Normal respiration

Objective

• To deliver enough fresh gas to the alveoli to meet the animal's oxygen requirement (VO2) and remove CO2, 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 manoeuvres 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.
• Post-operative 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) Lung: atelectasis Lung: pulmonary edema.

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 vasoconstriction 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

Modified ventilation cycles

Ventilatory support in veterinary practice

Techniques of ventilation

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.