Chapter 22 - Complications of inhalation anesthesia Flashcards
What effect does a decrease in body temperature have on the oxygen hemoglobin dissociation curve in anesthetized adult horses?
Shifts to left.
List two interventions for preventing hypothermia in anesthetized horses that are mentioned in the text.
Insulated padding
External heat sources
How much does the minimal alveolar concentration (MAC) decrease per degree Celsius drop in body temperature?
5%–8%
Why shivering postoperative may be observed?
Shivering during recovery
is commonly observed, and while there are likely other factors
causing this, hypothermia may contribute.
Why shivering is not desirable in the recovery?
Shivering can serve
to warm the patient but also increases oxygen consumption,
which may not be desirable.
What metabolic state, although rare, can be fatal in horses exposed to inhalation anesthetics?
Hyperthermia
What treatment is recommended for malignant hyperthermia, though it is challenging to administer?
Disconnect from machine
support with oxygen
ventilation
aactive cooling
Dantrolene sodium
What is the normal heart rate range in resting adult horses?
28–44 bpm
What heart rhythm condition is considered normal in athletic horses if it disappears during exercise?
Second-degree atrioventricular block
At what heart rate during inhalation anesthesia in adult horses is concern typically expressed?
Below 18–20 bpm
Which class of drugs potentiates bradyarrhythmias in horses during anesthesia?
α2-adrenoceptor agonists
In neonatal foals, what is primarily dependent on heart rate?
Cardiac output
Why are anticholinergics (such as atropine or hyoscine) not routinely used to treat bradycardia in horses?
Concern of ileus
What treatment should be considered if bradycardia is accompanied by hypotension and rhythm abnormalities?
anticholinergics (atropine)
Which anticholinergic has a more sustained effect on heart rate in horses, atropine or hyoscine?
Atropine
Which anticholinergic had longer-lasting effects on gastrointestinal motility, potentially causing colic?
Atropine
Which drug demonstrated improvement in cardiovascular variables when administered prior to xylazine?
Glycopyrrolate
What should be discontinued when using anticholinergics during anesthesia to avoid cardiac dysrhythmias?
Sympathomimetic drugs
Name two sympathomimetic drugs that may be used to treat bradycardia in horses.
Dobutamine
Ephedrine
Which electrolyte imbalance associated with hyperkalemic periodic paralysis warrants immediate treatment?
Hyperkalemia
What two treatments are used for hyperkalemia in horses with urinary tract obstruction or bladder rupture?
Intravenous calcium
Sodium bicarbonate
What nonspecific treatment can be administered for ventricular tachycardia during equine anesthesia?
Intravenous lidocaine
What condition may cause tachyarrhythmias in anesthetized, medically compromised horses?
Noxious stimulation
Which rhythm disturbance in anesthetized horses may compromise cardiovascular function depending on the ventricular rate?
Atrial fibrillation
What procedure may require anesthesia to treat atrial fibrillation in horses?
Electrical cardioversion
Which two electrolytes should be normalized prior to anesthesia in horses with atrial fibrillation?
- Potassium
- Magnesium
What is a common complication during inhalation anesthesia in horses?
Hypotension
What range of mean arterial blood pressure is recommended for horses during anesthesia?
70–90 mm Hg
Which drug is ideal for treating hypotension caused by decreased contractility during anesthesia?
Dobutamine
What is the recommended mean arterial blood pressure range for foals during anesthesia?
50–65 mm Hg
Which class of drugs can help improve blood pressure but may decrease cardiac output in horses?
α2-adrenergic agonists
What measurement tool provides continuous monitoring of oxygenation during anesthesia?
Pulse oximeter
What is the normal oxygen saturation range during anesthesia in horses breathing high FiO2?
98%–100%
At what saturation value is there cause for concern, indicating a PaO2 of approximately 60 mm Hg?
90%
What tool is used to measure arterial oxygen tension intermittently?
Blood gas analyzer
What PaO2 value is expected when horses are maintained with a fractional inspired oxygen (FiO2) of greater than 90% at sea level?
400–500 mm Hg
Which equation can be used to predict arterial oxygen tension?
