Evaluation of the Equine Upper Respiratory Tract

Question 1

What is the speed of horses like? What is required to reach this?

Answer 1

1200 lb horse can run ~55 mph (440 yards in 21 s)

efficient gas exchange

Question 2

Equine upper respiratory tract:

Answer 2

Question 3

What is labeled in this endoscopic view of the larynx?

Answer 3

Question 4

What is the normal resting an exercising respiration rates of horses?

Answer 4

RESTING = 8-20 bpm
- tidal volume = 5 L
- total lung capacity = 40 L
- minute ventilation = 40-100 L

EXERCISING = 80-100 bpm
- minute ventilation = ~1500 L

Question 5

How do horses breathe?

Answer 5

obligate nasal breather that exhales as forehand contacts the ground

Question 6

What limits horses’ exercise capacity?

Answer 6

URT resistance to airflow (highest at the nares!)

Question 7

What is the mechanics of airway flow like with inhalation and exhalation? What commonly increases pressure disparity?

Answer 7

INHALATION = negative pressure collapses URT and pushes air into lungs

EXHALATION = positive pressure dilates URT

Question 8

What is airway resistance primarily determined by?

Answer 8

airway diameter —> cutting the radius in half increases resistance by a factor of 16

R = (8ul)/r^4

Question 9

What are the 4 major areas of resistance in the upper airway?

Answer 9

1. external nares
2. nasal mucosa
3. pharynx - soft palate, pharyngeal walls
4. larynx - epiglottis, arytenoids

Question 10

Lower airway resistance:

Answer 10

Question 11

Airway resistance, upper vs. lower:

Answer 11

Question 12

What is the major contributor to airway stabilization? How does it maintain stability?

Answer 12

muscle tone

• nervous innervation of cranial nerves
• autonomic effects of adrenaline constricting blood vessels during exercise
  (enlarges the lumen!)

Question 13

Flexing the neck and airway resistance:

Answer 13

constricts larynx

Question 14

What is the normal inspiration to expiration ratio in horses?

Answer 14

1:1

Question 15

What muscle of the larynx should be palpated on URT evaluation?

Answer 15

cricoarytenoid dorsalis muscle

Question 16

What are some ancillary diagnostic aids for the URT?

Answer 16

• endoscopy - at rest, dynamic
• radiographs
• CT, MRI
• ultrasound
• blood work
• lower respiratory tract work up

Question 17

What is used for URT endoscopy? How is it performed?

Answer 17

1 m, 1-cm flexible endoscope

advanced through the ventral meatus (Y-shaped)

Question 18

What landmark is used for the pharynx?

Answer 18

dorsal pharyngeal recess

Question 19

Guttural pouch, endoscopic anatomy:

Answer 19

Question 20

Larynx, endoscopic anatomy:

Answer 20

Question 21

What is the preferred way to perform an URT endoscopy at rest? What is it used to diagnose?

Answer 21

no sedation —> drugs can alter arytenoid function (scope trachea and GP, too!)

static lesions

Question 22

What is the purpose of nasal occlusion during URT endoscopy?

Answer 22

creates negative pharyngeal pressure similar to exercise, which can help visualize arytenoid abduction

(abduction better correlated to swallowing)

Question 23

What is treadmill-dependent dynamic endoscopy preferred for? What are some concerns?

Answer 23

diagnosing dynamic conditions - DDSP occurs during fatigue, so horse needs to exercise for extended periods

• high intensity (220 bpm)
• safety
• doesn’t perfectly mimic real life

Question 24

What is considered the best method for performing dynamic endoscopy? How does it compare to the treadmill?

Answer 24

portable endoscope (brings scope to horse)

• more difficult to get to max intensity for racehorses
• mimics real life
• able to review in slow motion (120 breaths/min!)

Question 25

How do dynamic and resting endoscopies compare?

Answer 25

• start with resting for baseline
• dynamic to make definitive diagnosis, since many lesions are not apparent at rest —> maximum intensity, flexion, tack

Question 26

URT radiographs:

Answer 26

challenging to interpret!

Question 27

What is indicated in this radiograph?

Answer 27

fluid lines within sinuses

Question 28

What is indicated in this radiograph?

Answer 28

cyst on the epiglottis

Question 29

What is the best mode of diagnosis for the sinuses?

Answer 29

CT