Problems in equine anaesthesia- ventilation and blood pressure Flashcards
What are some problems associated with equine anaesthesia?
- Hypercapnia
- Hypotension
- Neuropraxias
- Corneal abrasions
- Equine post-anaesthetic myopathy
What is tympany and how can you minimise the effects during equine anaesthesia?
gas accumulation in guts
may be prevalent with high energy feed
try to let down before anaesthesia- reduce energy feed
What are the 2 effects of hypoventilation?
hypercapnia
hypoxaemia
Describe hypercapnia
reduced exhalation of carbon dioxide (failure to eliminate adequate C02)- measured value of capnography
life threatening
Describe hypoxaemia
side effect of hypoventilation
reduced uptake of O2
life threatening
What are reasons why horses hypoventilate?
anaesthetic agents
positioning
What are the effects of inhalants on ventilation?
- decreased ventilatory drive
- desensitise medullary and carotid body chemoreceptors
- hypoxaemia
- respiratory acidosis
- increased atelectasis and V/Q mismatch
- reduced minute ventilation
What is the normal alveolar C02 measure?
approx 40 mmHg
How can the positioning of the horse for anaesthesia impact?
the magnitude of V/Q mismatch
leads to change in perfusion and ventilation
added impairment of the weight of the abdominal contents which restricts diaphragmatic movement
Where is the greatest proportion of lung mass in the horse?
along the dorsum
What does decreased alveolar elimination of C02 cause?
an increase in arterial concentration (PaC02)
leads to development of acidaemia (decreased pH)
How is C02 usually measured?
- as end-tidal
measured by sampling of airways gases (PaC02) by capnography - also by analysis of blood from an arterial cannula
Describe the use of capnography in the horse
Continuous measurement
Useful to identify hypoventilation and hyperventilation trends
Allows the integrity of the airway and anaesthetic circuit to be checked
When assessed with PaC02- anatomic and alveolar dead space can be assessed, can also indicate metabolism , perfusion and Cardiac output
How can hypoventilation be treated?
first check the depth of anaesthesia- if too deep change the suppression of the anaesthetic or provide IPPV
What are the 5 main causes of hypoxaemia?
- inadequate inspired oxygen
- impaired diffusion across alveoli
- hypoventilation- common under anaesthesia
- VQ mismatch
- shunting of blood- due to V/Q mismatch
When does ventilation perfusion matching occurs?
when the amount of blood delivered to each alveolus is appropriate to maximally exchange the oxygen and C02 within that alveolus
What does High V/Q mean?
Means that not enough blood reaches well ventilated alveoli- Pa02 is high but PaC02 is low
What does low V/Q mismatch mean?
Not enough gas reaches over perfused vessels- PA02 is low and so Pa C02 is low.
Both High V/Q mismatch and Low V/Q mismatch therefore produce a reduction in PaO2, therefore both predispose to hypoxaemia.
Describe hypoxic pulmonary vasoconstriction
Compensatory vascular response that shunts blood flow away from unventilated alveoli
Reduces low V/Q mismatch
Compensatory response is abolished by inhalant anesthetics
What can prevent V/Q mismatch?
Improve the position of patient- lateral is better than dorsal but still get atelectasis
IPPV from beginning of anaesthetic
Air:oxygen mixture for the delivery of gas- lowers FI 02 but has a risk of hypoxaemia
Nitrogen gas provides a scaffold for alveoli and reduces absorption atelectasis
What are the benefits of using IPPV?
- may prevent decrease in tidal volume
Will help control carbon dioxide levels (prevent hypercapnia and acidosis)
Use of PEEP (positive end expiratory pressure)
Recruitment manoeuvre to reopen alveoli
What are the detriments to IPPV?
Gas will take path of least resistance
More gas may just flow to already inflated areas so increase V
May impede venous return reducing Q
May not improve V/Q mismatch by very much
How can V/Q mismatch by treated by drugs?
Bronchodilators- salbutamol and clenbuterol
Can affect CV system however leading to vasodilation and excessive sweating
Define hypotension
Reduction in mean arterial blood pressure
Ideally 70-90 mmHg (concerning= <60mmHg)
Leads to reduced perfusion of organs
Can lead to reduced oxygen delivery to tissues, increased lactate build up and equine post-anaesthetic myopathy
Common complication of inhalant anaesthesia
