Felis ISSN 2398-2950

Anesthetic ventilators: overview

Contributor(s): Daniel H Lewis, Claire Waters

Normal respiration

Objective

  • To deliver enough fresh gas to the alveoli to meet the animal's oxygen requirement (VO-2) and remove CO-2, thus maintaining acid-base balance. (NB: Also affected by concurrent gas diffusion and pulmonary blood flow.)

Cardiovascular effects of inspiration

  • Decreased intrapleural pressure creates a pressure gradient which favors gas flow from the atmosphere to the distal airway.
  • Increased intraperitoneal pressure creates a pressure gradient between the peritoneal cavity and the thorax, compressing splanchnic veins and increasing venous return.

Effect of respiration on anesthesia

  • A major factor governing the pharmacokinetics of inhaled anesthetics.
  • Inevitably impaired during surgery and predisposes to hypercapnia Hypercapnia and hypoxemia Hypoxemia   →   cardiac arrest.

Effects of anesthesia on the respiratory system
Breathing pattern

  • Regular respiration is the hallmark of light anesthesia: inspiration   →   inspiratory pause   →    expiration   →   expiratory pause.
  • Anesthesia inhibits lung-expanding maneuvers such as yawning, sighing and vocalizing.
  • The majority of anesthetic drugs cause a dose-dependent reduction in the chemoreceptor response to altered blood levels of carbon dioxide and oxygen.
  • Postoperative pain may inhibit normal breathing or coughing.

Lung volumes
Anesthesia and postural changes reduce lung volumes

  • Functional residual capacity (FRC).
  • Expiratory reserve volume (ERV).
  • Inspiratory reserve volume (IRV).
  • Vital capacity.
  • Expiratory flow rate.

Compliance

  • During anesthesia, lung compliance (the change in volume due to a change in pressure) decreases and airway resistance increases   →   greater airway pressures are needed to inflate the lungs.

Atelectasis

  • Prolonged immobility leads to dependent atelectasis and pulmonary edema (aggravated by crystalloid therapy).

Mucociliary function

  • Is decreased by:
    • Poor systemic hydration.
    • Low inspired humidity.
    • High inspired oxygen concentration.
    • Endotracheal tube cuff.
    • Some anesthetics.
    • The accumulation of mucus may lead to peripheral airway closure and the subsequent collapse of distal alveolar units.

Pulmonary hypoxic vasoconstriction

  • Normally, hypoxic pulmonary vasoconstrictyion reduces blood flow to poorly ventilated alveoli
  • The majority of inhalational (but not injectable) anesthetic agents appear to abolish this protective mechanism. This therefore allows venous blood to flow through hypoxic or atelectic lung regions   →   impaired arterial oxygenation.

Types of ventilation

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Modified ventilation cycles

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Ventilatory support in veterinary practice

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Techniques of ventilation

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Further Reading

Publications

Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Lee J A, Drobatz K J, Koch M W & King L G (2005) Indications for and outcome of positive-pressure ventilation in cats: 53 cases (1993-2002 ). JAVMA 226(6), 924-931 PubMed.


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