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Basics ⬇

Basics

Definition

  • A decrease in PaO2, resulting in marked desaturation of hemoglobin.
  • PaO2 at sea level ranges from 80 to 100 mmHg in normal animals.

Pathophysiology

Six physiologic causes

  • low PIO2
  • hypoventilation (increase in PaCO2)
  • mismatching of alveolar ventilation and perfusion so that areas of the lung that are not ventilated properly are still perfused adequately
  • alveolar–capillary membrane diffusion defect
  • right-to-left cardiac or pulmonary shunting

Systems Affected

  • All organs-oxygen essential for normal cellular function; individual tissue oxygen requirements vary by organ.
  • Cardiovascular-can result in focal or global ischemia; if prolonged, can develop arrhythmias and cardiac failure.
  • Nervous-brain and CNS most important; hypoxemia can result in irreversible brain damage because there are no large oxygen stores in brain tissue.

Signalment

Any breed, age, and sex of dogs and cats

Signs

Historical Findings

  • Episodes of coughing
  • Breathing problems-especially open-mouth breathing
  • Trauma
  • Gagging
  • Exercise intolerance
  • Cyanosis
  • Collapse

Physical Examination Findings

  • Tachypnea
  • Dyspnea
  • Orthopnea
  • Pale mucous membranes
  • Cyanosis
  • Coughing
  • Open-mouth breathing
  • Tachycardia
  • Poor peripheral pulse
  • Abnormal thoracic auscultation

Causes

  • Low PIO2-high altitude (the higher the elevation, the lower the barometric pressure, which results in a decrease in PIO2; FIO2 is fixed at 0.21); suffocation; enclosure in small areas with improper ventilation.
  • Hypoventilation-result of inadequate alveolar ventilation; muscular paralysis; upper airway obstruction; air or fluid in the pleural space; restriction of the thoracic cage, diaphragmatic hernia; CNS disease.
  • Mismatching of alveolar ventilation and perfusion-most common cause of hypoxemia and occurs with virtually any lung disease: pulmonary thromboembolism; pulmonary parenchymal disease (infectious or neoplastic); lower airway disease; pneumonia; pulmonary contusions; pulmonary edema; also during anesthesia or prolonged recumbency when a large region of lung becomes atelectatic.
  • Alveolar–capillary membrane diffusion impairment-rarely clinically important.
  • Right-to-left cardiac or pulmonary shunting-tetralogy of Fallot; ventricular septal defect; reversed patent ductus arteriosus; intrapulmonary arteriovenous shunt.

Risk Factors

  • Sudden move to higher elevation
  • Trauma
  • Bronchopneumonia
  • Pleural disease
  • Anesthesia
  • Cardiac disease
  • Bronchial disease-chronic bronchitis, feline asthma
  • Geriatric pulmonary or cardiac changes
  • Diseases associated with risk of embolization, e.g., immune mediated hemolytic anemia, hyperadrenocorticism, neoplasia, pancreatitis, sepsis.

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

  • Signs of tachypnea and/or dyspnea
  • Excitement or anxiety
  • Hyperthermia
  • Pyrexia
  • Head trauma
  • Pain

Laboratory Findings

Drugs That May Alter Laboratory Results

N/A

Disorders That May Alter Laboratory Results

  • Air bubbles in the arterial blood sample-falsely high PaO2 values.
  • Improper packaging of the arterial blood sample-falsely high PaO2 values after approximately 30 minutes at room temperature.

Valid if Run in Human Laboratory?

Yes

CBC/Biochemistry/Urinalysis

  • PCV-can be high with chronic condition; can be low if inflammatory or neoplastic.
  • Liver enzyme elevation common with organ hypoxia.

Other Laboratory Tests

Arterial Blood Gases

  • Collect arterial blood sample in an anaerobic manner, as follows: Use enough heparin to coat the needle and the inside of the syringe. Collect sample from femoral or dorsal pedal artery. Place a rubber stopper on the needle or covering the hub of the syringe, to prevent room air from entering the sample. Analyze sample within 15 minutes if left at room temperature; place sample on ice to extend safe time for analysis to 2–4 hours.
  • Bedside or portable blood gas analyzers-several models available; make analysis more convenient.

Imaging

Thoracic radiographs and echocardiography-evaluate intrathoracic disease; differentiate pulmonary and cardiac disease.

Diagnostic Procedures

Pulse Oximetry

  • Indirectly determines SaO2; relation between PaO2 and SaO2 based on the oxyhemoglobin dissociation curve: SaO2 >90% when PaO2 >60 mmHg.
  • SaO2 <95%-considered abnormal, indicates PaO2 <80 mmHg.
  • Best results when probe used on the tongue of animals; thus may be limited to anesthetized, heavily sedated, or seriously ill patients with a low level of consciousness; keep tongue moistened for most accurate readings.
  • Other successful probe sites-lip, ear; vulva (female), and prepuce (male); skin between toes; thin skin in the flank area.
  • Poor results-least accurate in low-flow states such as hypotension (global low flow) or hypothermia (low flow to skin); falsely low values (usually <85%) during carboxyhemoglobinemia (smoke inhalation).
  • Rectal probes-should become available; will allow readings in awake patients.

