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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • Hypoxemia is a decreased amount of dissolved O2 carried in blood
  • PaO2 <80 mmHg, PvO2 <40 mmHg, or SO2/SpO2 <95% indicate hypoxemia

Pathophysiology!!navigator!!

  • Hypoxemia may cause hypoxia, which is decreased O2 delivery or utilization by tissues. Cellular function is adversely affected at a PaO2 of <60 mmHg
  • Hypoxia results from low levels of inspired O2 (atmospheric hypoxia); impaired delivery of air to alveoli due to respiratory obstruction or failure of the lungs to inflate or deflate (tidal hypoxia); decreased alveolar function due to abnormal alveoli, pulmonary interstitium, or pulmonary blood flow (alveolar hypoxia); or abnormal hemoglobin function (hemoglobic hypoxia)
  • More than one mechanism often is present
  • Anemia may cause hypoxia, but does not cause hypoxemia

Systems Affected!!navigator!!

All (hypoxia).

Genetics!!navigator!!

N/A

Incidence/Prevalence!!navigator!!

N/A

Geographic Distribution!!navigator!!

At high altitudes, FIO2, PaO2, and SO2 decrease.

Signalment!!navigator!!

Any breed, age, or sex.

Signs!!navigator!!

Historical Findings

History may include exercise intolerance, respiratory distress, coughing, lethargy, and other signs referable to the primary problem.

Physical Examination Findings

  • Tachypnea, tachycardia, coughing, reluctance to move, abducted elbows, outstretched neck, nostril flare, fever
  • Cyanosis develops when PaO2 is <40 mmHg

Causes!!navigator!!

  • Diffusion impairment
    • Pneumonia (e.g. Rhodococcus equi in foals, aspiration, pleuropneumonia, esophageal rupture)
    • Pulmonary edema (e.g. smoke inhalation, anaphylaxis, cardiac failure, volume overload)
    • Interstitial disease (acute respiratory distress syndrome, mineral oil pneumonitis, silicosis, EMPF)
    • Severe equine asthma (i.e. heaves, recurrent airway obstruction)
    • Prematurity
  • Hypoventilation (e.g. perinatal asphyxia, botulism, anesthesia, thoracic trauma, brainstem lesion). A PaO2 <60 mmHg stimulates respiration and increases minute ventilation. This may correct hypercapnia but often does not improve O2 levels because O2 does not diffuse as quickly as CO2 and is more dependent on the matching of blood flow to ventilation to maintain normal levels
  • V/Q mismatch (e.g. pulmonary hypertension, systemic hypotension, atelectasis)
  • Cardiac failure or right-to-left cardiac shunts (e.g. tetralogy of Fallot, truncus arteriosus, tricuspid atresia)
  • High altitude reduces the inspired O2 tension
  • Methemoglobinemia (e.g. red maple leaf toxicity) may decrease SO2, but does not directly affect PaO2 and SpO2

Risk Factors!!navigator!!

  • General anesthesia
  • Foals born from dams with dystocia might develop perinatal asphyxia (hypoventilation)
  • Foals on farms with endemic R. equi
  • Young performance horses (e.g. racehorses) and horses that are trailered long distances are at higher risk of developing pleuropneumonia
  • Infection with equine herpesvirus 5 is associated with development of EMPF
  • Environmental dust and mold can trigger exacerbation of severe equine asthma

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

See Causes.

Laboratory Findings!!navigator!!

Disorders That May Alter Laboratory Results

  • With poor peripheral perfusion or cardiovascular shunt, results of blood gas analysis on samples taken from peripheral arteries may not reflect the patient's overall systemic condition
  • Falsely increased PO2 results may occur with sample exposure to room air, excessive heparin dilution, and standard plastic syringes
  • Falsely decreased PO2 results may occur with delayed sample analysis, failure to adequately chill the sample, and severe neutrophilia

Valid if Run in a Human Laboratory?

Yes

CBC/Biochemistry/Urinalysis!!navigator!!

N/A

Other Laboratory Tests!!navigator!!

  • Arterial blood gas analysis is the definitive method for documenting hypoxemia
    • Take a heparinized blood sample anaerobically, cap with a rubber stopper, and analyze within 15–20 min
    • If the sample is stored on ice and collected in syringes dedicated for this use and containing powder heparin, results are valid for up to 3–4 h
  • Blood lactate concentrations can be measured with a handheld analyzer; hyperlactatemia is consistent with hypoxia

Imaging!!navigator!!

Radiography or ultrasonography to evaluate cardiac and pulmonary disease.

Other Diagnostic Procedures!!navigator!!

  • Pulse oximetry measures SpO2
  • The oximeter calculates the amount of oxygenated versus deoxygenated hemoglobin in blood based on light absorption
  • Decreased blood flow to the area of oximeter probe attachment invalidates the results (e.g. hypotension, vasoconstriction, hypothermia)
  • Oximeter probes can be used on the tongue of anesthetized horses
  • Conscious horses require a nasal oximeter probe

Treatment ⬆ ⬇

TREATMENT

Resolution of the primary cause of hypoxemia is paramount.