PAO2 = (BP – H2Ovapor pressure × FiO2) – (PaCO2/RQ) (BP: barometric pressure, usually around 760 mm Hg at sea level; H2Ovapor pressure is 47 mm Hg; FiO2 is 0.21 for normal air; RQ: respiratory quotient defined as amount of CO2 exhaled/O2 inhaled in a defined period of time; the RQ is dependent on the substrate that is metabolized and is usually 0.8)
What is the RQ (respiratory quotient) usually assumed to be in the ideal alveolar partial pressure of oxygen equation?
0.8
What can cause hypoxemia in horses during anesthesia, often explained by postural influences on ventilation and perfusion?
Postural influences
What should be increased if severe hypoxemia remains unresolved during anesthesia?
Cardiac output
Name two β2-agonists that may be used as bronchodilators to improve oxygenation during anesthesia.
Albuterol
Salbutamol
What is the PaO2 in mm Hg at sea level, assuming normal lung function and FiO2 of 21%?
~80–105 mm Hg
Name two methods commonly attempted to counter arterial hypoxemia during anesthesia.
Use of positive end-expiratory pressure (PEEP)
Recruitment maneuvers
What is the main effect of inhalation anesthetics on myocardial function in horses?
Reduces contractility
Electrocardiogram recorded from a horse in atrial fibrillation.
Notes: Note the irregularly spaced QRS complexes, the absence of a P wave, and the lack of a smooth baseline. The coarse undulations that are occurring throughout the recording are F (fibrillation) waves. The green diamond indicates a QRS complex, the red square a T wave, and the gray arrow is pointing to an F wave. McGurrin 2015
Electrocardiogram recorded from a horse in atrial fibrillation.
Notes: The QRS complexes, indicated by the black triangles, are irregularly spaced. In this recording, there are no P waves, and the F waves are fine. McGurrin 2015
Fig. 7. Atrial fibrillation is characterized by absence of P waves, presence of fibrillation (f) waves, and irregular RR intervals with normal (supraventricular) QRS morphology. As always, for short RR intervals, the T wave has opposite polarity to the QRS complex (arrow). This pattern should not be misinterpreted as a ventricular ectopic beat. Gunther van Loon 2019
Fig. 18. Third-degree atrioventricular block resulted in atrioventricular dissociation and a slow (junctional) escape rhythm at a rate of 25 bpm. The atrial rate is increased to about 60 bpm, probably as a reflex to systemic hypotension. Note that the term atrioventricular dissociation is a purely descriptive term that simply describes that the P waves are not associated with the QRS complex. This can also be caused by ventricular tachycardia (as shown in Fig. 15). Gunther van Loon 2019
Difference between first and second AV blocks
First-degree AV block shows a prolonged PQ interval that has been shown to depend on body weight.
In second-degree AV block, there is intermittent failure of the atrial impulse to be conducted through the AV node toward the ventricle, resulting in a P wave without a QRS complex
Fig. 2. Second-degree atrioventricular (AV) block is characterized by intermittent failure of the atrial electrical impulse to be conducted through the AV node to the ventricles. The electrocardiogram is characterized by regularly occurring P waves, of which some are not followed by a QRS-T complex. This usually results in a regular irregularity on auscultation, for example, a pause occurring after every third heartbeat.
What happens to pH when carbon dioxide increases by 20 mm Hg?
Reduces by 0.1 unit
At what carbon dioxide tension do sedative and anesthetic effects increase significantly?
90 mm Hg
What is the normal arterial carbon dioxide range for unsedated, calm, air-breathing horses at sea level?
45–50 mm Hg
What arterial carbon dioxide values are typically reached with modern inhalation anesthetics like isoflurane in horses?
65–75 mm Hg
What are the common contributing factors to compromised ventilation in anesthetized horses? (3)
- Lack of preanesthetic fasting
- Abdominal distention
- Recumbency
What level of permissive hypercapnia is considered acceptable in healthy horses during anesthesia?
55–60 mm Hg
Why is permissive hypercapnia used in anesthetized horses?
To maintain pH and minimize cardiovascular effects
What is the tidal volume range per breath for anesthetized horses?
