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Clinical Assessment of the Respiratory System in Horses

I. History

A. Age: certain diseases are confined to age ranges
1. Pneumonias common in foals
2. COPD of horse

B. Species and breed
1. CID: Arabians only

C. Past medical history

D. Complaint
1. Cough, nasal discharge and increase RR most common complaint
2. Duration of signs, getting better or worse?
3. Any previous therapy? Response to therapy?

E. Environment
1. Extremely important! Need a working knowledge of environmental influences detrimental to the respiratory system.

F. Exercise tolerance - especially horses.

G. General health: chronic respiratory diseases may present with a primary complaint of weight and condition loss

II. General examination

  • A. Observe breathing: characterize effort, pattern
  • B. Movement of nares
  • C. View each side of the animal to observe thoracic and abdominal components of the breathing effort
  • D. Palpate lymph nodes
  • E. Feel air flow from each nostril
  • F. Smell -- foul odor?
  • G. Complete physical examination

    III. Examination of the Upper Respiratory Tract

    A. Anatomy: All structures rostral to the caudal border of the larynx
    1. Nares and nasal passages
    2. Turbinates
    3. Paranasal sinuses
    4. Guttural pouches (equine)
    5. Larynx

    B. Client complaint

    1. Cough: related to exercise, eating or spontaneous?

    a. Coughing when eating suggests pharyngeal disease
    b. Coughing with mild laryngeal manipulation suggests URD [ Upper Respiratory Disease ]
    c. Coughing associated with thoracic percussion suggests LRD [ Lower Respiratory Disease ]
    d. Character of Cough :
  • 1. Soft, short, hacking, deep
  • 2. Productive or nonproductive (moist or dry)
  • 3. Painful or nonpainful

    2. Nasal discharge

    a. Unilateral - Upper Respiratory Disease [see picture below ]
    1. Sinusitis
    2. Guttural pouch
    3. Nasal mass
    4. Progressive hematoma

    b. Bilateral: either URD and/or LRD

    c. Serous discharge [ see picture ]
    1. Cold weather, allergic or irritant rhinitis, viral infection.
    2. Mild rhinitis

    Mostly equine URD

    d. Mucopurulent: often suggests bacterial infection [ see picture below]
    1. Increased exudate when lowering of the head suggests guttural pouch infection in the horse

    e. Odor
    1. Necrotic, fetid smell of unilateral nasal discharge suggests neoplasia, sinusitis, foreign body, nasal granuloma.
    2. Fetid bilateral nasal discharge suggests the same as above and aspiration pneumonia, necrotizing pneumonia, pharyngeal abscesses.

    f. Epistaxis [ see picture below]
    1. Red-blood (epistaxis: def. nosebleed or hemorrhage from the nose). Free blood at nostrils may occur which has originated deep in the lung. It does not have to be frothy to originate from the lung because of horizontal position of trachea.

    2. Examples:
    a. Guttural pouch mycosis [horses]
    b. Ethmoid hematoma [horses]
    c. Tumor, granuloma [all species]
    d. Necrotic pneumonia (lung abscesses) [all species]
    e. Exercise induced pulmonary hemorrhage (bleeders) [horses}
    f. Trauma [all species]
    g. Green tinged: Feed materials mixed in: choke, pharyngeal paralysis, vomition in the ruminant.

    Respiratory noise

    a. UR obstruction - inspiratory stertor as increased negative pressure results in the collapse of soft tissues and a decreased diameter of the airway.
    b. Nasal cavity obstruction - both inspiratory and expiratory noise
    c. Horse limited to nasal breathing because of long soft palate
    d. Greatest amount of resistance to breathing in URD

    Exercise intolerance

    Weight loss does not commonly occur in URD, but does with LRD

    C. Physical exam

    1. Observation

    a. Respiratory rate at rest
    i. Age dependent
    ii. Environmental conditions: increased temperature and humidity results in increased respiratory rate.
    iii. Younger animals have higher respiratory rates.

