Equis ISSN 2398-2977

Anesthesia: ventilators - overview

Contributor(s): Dennis R Gieser, Craig Johnson, Mark Senior

Introduction

  • Ventilation of the lung is defined as the mass movement of gas into and out of the lung.
  • It is essential that the horse has sufficient alveolar ventilation, in other words, that the volume of inspired gas is sufficient to maintain normal partial pressures of arterial oxygen and carbon dioxide   Anesthesia: monitoring - respiratory management  .
  • In the conscious horse, adequate ventilation is achieved by active use of respiratory muscles in both inspiration and expiration.
  • General anesthesia has depressive effects on the respiratory system which are more marked in the horse than in other species. This problem is compounded by long periods of recumbency.
  • Mechanical ventilation may be provided manually by compressing the reservoir bag   Anesthesia: circuits - overview  intermittently, but in horses, where the bag can be 30 liters capacity, it is more commonly carried out using a mechanical ventilator.
  • Because positive pressure is required to force gas into the lungs, this is usually referred to as intermittent positive pressure ventilation (IPPV).
    Objectives of IPPV
  • Hypercapnia, respiratory acidosis and hypoxia   Blood: gas analysis  are all commonly encountered in equine anesthesia, especially where the patient is kept anesthetized for extended periods.
  • By using controlled ventilation, arterial blood gas levels can be maintained at near-normal levels.
  • Arterial hypoxemia, especially when the patient is in dorsal recumbency, can still be a problem, but the acid-base status is improved.
  • A problem with IPPV is the effect it has on the cardiovascular system.
    Effects of IPPV on the circulation
  • During normal (conscious) breathing, the negative pressure in the thorax which causes air to enter the lungs in inspiration also helps blood to move into the right atrium by helping to dilate the vena cava. This is known as the thoracic pump mechanism.
  • When IPPV is used, the pressure in the lungs and pleural space becomes positive and so interferes with the normal filling of the vena cava and right atrium.
  • This decrease in venous return   →   decreased cardiac output and decreased arterial blood pressure   Anesthesia: monitoring - cardiac output and blood pressure  .
    Indications for IPPV
  • Use of muscle relaxants will necessitate IPPV.
  • Apnea immediately after induction   Anesthesia: induction - overview  can prevent an adequate concentration of inhalant anesthetic from reaching the lungs, so either manual or mechanical ventilation may be necessary to increase the alveolar concentration of the drug and allow a smooth transition from intravenous to inhaled anesthesia   Anesthesia: maintenance - overview  .
  • Abdominal tympany can cause severe respiratory impairment and so ventilation may be required in these cases.
  • Since hypoxemia is difficult to correct once it has occurred, if there is any indication that the patient may require IPPV, this should be initiated as soon as possible into the anesthetic to prevent hypoxemia developing.
    Assessing and monitoring mechanical ventilation
  • Serial blood gas analysis   Blood: gas analysis  , and analysis of respiratory gases   Anesthesia: monitoring - respiratory management  will help to monitor the adequacy of mechanical ventilation.

Mechanical ventilators

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Using mechanical ventilators

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Problems with mechanical ventilation

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Further Reading

Publications

Refereed papers
  • Eicker S W & Cuvelliez S (1990)Equipment for inhalation anesthesia. Vet Clin North Am6(3) 543-549.
  • Shawley R V & Mandsager R E (1990)Clinical use of positive pressure ventilation in the horse. Vet Clin North Am6(3) 575-585.

Other sources of information

  • Steffey E P (1990)Positive pressure ventilation.In: Current Practice of Equine Surgery. Eds: N A White and J N Moore. J B Lippincott Company, Philadelphia. pp83-86.
  • Hall L W & Clarke K W (1983) Veterinary Anesthesia, 8th edn. Bailliere Tindall, London, pp149-155.


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