10–15 mL/kg
What is the recommended respiratory rate for anesthetized horses?
4–8 breaths per minute
What peak inspiratory pressure is considered appropriate for adult horses during anesthesia?
20–30 cm H2O
Which electrolytes are crucial to monitor during anesthesia due to their impact on hypotension and heart rhythm (4)?
- Sodium
- Potassium
- Calcium
- Magnesium
How do pH changes influence ionized calcium and potassium levels?
Falsely elevated
What is the maximum safe administration rate of potassium IV in horses?
0.5 mEq/kg/h
What is the conservative IV dose of calcium gluconate for treating low ionized calcium in horses?
30 mg/kg
What is the conservative IV dose of calcium chloride for treating low ionized calcium in horses?
10mg/kg in 20-30 min
What condition may occur following nasal congestion or upper airway obstruction during equine recovery from anesthesia? treatment?
Pulmonary edema
furosemide (0.5–1 mg/kg IV) may be administered to help manage this.
What drug is commonly used to reduce nasal congestion during recovery from long anesthesia?
Phenylephrine
Which horses are more prone to upper airway obstruction following extubation? Name 3
- Draft breeds
- Horses with HYPP
- Post-surgical horses (e.g., sinus surgery)
What is the immediate treatment required if a horse experiences upper airway obstruction post-extubation? 3 points
Keep horse calm
Secure airway
Administer oxygen
What is a potential complication of extreme carbon dioxide tension in anesthetized horses?
Increased intracranial pressure
What genetically or nutritionally based conditions may contribute to delayed recovery or poor standing post-anesthesia in horses?
- HYPP (Quarter Horses)
- Muscle/glycogen storage diseases (Draft breeds)
- Vitamin E and selenium deficiencies
In dorsal recumbency, what position should the horse’s limbs be kept in?
Neutral flexed position, with any extension kept to the shortest possible duration
In lateral recumbency, how should the horse’s lower forelimb and hind limb be positioned?
Lower forelimb should be pulled forward, and lower hind limb should be placed in a neutral (standing-like) position
Why is preanesthesia fasting controversial in horses?
It balances the benefit of reduced gastrointestinal volume and improved ventilation/oxygenation with the disadvantage of possibly negatively influencing gastrointestinal motility
Which drugs most notably reduce gastrointestinal motility in horses during sedation and anesthesia? (name 2)
- Opioids
- α2-adrenergic agonists
What complication is common in foals that are fasted or medically compromised during anesthesia?
Hypoglycemia
How should hypoglycemia be treated in foals during anesthesia?
Administer 1% to 5% dextrose in a balanced electrolyte solution, depending on the fluid administration rate
What effect do α2-adrenoceptor agonist drugs have on blood glucose levels in horses?
They can cause hyperglycemia
What effect do xylazine and detomidine have on urine production in standing horses?
They increase urine production multiple-fold over normal rates
Why should an anesthetist be aware of hyperglycemia induced by α2-adrenoceptor agonists in horses?
Though no adverse consequences are documented in horses, in other species, hyperglycemia can result in diuresis
How does general anesthesia affect urine production in horses?
It reduces urine production but remains above normal rates even in water-deprived animals
Why is catheterization of the urinary bladder recommended during longer surgical procedures in horses?
To minimize bladder distention and its stimulus on equine recovery
What complications may arise from increased diuresis during anesthesia?
Dehydration and the potential need for adjusting intravenous fluid administration rates
What is the normal urine production rate for horses?How does general anesthesia affect urine production in horses?
Approximately 0.5 mL/kg/hour
What are the two main physiological systems influenced by preanesthetic fasting in horses?
Gastrointestinal system
Respiratory system (ventilation and oxygenation)
What drugs are associated with increased urine production during the peri-anesthetic period in horses?
Xylazine
Detomidine
What signs should be monitored for to detect impaired gastrointestinal motility during the recovery period?
Abnormal behavior and decreased fecal production
How can diuresis during anesthesia contribute to complications, and what should be considered?
Diuresis can contribute to dehydration, so IV fluid administration rates may need adjustment