O2 Therapy

  • O2 therapy via nasal insufflation can be effective in elevating the PaO2. Inspired concentrations are limited to 30–45% with nasal insufflation. Higher levels may be obtained via insufflation directly into the trachea
  • Inspired gases must be humidified to avoid damage to mucous membranes from desiccation. This is accomplished by use of a humidifier or passing the O2 through a bottle of sterile water before exposure to the airway. The bottle must be secured in an upright position to prevent inspiration of fluid
  • To avoid O2 toxicity, maintain the FIO2 at the lowest level that produces a PO2 of >80 mmHg. If insufflation eliminates hypoxemia, PaCO2 levels may increase if the low PO2 was the primary stimulus of respiratory drive
  • Begin insufflation at 3–5 L/min in foals and 5–10 L/min in adults
  • Not useful if right-to-left cardiac shunt, persistent fetal circulation, or severe V/Q mismatch is present

Postural Therapy and Thoracic Percussion

  • Helpful to improve ventilation and drainage of secretions, especially in foals
  • Maintenance in sternal recumbency helps prevent atelectasis; turning every few hours is necessary for those in lateral recumbency

Mechanical Ventilation

  • Necessary in patients with severe hypoventilation (PaCO2 persistently >65 mmHg) and hypoxemia, and feasible in foals and anesthetized adults
  • Conscious foals can be intubated nasotracheally and connected to the rebreathing circuit of a small-animal anesthesia machine or a human ventilator
  • 2 flowmeters (or one that allows mixing of O2 and room air) are necessary, as is a monitor that can measure the FIO2 level
  • Assisted rather than controlled ventilation is better, because most foals are more comfortable when respiratory drive is not eliminated
  • Sedation may be necessary in some patients but many relax once ventilation improves
  • Periodic suctioning of the nasotracheal tube is necessary to prevent obstruction from accumulated secretions
  • After weaning from mechanical ventilation, temporary nasal insufflation of O2 is recommended in foals because their functional residual capacity will decrease and hypoxemia may recur

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

Dependent on the underlying cause.

Contraindications!!navigator!!

  • Do not use doxapram to improve respiratory function in healthy anesthetized patients, especially during weaning. Its effects are temporary, and if the patient's PaO2 levels are low or the patient remains depressed once it wears off, apnea may occur
  • Mechanical ventilation for meconium aspiration might cause alveolar damage because of air-trapping behind obstructed bronchioles

Precautions!!navigator!!

  • Do not allow combustible materials or smoking near the patient or the O2 tanks
  • Securely attach tanks to an immovable structure because they can rupture or explode violently if knocked over. Wear safety glasses and keep face out of range of the valves during setup and disconnection
  • Use aseptic techniques in handling endotracheal tubes
  • Use fluid therapy carefully in neonates, cardiac patients, and patients with renal failure to avoid volume overload
  • O2 toxicity can occur with administration of >50% FIO2 or maintenance of a PaO2 >100 mmHg for long periods
  • O2 therapy may decrease the ventilator response and worsen hypoventilation

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Serial arterial blood gas evaluations
  • Pulse oximetry can be used to monitor hemoglobin saturation
  • As PaO2 improves, provide decreasing levels of FIO2. Eventually, periodic trials on room air can be attempted
  • Evaluate patient demeanor and degree or quality of respiratory effort as blood gas levels change. When the patient can maintain a PO2 of >70 mmHg (>60 mmHg in premature neonates) on room air, O2 therapy can be discontinued

Prevention/Avoidance!!navigator!!

N/A

Possible Complications!!navigator!!

  • Damage to nervous tissue from prolonged periods of hypoxemia may result in brain damage (e.g. altered consciousness, blindness, seizures)
  • Cardiac arrhythmias may be caused by hypoxemic damage to the myocardium

Expected Course and Prognosis!!navigator!!

Dependent on the underlying cause.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

Metabolic acidosis caused by accumulation of lactic acid from anaerobic glycolysis may develop with prolonged hypoxemia, especially if hypotension exists.

Age-Related Factors!!navigator!!

  • Neonatal foals normally have lower PaO2 (<60 mmHg) and higher PaCO2 (>50 mmHg) values during the first hour of life. PaO2 values increase to >80 mmHg by 2–4 days. PaCO2 values decrease to adult reference intervals by 4–7 days
  • Premature neonates are highly predisposed to hypoxemia

Pregnancy/Breeding/Fertility!!navigator!!

  • Heavily pregnant mares are at greater risk of hypoxemia under general anesthesia
  • Prolonged hypoxemia during gestation may result in fetal growth retardation and perinatal asphyxia

Abbreviations!!navigator!!

  • EMPF = equine multinodular pulmonary fibrosis
  • FIO2 = percent fractional inspired oxygen concentration
  • PaCO2 = partial pressure of arterial carbon dioxide tension, mmHg
  • PaO2 = partial pressure of arterial oxygen tension, mmHg
  • PvO2 = partial pressure of venous oxygen tension, mmHg
  • SO2 = percent hemoglobin saturation with oxygen
  • SpO2 = percent hemoglobin saturation with oxygen in peripheral tissues (pulse oximetry)
  • V/Q = ventilation–perfusion ratio

Suggested Reading ⬆ ⬇

Bettschart-Wolfensberger R. Anesthesia and analgesia for domestic species: horses. In: Grimm KA, Lamont LA, Tranquilli WJ, et al., eds. Veterinary Anesthesia and Analgesia, 5e. Ames, IA: Wiley Blackwell, 2015:857–866.

McDonnell WN, Kerr CL. Respiratory system: physiology, pathophysiology, and anesthetic management of patients with respiratory disease. In: Grimm KA, Lamont LA, Tranquilli WJ, et al., eds. Veterinary Anesthesia and Analgesia, 5e. Ames, IA: Wiley Blackwell, 2015:513–558.

Palmer J. Ventilatory support of the critically ill foal. Vet Clin North Am Equine Pract 2005;21:457–486.

Author(s) ⬆ ⬇

Author: Katie M. Boes

Consulting Editor: Sandra D. Taylor

Acknowledgment: The author and editor acknowledge the prior contribution of Jennifer G. Adams.

Additional Further Reading ⬆

Click here for Additional Further Reading