as a CRI.
3. Caffeine tablets orally-10 mg per kg initially then 2.5 mg/kg BID.
4. Recent work indicates that doxapram significantly decreased PaCO2 levels close to normal range in healthy anesthetized hypercapnic foals and in foals with HIE. Caffeine however failed to show any significant effect on PaCO2in the same studies [6,7].

3. See Respiratory Therapeutics (See Respiratory Therapeutics).

VIII. Clinical Signs and Blood Gas Abnormalities [a]

1. Foals showing no signs of respiratory distress or respiratory impairment with an arterial blood gas PaO2 <52 mmHg in lateral recumbency will benefit from nasal insufflation. To raise PaO2 to 80-100 mmHg, start with a flow rate of 2-5 liter/min and adjust rate based on blood gas values.
2. Foals with signs of respiratory difficulties and PaO2 <52mmHg will benefit from nasal insufflation of O2 as described in (1) above.
3. Foals with signs of respiratory distress with PaO2 <52 mmHg in lateral recumbency and PaCO2 >70 mmHg with a pH of ≤7.2 require chemical stimulation of ventilation or mechanical ventilation. (See Positive Pressure Ventilation, See Sedation and Anesthesia)

[a]. Modified from Kosch PC, Koterba AM, Coon JJ, Webb AI. Developments in management of the newborn foal in respiratory distress: Evaluation. Equine Vet J 16:312-318, 1984. - PubMed -

IX. How to Decide When the Foal no Longer Needs Oxygen

1. As the foal's respiratory condition improves with treatment and time the blood gas PaO2 levels will increase while on the same flow of oxygen.
2. If the PaO2 is above 80-100 mmHg, decrease the O2 flow rate by 1/2 and recheck the arterial oxygen in about 10-15 minutes. Following this method the foal can be weaned off O2.
3. If the PaO2 is >52 mmHg in small for gestation age or premature foals less than 48 hours of age, >58 mmHg in term foals when on room air, discontinue O2 therapy.
4. Continue to monitor foal and provide respiratory therapeutics.