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Lectures > Problems in equine anaesthesia- ventilation and blood pressure > Flashcards

Problems in equine anaesthesia- ventilation and blood pressure Flashcards

1
Q

What are some problems associated with equine anaesthesia?

A
  • Hypercapnia
  • Hypotension
  • Neuropraxias
  • Corneal abrasions
  • Equine post-anaesthetic myopathy
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2
Q

What is tympany and how can you minimise the effects during equine anaesthesia?

A

gas accumulation in guts

may be prevalent with high energy feed
try to let down before anaesthesia- reduce energy feed

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3
Q

What are the 2 effects of hypoventilation?

A

hypercapnia

hypoxaemia

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4
Q

Describe hypercapnia

A

reduced exhalation of carbon dioxide (failure to eliminate adequate C02)- measured value of capnography
life threatening

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5
Q

Describe hypoxaemia

A

side effect of hypoventilation
reduced uptake of O2
life threatening

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6
Q

What are reasons why horses hypoventilate?

A

anaesthetic agents

positioning

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7
Q

What are the effects of inhalants on ventilation?

A
  • decreased ventilatory drive
  • desensitise medullary and carotid body chemoreceptors
  • hypoxaemia
  • respiratory acidosis
  • increased atelectasis and V/Q mismatch
  • reduced minute ventilation
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8
Q

What is the normal alveolar C02 measure?

A

approx 40 mmHg

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9
Q

How can the positioning of the horse for anaesthesia impact?

A

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

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10
Q

Where is the greatest proportion of lung mass in the horse?

A

along the dorsum

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11
Q

What does decreased alveolar elimination of C02 cause?

A

an increase in arterial concentration (PaC02)

leads to development of acidaemia (decreased pH)

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12
Q

How is C02 usually measured?

A
  • as end-tidal
    measured by sampling of airways gases (PaC02) by capnography
  • also by analysis of blood from an arterial cannula
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13
Q

Describe the use of capnography in the horse

A

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

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14
Q

How can hypoventilation be treated?

A

first check the depth of anaesthesia- if too deep change the suppression of the anaesthetic or provide IPPV

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15
Q

What are the 5 main causes of hypoxaemia?

A
  • inadequate inspired oxygen
  • impaired diffusion across alveoli
  • hypoventilation- common under anaesthesia
  • VQ mismatch
  • shunting of blood- due to V/Q mismatch
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16
Q

When does ventilation perfusion matching occurs?

A

when the amount of blood delivered to each alveolus is appropriate to maximally exchange the oxygen and C02 within that alveolus

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17
Q

What does High V/Q mean?

A

Means that not enough blood reaches well ventilated alveoli- Pa02 is high but PaC02 is low

18
Q

What does low V/Q mismatch mean?

A

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.

19
Q

Describe hypoxic pulmonary vasoconstriction

A

Compensatory vascular response that shunts blood flow away from unventilated alveoli
Reduces low V/Q mismatch
Compensatory response is abolished by inhalant anesthetics

20
Q

What can prevent V/Q mismatch?

A

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

21
Q

What are the benefits of using IPPV?

A
  • 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
22
Q

What are the detriments to IPPV?

A

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

23
Q

How can V/Q mismatch by treated by drugs?

A

Bronchodilators- salbutamol and clenbuterol

Can affect CV system however leading to vasodilation and excessive sweating

24
Q

Define hypotension

A

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

25
Q

Describe the impact of inhaled anaesthetic agents

A

Decrease contractility (reduce stroke volume)
Decrease heart rate
Decrease systemic, vascular resistance

26
Q

What is cardiac output proportional to?

A

stroke volume X heart rate

27
Q

What is the mean arterial blood pressure proportional to?

A

Cardiac output x SVR

28
Q

What are some non-invasive methods of measuring blood pressure?

A

Cuff
Oscillometric
Used on tail or distal limb
Advantageous for ponies or foals as they are smaller so the machine is more accurate

29
Q

What are invasive methods of measuring blood pressure?

A

placement of a cannula in artery such as facial artery or metatarsal

30
Q

What are the advantages of the placement of the cannula?

A

Accuracy and beat to beat recording and analysis

Permit sampling of arterial blood for gas analysis

31
Q

What are the disadvantages of placement of a cannula?

A
  • haemorrhage
  • infection
  • damage to periosteum
32
Q

How can hypotension be treated?

A

Check depth of anaesthesia and reduce the administration to the minimum level to maintain the safety
If the animal is hypovolemic- use crystalloids, colloids or hypotonic saline
Pharmacological support: dobutamine infusion- increase contractility, ephedrine- increases contractility and SVR, phenylephrine- increased SVR

33
Q

Where are murmurs in horses commonly heard?

A

In the left heart base with a low grade

34
Q

What are the most common atrioventricular block arrhythmias in horses?

A

First and second degree
Usually low grade, high vagal tone and fitness
Persistent high grade can be used to drug induced or disease

35
Q

What can cause atrial fibrillation?

A

A large heart, ectopic electrical focus or increased automaticity (can be drug induced)

36
Q

What are normal heart rates for racehorses and small ponies?

A

racehorse= >40bpm

small pony= >60bpm

37
Q

What are potential reasons for tachycardia?

A

Hypovolaemia- low circulatory volume increases heart rate to preserve cardiac output
Hypoxaemia- causes increased heart rate as an increased cardiac output attempts to deliver more oxygen if less is in the blood
Pain and nociception- stimulate sympathetic drive and cause tachycardia
Drug induced

38
Q

At what levels of heart rate in a racehorse and a small pony under anaesthesia would you be concerned?

A

Race horse <24bpm

Small pony <30-35bpm

39
Q

What are potential reasons for bradycardia?

A

Hypertension- can be drug induced or as a result of pain and triggers the baroreceptor response to reduce heart rate
Hypoxaemia- can produce bradycardia when the myocardium itself becomes hypoxic and short of oxygen it ceases to function adequately
Drug induced- for example alpha 2 agonists

40
Q

Describe cardiac arrest of horses under anaesthesia

A

Stop administration of anaesthetic
Check patient has adequate airway
If animal is not breathing It should be ventilated with 100% oxygen at an appropriate rate and chest compressions at 40-60 per min
Atropine and adrenaline administered at 2 min cycles and possibly calcium if pulseless electrical activity is identified
ECG- check for restoration of electrical activity
After resuscitation occurs- fluids should be given to treat any volume deficits

Lectures (25 decks)

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