ISSN 2398-2977      

Anesthesia: peri-operative complications - hypoxemia

pequis

Introduction

  • Oxygen exists in the blood in 2 forms: a small amount is dissolved in plasma (PaO2 ) and the vast majority is bound to hemoglobin (SO2 ).
  • Hypoxemiais defined as low levels of oxygen in the blood.  
  • Hypoxiais defined as decreased oxygen and may be described in the context of organs, tissues or cells tissue level andmaybe a result ofhypoxemia. However, hypoxia is not always associated with hypoxemia. 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).
  • Hypoxemia and hypoxia may be implicated in the development of complications such as arrhythmias   Anesthesia: peri-operative complications - cardiovascular  and post-anesthetic myopathy   Anesthesia: peri-operative complications - post-operative myopathy  , and therefore its recognition, causality and treatment are essential during anesthetic management.
  • When hypoxemia is suspected in the conscious patient, blood gas analysis   Blood: gas analysis  will confirm the diagnosis. Pulse oximetry is difficult in the conscious horse and may not 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 horses presents the clinician with problems, mainly because of their thoracic size making radiographic (or other) examination challenging. Oxygen supplementation in conscious adult horses is impractical and of dubious value.
  • Tissue oxygenation is difficult to measure but oxygenation of blood can be measured with a variety of instruments and can be used give an indication of how well tissues are being oxygenated (providing those tissues are being perfused adequately).

Measurement of oxygen in blood

  • Pulse oximetry (SpO2 ) or co-oximetry (SaO2 )   Anesthesia: monitoring - respiratory management  :
    • techniques which measure the saturation of hemoglobin with oxygen in arterial blood.
    • A pulse oximeter is a portable, non-invasive instrument which utilizes the differing absorptions of light by oxygenated and reduced - hemoglobin species.
    • A pulse oximeter is an essential piece of equipment and should be considered mandatory during anesthetic management.
    • A co-oximeter is a laboratory based instrument that is used to differentiate other species of haemoglobin, eg methemoglobin, carboxyhemoglobin, in addition to oxygenated and reduced hemoglobin, and requires a blood sample to be taken from the patient.
    • Co-oximeters are rare in veterinary practice.
  • Arterial blood gas analysis   Anesthesia: monitoring - respiratory management  :
    • Measures the partial pressure of oxygen in arterial blood, ie the amount of oxygen dissolved in the plasma.
    • This technique also requires a blood sample to be taken from the patient.
    • Many types of analyzers are available, both portable and bench top.
    • Blood gas analysis also give information on carbon dioxide (CO2 ) tension and acid-base status.
    • Blood gas analyzers are readily available in veterinary practice.

The oxygen-hemoglobin dissociation curve

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Causes of hypoxemia

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Hypoventilation

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Ventilation (V) perfusion (Q) inequality

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Treatment of hypoxemia

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Moens Y & Böhm S (2011) Ventilating horses: moving away from old paradigms. Vet Anaes Analg 38 (3), 165-168 PubMed.
  • Hopster K, Kästner S, Rohn K et al (2011) Intermittent positive pressure ventilation with constant positive end-expiratory pressure and alveolar recruitment manoeuvre during inhalation anaesthesia in horses undergoing surgery for colic, and its influence on the early recovery period. Vet Anaes Analg 38 (3), 169-177 PubMed.
  • Patschova M, Kabes R & Krisova S (2010) The effects of inhalation salbutamol administration on systemic and pulmonary hemodynamic, pulmonary mechanics and oxygen balance during general anaesthesia in the horse. Veterinari Medicina 9, 445-456.
  • Mansel J C & Clutton E (2008) The influence of body mass and thoracic dimensions on arterial oxygenation inanaesthetised horses and ponies. Vet Anaes Analg 29, 159-170 PubMed.
  • Levionnois O, Iff I & Moens Y (2006) Successful treatment of hypoxemia by an alveolar recruitment maneuver in a horse during general anaesthesia for colic surgery. Pferdeheilkunde 22, 1-3.
  • Wettstein D, Moens Y, Jaeggin-Schmucker N et al (2006) Effects of an alveolar recruitment maneuver on cardiovascular and respiratory parameters during total intravenous anesthesia in ponies. Am J Vet Res 67, 152-159 PubMed.
  • Marntell S, Nyman G & Hedenstierna G (2005) High inspired oxygen concentrations increase intrapulmonary shunt in anaesthetized horses. Vet Anaesth Analg 32, 338-347 PubMed.
  • Robertson S A & Bailey J E (2002) Aerosolized salbutamol (albuterol) improves PaO2 in hypoxaemic anaesthetized horses a prospective clinical trial in 81 horses. Vet Anaesth Analg 29, 212-218.
  • Nyman G, Funkquist B, Kvart C et al (1990) Atelectasis causes gas exchange impairment in the anaestehtised horse. Eq Vet J 22, 317-324 PubMed.

Other sources of information

  • Doherty T & Valverde A (2006) Equine Anesthesia & Analgesia. Blackwell Publishing Ltd, UK.
  • Lumb A B (2005) Nunns Applied Respiratory Physiology. 6th edn. Elsevier, USA.
  • West J B (2005) Respiratory Physiology: The Essentials. 7th edn. LippinCott Williams & Wilkins, UK.
  • Power I & Kam P (2001) Principles of Physiology for the Anaesthetist. Arnold, UK.

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