Endoscopy or Lung Biopsy

Airway sampling often required to determine primary abnormality resulting in hypoxemia.

Treatment ⬆ ⬇

Treatment

Must identify and correct the primary cause.

Oxygen Therapy

  • Most common supportive treatment.
  • Corrects low-inspired oxygen, hypoventilation, and alveolar–capillary membrane diffusion defects; may not fully correct mismatching of ventilation and perfusion; does not correct right-to-left cardiac or pulmonary shunts and low cardiac output.
  • May not be completely beneficial until adequate blood volume is established.
  • Delivery-directly from an oxygen source from the anesthetic machine via a face mask placed securely around the muzzle or from an E-tank fitted with an oxygen regulator through a face mask, intranasal catheter, or oxygen cage.
  • Increase in FIO2-determined by the oxygen flow rate and the amount of oxygen mixed with room air.
  • PPV-may be needed for ARDS or severe hypoventilation.

Fluid Therapy

  • Low cardiac output-fluid administration and inotropic support (e.g., dobutamine or dopamine) important.
  • Cardiac failure-requires aggressive medical treatment; diuretics; afterload and preload reduction; inotropic support; oxygen administration; fluids indicated after institution of primary treatment; use caution with type and rate of fluids after initial stabilization.
  • Hypovolemic, hemorrhagic, traumatic, or septic shock-requires aggressive fluid administration; crystalloids (90 mL/kg as fast as possible), hypertonic solutions (7% NaCl, 4 mL/kg), colloids (hetastarch, 20 mL/kg), hemoglobin-based oxygen-carrying solutions, or combination.
  • Severe pulmonary contusion-hypertonic fluids or colloids, or combination preferred.

Medications ⬆ ⬇

Medications

Drug(s) Of Choice

For bronchospasm-bronchodilators; terbutaline (0.01 mg/kg SC, IM, or IV q8h).

Contraindications

  • Aggressive fluid administration-not indicated for cardiac failure and pulmonary edema.
  • Diuretics-not indicated for shock, low PIO2, alveolar–capillary membrane diffusion defects, mismatching of alveolar ventilation and perfusion, and right-to-left shunts.

Precautions

  • Inotropic drugs-arrhythmias may develop.
  • Oxygen toxicity-from prolonged (>12 hour) exposure to high-concentration (>70%) oxygen; pulmonary edema, seizures, and death.

Possible Interactions

N/A

Follow-Up ⬆ ⬇

Follow-Up

Patient Monitoring

  • Decrease in respiratory effort and a decrease in cyanosis (if initially noted)-check efficacy of treatment and support.
  • Arterial blood gas-determine resolution.
  • Pulse oximetry-alternative; interpret results cautiously with hypotension, hypothermia, smoke inhalation, and non-tongue probe site.

Possible Complications

  • Brain damage-depends on severity and duration of hypoxemia; partial or complete loss of neuronal function; dementia; seizures; loss of consciousness.
  • Arrhythmias-may develop secondary to myocardial hypoxia; may be very difficult to treat effectively.

Miscellaneous ⬆

Miscellaneous

Associated Conditions

N/A

Age-Related Factors

N/A

Pregnancy/Fertility/Breeding

May adversely affect fetuses, especially during the first trimester of pregnancy.

Abbreviations

  • ARDS = acute respiratory distress syndrome
  • CNS = central nervous system
  • FIO2 = fraction of oxygen in inspired air
  • PaCO2 = partial pressure of carbon dioxide in arterial blood
  • PaO2 = partial pressure of arterial oxygen
  • PCV = packed cell volume
  • PIO2 = partial pressure of inspired oxygen
  • PPV = positive-pressure ventilation
  • SaO2 = saturation of arterial blood with oxygen

Suggested Reading

Barton L. Respiratory failure. In: Silverstein DC, Hopper K, eds. Small Animal Critical Care Medicine. St. Louis, MO: Saunders Elsevier, 2009, pp. 64–66.

Haskins SC. Monitoring anesthetized patients. In: Tranquilli WJ, Thurmon JC, Grimm KA, eds. Lumb and Jones; Veterinary Anesthesia and Analgesia, 4th ed. Ames, IA: Blackwell Publishing, 2007, pp. 547–552.

McDonell WN, Kerr CL. Respiratory system. In: Tranquilli WJ, Thurmon JC, Grimm KA, eds. Lumb and Jones; Veterinary Anesthesia and Analgesia, 4th ed. Ames, IA: Blackwell Publishing, 2007, pp. 117–139.

West JB. Pulmonary Pathophysiology, 8th ed. Philadelphia: Lippincott Williams & Wilkins, 2011, pp. 141–152.

West JB. Respiratory Physiology: The Essentials, 9th ed. Philadelphia: Lippincott Williams & Wilkins, 2012, pp. 56–76.

Author Thomas K. Day

Consulting Editor Lynelle R. Johnson