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Ventilators

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Introduction

  • Ventilation of the lung is defined as the mass movement of gas into and out of the lung.
  • It is essential that the cow 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.   
  • In the conscious cow, adequate ventilation is achieved by active use of respiratory muscles in both inspiration and expiration.
  • General anesthesia General anesthesia has depressive effects on the respiratory system. However, in contrast to horses, cattle frequently manage to maintain a relatively high respiratory rate although their breaths are usually relatively shallow so that Hypercapnia Hypercapnia may still occur. However, as with horses, Hypoxemia Hypoxemia can occur secondary to ventilation-perfusion mismatch and this problem is compounded by long periods of recumbency.
  • Mechanical ventilation may be provided manually by compressing the reservoir bag of the anesthetic breathing system intermittently. Mechanical ventilation can be performed if required.
  • 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 hypoxemia are all encountered in bovine anesthesia (although it is rarely performed), especially where the patient is kept anesthetized for extended periods.
  • By using controlled ventilation, arterial blood gas levels can be maintained at near-normal concentrations.
  • Arterial hypoxemia, especially when the patient is in dorsal recumbency, can still be a problem, but the acid-base status is improved after institution of IPPV.
  • 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 for augmentation of venous return.
  • 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.
 Indications for IPPV
  • Apnoea immediately after induction 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.
  • Abdominal tympany can cause severe respiratory impairment and so ventilation may be required in these cases. [insert link]
  • 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 in the anesthetic to try to prevent hypoxemia developing.
Assessing and monitoring mechanical ventilation
  • Serial blood gas analysis and analysis of respiratory gases will help to monitor the adequacy of mechanical ventilation if available.

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

  • Recent references from PubMed and VetMed Resource.
  • Eicker S W & Cuvelliez S (1990) Equipment for inhalation anesthesia. Vet Clin North Am (3) pp 543-549.

Other sources of information

  • Valverde A & Sinclair M (2015) Ruminant and Swine Local Anesthetic and Analgesic Techniques. In: Veterinary Anesthesia and Analgesia. Blackwell Publishing.
  • Clarke K & Trim C (2014) Pulmonary gas exchange: Artificial ventilation of the lung, Patient Monitoring and Clinical Measurement and Anaesthesia of Cattle. In: Veterinary Anaesthesia. 11th Edn. Elsevier.

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