    Best to count for one minute by observation of the resting animal rather than by auscultation, which may cause excitement
    Rate/min at rest, cool environment: 10-14

    b. Flaring of nostrils

    c. Dyspnea or difficult breathing
    i. Inspiratory: URD, alveolar disease, pulmonary edema, or internal respiratory disorder
    ii. Expiratory: small airway disease
    iii. Mixed: pneumonia, severe airway obstruction

    d. Noise, cough, nasal discharge

    2. Palpation

    a. Airflow through nostrils (Obstructed airflow suggests a nasal mass or nasal septal disorder )
    b. Lymph nodes: intermandibular and retropharyngeal enlargement suggests URD
    c. Larynx

    i. Elicit cough

    ii. Horse: squeeze to decrease luminal diameter in space between caudal border of larynx and first tracheal ring

    3. Percussion

    a. Symmetry of sound and shape of paranasal sinuses

    4. Auscultation

    a. UR sounds may be referred into the chest cavity. Careful auscultation all areas to determine the region of maximum intensity.
    b. Stertorous sounds characteristic of URD.
    NOTE:Anytime URD is determined, a detailed examination of the LRT is always indicated. Always do a complete physical to determine if there is a polysystemic disease.

    5. Ancillary tests: Upper Respiratory System

    a. Endoscopic exam
    b. Radiography
    c. Biopsy
    d. Catherization guttural pouch
    e. Culture
    f. Viral isolation
    g. Serology
    h. Pass stomach tube

    IV. Definitions relating to respiration

    A. Dyspnea:
    difficult or labored breathing. Any subjectively assessed difficulty in respiration causing apparent distress to the animal. A normal animal should recover in 5-10 minutes following exercise.

  • 1. Three types of dyspnea
    a. Inspiratory dyspnea - impaired entry of air to lung or transfer of 02 to RBC inhibited. Most widely caused by upper respiratory disease or alveolar disease.
    Typical attitude - head and neck extended, open mouth breathing, outward rotation of elbows.

    b. Expiratory dyspnea - escape of air from the lung is impeded. Most likely caused by airway disease within the thorax. Marked abdominal movements result.

    1. Horses: heave line (hypertrophy of the external abdominal oblique muscle), "Pumping anus".

  • 2. Pigs: "thumps"

    c. Mixed dyspnea most common

    2. Determine which type of dyspnea is present by assessing the time it takes for inspiration and expiration to occur. Ratio inspiration: expiration = 0.7

    3. Diseases where dyspnea may occur:
    a. Stenosis of air passages
    b. Bronchopneumonia
    c. Pulmonary edema
    d. Ruptured diaphragm
    e. Pleuritis

    B. Eupnea:
    easy or normal respiration

    C. Hyperpnea:
    abnormal increase in rate and depth of the respiratory movements.

    D. Tachypnea:
    the rate of respiration is increased

    1. Examples where one may have tachypnea:
    a. Exercise
    b. Excitement, fear
    c. High environmental temperature
    d. Obesity

    e. Disease states:
    pulmonary, cardiac, painful conditions, anemia, nitrate or cyanide poisoning

    VI. Type or character of respiration

    A. Abdominal breathing:
    Characterized by a visible movement of the abdomen with respirations, the abdomen is protruded during inspiration and recoils with expiration.
    1. Normal cow, sheep, goat

    B. Costal breathing:
    characterized by rib movements during respiration. If thoracic movement occurs, without abdominal movement, the action of the diaphragm is impaired, eg, ruptured diaphragm; accumulation of gas or fluid in an abdominal viscus or peritoneal cavity.

    C. Costoabdominal breathing:
    equal movement of the abdominal and thoracic wall during respiration. During normal breathing the diaphragm is the principle respiratory muscle, therefore, abdominal breathing predominates with most animals at rest. Depth describes the amount of movement of the ribs and abdomen. It is described as shallow, normal or deep.
    1. Normal in the horse

    D. Respiratory rhythm

    1. Phases of the respiratory cycle
  • a. Inspiration: active movement of the diaphragm, costal, and/or abdominal muscles.
  • b. Expiration: Passive, longer than inspiration at rest. When expiration becomes excessively labored or longer than inspiration at increased respiratory rate, escape of air from lungs impeded. Ex. COPD.
  • c. Pause: variable, longer at rest, will shorten when the rate is increased.

