Introduction

• Oxygen exists in the blood in 2 forms: a small amount is dissolved in plasma (PaO2) whereas the vast majority is bound to hemoglobin (SO2).
• Hypoxemia is defined as low concentrations of oxygen in the blood.  
• Hypoxia is defined as decreased oxygen content and may be described in the context of organs, tissues or cells. 
  • Low oxygen at tissue level may be a result of hypoxemia. 
  • However, hypoxia is not always associated with hypoxemia. 
  • There are, classically, 4 causes of hypoxia:
    • Hypoxemic hypoxia: poor oxygen content in the blood supply.
    • Anemic hypoxemia: due to lack of functional Hb in the blood, is a subtype of hypoxemic hypoxia.
    • Stagnant or ischemic hypoxia: lack of perfusion.
    • Histotoxic hypoxia where the cells are unable to utilise the oxygen being carried to them, which can occur with endotoxemia. 
  • Cellular hypoxia results in a switch to anaerobic metabolism leading to intracellular acidosis and cell damage.
• Most clinicians define hypoxemia as a hemoglobin saturation <90% or a partial pressure of oxygen in arterial blood (PaO2) of <60 mmHg (8 kPa) when breathing room air.
• Hypoxemia and hypoxia may be implicated in the development of complications such as dysrhythmias, post-anesthetic myopathy-neuropathy and other morbidities. Its recognition, causality and treatment are thus important to monitor and address.
• When hypoxemia is suspected in the conscious patient, blood gas analysis will confirm the diagnosis. Pulse oximetry is difficult in the conscious cow and will unlikely be reliable due to movement. Diagnosis of the underlying cause of hypoxemia (see below) is essential to be able to devise a suitable treatment plan and reach a prognosis. However, diagnosis of lung disease in cattle can be more challenging than in small animals.

• Pulse oximetry (SpO2).
  • Techniques which measure the saturation of hemoglobin with oxygen in arterial blood.
  • Difficult to get reliable, long-lasting readings in cattle and of limited value in animals breathing 100% oxygen.
• Arterial blood gas analysis:
  • Many types of analyzers are available, both portable and bench top.
  • Blood gas analyzers are readily available in veterinary practice, but often not in large animal practice.
  • Measures the partial pressure of oxygen in arterial blood, ie the amount of oxygen dissolved in the plasma.
  • Blood gas analysis also gives information on carbon dioxide (CO2) tension and acid-base status.

The oxygen-hemoglobin dissociation curve

Causes of hypoxemia

Hypoventilation

Ventilation (V) perfusion (Q) inequality

Treatment of hypoxemia

Further Reading

Publications

Refereed Papers

• Recent references from PubMed and VetMedResource.

Other sources of information

• Valverde A & Sinclair M (2015) Ruminant Anesthesia and Ruminant and Swine Local Anesthetic and Analgesic Techniques. In: Veterinary Anesthesia and Analgesia. Blackwell Publishing.
• Clarke K & Trim C (2014) Patient Monitoring and Clinical Measurement and Anaesthesia of Cattle. In: Veterinary Anaesthesia. 11th edn. Elsevier.
• West J B (2005) Respiratory Physiology: The Essentials. 7th Edn. Lippin Cott Williams & Wilkins, UK.