The respiratory system in the equine athlete Flashcards
List the factors that contribute to successful performance in the equine athlete
Biomechanics
Anaerobic capacity
Haemoglobin conc
Skeletal muscle properties
Gas exchange
Heart rate
Describe the pathway of oxygen in the equine athelete
- Inspired O2 in air
- Upper Respiratory Tract: nares, nasal passages, larynx, trachea
- Lower Respiratory Tract: intrathoracic trachea, bronchial tree, alveoli
- Cardiovascular System: pulmonary capillary, haemoglobin, heart (CO), tissue perfusion
- Tissue mitochondria
Describe energy production for exercise in horses
Depends on the availability of oxygen (substrate)
Aerobic - clean efficient, slow
Anaerobic - generates lactate, rapid, simple but much less efficient than aerobic energy supply
Describe the adapted kinetics of oxygen in an equine athlete
- 20 seconds in horse vs 2 minutes in human athlete to 75% VO2max
- Improves with training
- Less O2 debt (i.e. back to sustaining high speeds)
Why is the pathway of oxygen important to consider in the equine athlete?
Problem anywhere along the pathway will affect O2 (and CO2) delivery & gas exchange
What is VO2 max?
- Maximal aerobic metabolic rate
- Measurable
- Closely related to (endurance) performance ability
What is the primary function of the respiratory system?
Gas exchange
List the secondary functions of the respiratory system
- Humidification, filtering & warming of air
- Thermoregulation
- Phonation & olfaction
- Acid-base regulation
- Blood filtering & pulmonary defence mechanisms
What is ventilation?
How air gets into alveoli
What is dead space?
Ventilation that doesn’t contribute to gas exchange
Minute ventilation = … X ….
Tidal volume x breaths/min
What is perfusion?
How gas is removed from the lungs by blood
What is pulmonary perfusion?
Low resistance high pressure esp. horses during exercise (100mmHg)
What is shunting perfusion?
Perfusion of poorly ventilated alveoli
Describe the ventilation perfusion ratio
How matching of air and blood in the lung influences gas exchange
Describe diffusion and what it is affected by
- Passive exchange of O2 and CO2
- CO2 is 25 times more diffusible than O2
- Diffusion depends on pressure gradient, transit time
- Rate also depends on thickness of alveolar-capillary barrier
How are oxygen and carbon dioxide gases transported?
- Oxygen is partly dissolved but mostly bound to Hb: oxyhaemoglobin dissociation curve
- CO2 mostly transported as HCO3-
Changes in pleural pressure and determined by?
Change in lung volume
Lung compliance
Airflow
Respiratory resistance
Volume acceleration and inertance
Where is 80-90% of resistance to airflow located?
Upper airways
Why is resistance to airflow so important in horses?
They are obligate nasal breathers
Describe the respiratory functions during exercise
- Increased ventilation = increased tidal volume, frequency, decreased dead space
- Increased perfusion = cardiac output
- Increased diffusion: gradient, blood flow
- Increase Hb conc: oxygen carrying capacity
- Increased diffusions at tissues
Rate of oxygen dissociation at tissues is increase by?
Shifting oxyhaemoglobin curve to the right with hypercapnia, acidosis and hyperthermia
List the factors that cause decreased pulmonary gas exchange in horses
- Increased pulmonary resistance
- Decreased alveolar/pulmonary compliance
- Dynamic airway collapse
- Respiratory muscle/chest wall disease
- Decreased Cardiac Output
- Decreased Haemoglobin
Describe exercise induced hypoxaemia in horses
- normal during maximal performance in athletic breeds
- 75% due to diffusion limitation, 25% V/Q mismatch, minimal shunting
- Despite increased haemoglobin, increased gradient for diffusion
- Horse extremely high pulmonary vascular pressures and rel. thick diffusion barrier
What are the causes of increased pulmonary resistance in horses?
- Upper respiratory tract disorders
- Resistance, turbulence
- Small airways - hypersecretion, blood, inflammation
What are the causes of decreased alveolar/pulmonary compliance?
Oedema
Hypertension
Fibrosis
Interstitial disease
What are the two causes of dynamic airway collapse in horses?
Inflammatory airway disease
Tracheal collapse
EIPH stands for?
Exercise induced pulmonary haemorrhage
What is Exercise induced pulmonary haemorrhage?