    2. Abnormal respiratory rhythms: think polysystemic disease.
  • a. Kussmaul's breathing: Regular pattern with an increased rate and large tidal volumes performed without a great deal of effort. Ex: exercise and metabolic acidosis.
  • b. Biot's respiration: Irregular periods or apnea alternating with 4 or 5 breaths of equal depth: seen in animals with increased intracranial pressure, ie meningitis, disease of medulla, septic cows.
  • c. Cheynes -Stokes respiration: Regular periods of apnea alternating with periods of respiration in which the depth gradually increases and then decreases. Seen in advanced renal and cardiac diseas e, toxemia and meningitis. May occur to a mild degree in normal horses.
  • d. Syncoptic respiration: Periods of apnea followed by deep breathing then gradually becomes more shallow until apnea occurs.

    Biot's, Cheyne-Stokes and Syncoptic respiration occur when the regulatory centers within the medulla have become less sensitive to CO2 tension in the blood and are associated with a grave prognosis. If one of these 3 rhythms are present, suspect neurologic involvement.

    VII. Auscultation of the thorax and trachea:

    A. Definition auscultation: Examination by listening to sounds within the body. Direct or immediate auscultation is performed by placing the ear against the part of the body to be examined and is rarely used. Mediate auscultation is performed with the aid of an instrument (stethoscope) interposed between the ear and the part being examined. Three important factors are essential to make this form of examination valuable.

    1. Practice in listening and careful analysis of all sounds heard.
    2. Examination in quiet surroundings and care to prevent extraneous sounds such as friction from the diaphragm of the stethoscope on the hair from distorting sounds.
    3. Adequate time to listen.
    The entire thorax on both sides and the trachea should be ausculted.

    B. Normal respiratory sounds are dependent on the velocity of air flow and diameter and elasticity of the airway. These sounds are heard during inspiration in most healthy animals and are generated from the turbulence of air within small airways. It may be necessary to increase the depth of respiration in order that these sounds may be heard over the entire lung field in some normal animals by exercise, the use of a rebreathing bag, or holding off nostrils for a period of time.

    Normal breath sounds are softer in the horse than in the cow. The presence of these sounds are equated with the normal passage of air through small airways and into alveoli. Increased breath sounds are heard following exercise or in compensating areas of the lung. Breath sounds are normally louder in the ventral region due to increased perfusion and resulting increased ventilation. Breath sounds are louder during the active phase (inspiration).

    Bronchial sounds are sounds heard over the trachea and large bronchi. These sounds are louder and harsher than breath sounds.

    C. Abnormal respiratory noises

    1. Stertorous (upper airway sounds): loudest on inspiration, may be heard without stethoscope. Referred down the bronchi.
    a. Sneezing, due to nasal irritation
    b. Snoring and honking, caused by pharyngeal occlusion or tracheal occlusion. Ex: Congenital tracheal collapse.
    c. Roaring, due to paralysis of intrinsic muscles of larynx
    d. Grunting, forced expiration against a closed glottis. Associated with painful conditions.
    e. Nasal stertor (sniffling). Obliteration of nasal passage. The horse cannot breath through the mouth. Problems of the equine sinuses and turbinates cause severe respiratory distress. The horse normally has very little resistance in nasal passage compared to man.

    2. Abnormal lower respiratory sounds

    a. Musical wheezes are produced when there is shortening or narrowing of the airways with an increased resistance to airflow, especially when velocity is increased. Wheezes are common in disease primarily affecting the small conducting airways. Deep breathing and/or coughing usually exacerbate wheezes.
    b. Air moving in large airways with large amounts of exudate may produce bubbling sounds during inspiration.
    c. Crackles are nonmusical explosive sounds similar to crinkling cellophane. They usually result from a sudden forceful opening of a small airway and therefore are heard toward end inspiration. They are not constant and may be silenced or changed in location or intensity following coughing. Most often these sounds originate from intraluminal exudate although they may be heard in emphysema or other elastic changes in the lung.
    d. Pleural friction "rubs". These sounds are generated by the sudden movement of inflamed "roughened" areas of the pleural layers due to fibrinous exudate. They are generally in a small area and are heard during inspiration and expiration and are constant in location.
    e. Pleural effusion dampens breath sounds and only loud large airway sounds may be heard in the region of effusion. A sharp line of demarcation between the ability to hear breath sounds and bronchial sounds occurs.
    f. Factors affecting respiratory sounds:
  • 1. Referred GI sounds
  • 2. Stethoscope movement or dry, coarse hair
  • 3. Subcutaneous emphysema
  • 4. Muscle fasiculations
  • 5. Obesity

    g. It may be necessary to increase the depth of respiration or accentuate normal and abnormal lung sounds.

    1. Rebreathing bag: use of a large plastic bag to force inhalation of CO2 and trigger greater depth of respiration. The bag should be large enough to hold the vital capacity of the animal (eg., tall kitchen garbage sack is the right size for the 1000 lb horse). [ see picture below ]
    2. Holding off the nostrils may cause several deep breaths upon release.

    h. Silence. Only rarely are consolidated lungs silent, instead harsh bronchial sounds are more common. More commonly silence suggests pleural effusion, masses, pulmonary edema, or pneumothorax. Percussion of the thorax and auscultation of the heart are useful.

    VIII. Percussion of the chest

    A. Method: This technique relies upon gentle tapping to resonate the chest wall and lungs. Thumping too heavily resonates GI viscera and invalidates the results.
    1. Finger method: Middle finger is run down intercostal spaces while beating upon it with tips of the other fingers.
    2. Pleximeter: same as above, only a plexor and pleximeter are used.

    B. Boundaries of the lungs [ see picture below]
    1. Horse - 6th rib (at the olecranon)
    a. Ventral border on right is at olecranon
    b. On left is 4 cm above olecranon (above heart)
    c. Middle of eleventh rib
    d. Sixteenth intercostal space at epaxial muscle
    e. Position may vary with visceral pressure on diaphragm

    C. Abnormal percussion
    1. Hyperresonance (drum beat, expanded border): emphysema or collapsed lung. Expanded border often found with chronic obstructive pulmonary disease in the equine and is indicative of increased functional residual capacity.
    2. Decreased pulmonary areas (solidification) nearly always occurs anteroventral. This is a common finding in all species. Anteroventral dullness usually is caused by:

  • a. Consolidation from bronchopneumonia, not always easy to evaluate.
  • b. Fluid accumulation from pleuritis (usually can elicit pain with percussion).
  • c. Pericardial effusion
  • d. Thoracic tumors - thymic lymphosarcoma (rare)

    3. Pleurodynia is pain within the pleura or intercostal muscles and may be marked in diseases affecting the pleura.

    IX. Cough: A nonspecific sign of respiratory disease in large animals.

    A cough may be induced by inhaled irritants such as lime, hay dust, and noxious gases. Any disease that irritates the trachea, bronchioles or larynx also will produce a cough. In these cases, the cough arises spontaneously and is repetitive despite environmental influences.

    A. Inducible cough:
    Always squeeze the trachea when examining an animal. If a cough can be easily induced in an adult, the animal has respiratory irritation. In small ruminants, calves and foals over zealous compression of the trachea may induce coughing in normal animals.

    B. Spontaneous repeating cough:
    The repetition suggests respiratory disease. In calf pneumonias, these often are the only physical signs of respiratory disease.