It is haemorrhage into the airways that occurs in horses that are exercising at high intensity in a wide range of sports
Blood present in the airways after exercise
How is exercise induced pulmonary haemorrhage diagnosed?
Post exercise endoscopy
Bronchoalveolar lavage
Describe the lesions in the lungs seen with EIPH
- Blue discolouration of the lungs is a result of the accumulation of haemosiderin (an iron pigment) within the damaged areas of lung
- The lesions start caudally and progress craniodorsally
- Histologically the damaged lung has peribronchial inflammation and fibrosis
Compare epistaxis to EIPH
- The prevalence of epistaxis is only approx.1% while EIPH is >90%.
- Epistaxis results from a far greater amount of bleeding than EIPH.
- Epistaxis may have a different aetiology than EIPH
- Consensus that it is not merely a more severe form of EIPH
Epistaxis has been seen to correlate with which conditions?
- Pulmonary abscesses and rupture of large vessels
- Atrial Fibrillation producing high left atrial pressures
How is endoscopy performed to diagnose EIPH?
- Endoscopy is used to visualise the tracheal blood.
- Endoscopy is performed 30-60 minutes after exercise.
- The haemorrhage is graded on a scale of 0 = no blood to 4
Describe grades 1 - 4 of EIPH
Grade 1 - flecks of blood or a single short stream of blood extending less than a quarter of the tracheal length. Flecks of blood in the trachea
Grade 2 - one continuous stream of blood extending at least one half the length of the trachea or multiple streams of blood covering less than one third of the tracheal surface. More than flecks but less than a continuous stream
Grade 3 - multiple streams of blood covering more than one third of the tracheal surface. Continuous stream less than half the trachea width
Grade 4 - abundant blood in the trachea, completely covering the tracheal surface and pooling at the thoracic inlet. Continuous stream greater than half the tracheal width
Describe the use of tracheal wash and bronchiolar lavage in EIPH
Can be used to detect free erythrocytes and haemosiderin in macrophages (‘haemosiderophages’ – orangey colour) for weeks to months following an episode of EIPH
Describe the pathophysiology of EIPH
- EIPH is a clinical sign or disorder with many causes
- High pulmonary vascular pressures appear to be necessary to predispose a horse to EIPH
Some of the events that cause EIPH may include:
- Extreme vascular pressures
- High inspiratory pressures
- Inflammation
- Locomotory shockwaves
- Regional differences in dynamic compliance
How does age influence the prevalence on EIPH?
It is a progressive lesion that increases in prevalence and severity as the horse gets older and continues to race.
Flat racing horses in UK
- 2yo – 40%
- 3yo – 65%
- 4yo – 82%
How does the grade of EIPH affect performance?
Grade 1 and 2 associated with normal performance.
Grade 3&4 associated with poor performance
Describe the progression of inflammation seen in EIPH
- Blood within the airways is removed quite slowly.
- By 3 days airway inflammation develops and lasts weeks
- Initially this response is neutrophil dominated, but then a more chronic and persistent phase characterized by increased macrophage numbers and marked macrophage activation and erythrophagocytosis.
How do the inflammatory findings influence management of a horse following EIPH?
Suggests that horses require a period of rest to remove haemorrhage and that they may benefit from anti- inflammatory therapy during that time.
How is EIPH linked to fibrosis in the lungs
- Intrapulmonary blood provokes macrophage influx and activation.
- The influx of inflammatory cells results in reversible disruption of the alveolar septal architecture.
- The chronic macrophage activity coincides with the development of alveolar septal wall thickening and fibrosis.
What are the consequences of alveoli fibrosis?
Permanent alterations to the alveolar blood-air barrier and reduced local pulmonary compliance
How is EIPH treated?
- No single therapy has been found to stop EIPH
- Small amounts of EIPH (Grade 1 or 2) may be physiologically inevitable in racing horses and may not need treatment
- Future goal is not necessarily to stop EIPH but to reduce severity and thereby removing the performance limiting effect of EIPH
Name and describe the action of the only medication found to effect EIPH
Furosemide
- good evidence that it decreases severity and incidence of EIPH
- potent bronchodilator
- drops pulmonary blood pressure via hypovolaemia
- banned in UK
Why is furosemide banned in the UK?
Makes horses run faster – altering power to weight ratio Alkalinizes blood and alters drug excretion rates