    C. Significant characteristics of coughs
  • 1. Moist (productive). Indicates the animal has inflammatory lung disease producing large amounts of mucus and/or exudate. Important in clearing lungs and should not be suppressed. May be recognized by swallowing after cough.
  • 2. Dry (nonproductive). Small amounts of thick mucus or no mucus is being produced. The very large interpleural pressures producing in such coughing may lead to lung damage (emphysema).
  • 3. Soft. Suggests that the animal has a painful disease. It usually means that there is inflammation in the visceral and parietal pleura. When you hear a soft cough, attempt to evaluate the pain response over the ribs by pressing the rib cage with palm. Pleural pain also causes a splinted thorax and "abdominal" breathing. A horse with pleuritis will sometimes paw after coughing.

    D. Cough mechanisms
    1. Receptor end organs are located in the respiratory mucosa.
    2. Impulses are transmitted through the vagus nerve to the nucleus ambiguus.
    3. Reflexes between motor center of cranial nerves 9, 10, and 11 nerves to the respiratory and upper motor neuron centers occur. Efferents go to the diaphragm, intercostals and the larynx.
    4. The cough consists of two phases:

    a. Closure of the glottis and sudden expulsion of the air (velocity can exceed 100 mph).
    1. Intense collapse of lungs and a "milking" effect on brochiolar exudate occurs.
    2. If bronchioles are obstructed during a cough, the intense pressure may rupture distended alveoli.

    5. Rules:
    a. Not all coughs are bad. If the cough is productive, provide postural drainage and induce coughing.
    b. If coughs are nonproductive, frequent and severe, suppression with medication may be indicated.
    c. Suppress coughs in animals with pulmonary obstruction.
    d. Cough does not mean pneumonia. In the horse, the most frequent cause of cough is upper respiratory disease.

    X. Further diagnostic aids in evaluating respiratory diseases

    A. Endoscopic examination
    1. Allows direct visualization of the nasal passages, ethmoid turbinates, nasopharynx, guttural pouches, laryngopharynx, larynx, and tracheobronchial tree.
    2. Can be performed either during (if horse on a treadmill) or after exercise to evaluate functional disorders.

    B. Transtracheal aspiration:
    One of the most useful diagnostic aids in evaluating LRD, particularly when looking for an infectious etiologic agent.

    1. Technique

    a. Adult horse
    1. Tubing should be as large as possible and still fit within the lumen of a 10 gauge trocar as thick exudates will not be obtained if the tubing is too small. Relatively stiff tubing will prevent coughing of the canula into the pharynx.
    2. Up to 60-90 cc saline can be administered.

    b. Foals
    1. Adequate restraint or sedation necessary to prevent the foal from rearing when the catheter reaches tracheal bifurcation.
    2. Up to 30 cc saline can be administered.

    Transtracheal Aspirate in the Adult Horse

    1. Bleeding trochar
    2. 280 polyethylene tubing (100 cm)
    3. 14 gauge needle
    4. 30-60 cc. Lactated ringers or physiologic saline
    5. Aspirate as catheter is pulled out

    Transtracheal Aspirates in the Foal

    1. Sovereign Indwelling Catheter - Sherwood Medical Industries
    2. 22" 5 French Sovereign polypropylane catheter
    3. Sedation

    2. Sample handling
    a. Make glass slides at the time procedure performed. Smear material between two slides and air dry. Or preserve cells in EDTA tube. If long delay before slide preparation best to air dry slides.
    b. Cap syringe with clean needle. Refrigerate until delivered to microbiology lab for culture and sensitivity.

    3. Cytology: Wright's stain: gram stain
    Normal cytology from TTW in a horse. Not ciliated columnar epithelial cells, mucus and nondegenerate neutrophils. Wright's stain. [ see picture ]

    TTW from a horse with S. zooepidemicus bronchopneumonia. Gram stain.
    [ see picture below]

    TTW cytology from a horse with COPD. Note the eosinophil, macrophages, nondegenerate neutrophils and mucus. Wright's stain. [ see picture ]

    a. Normal
    1. Columnar ciliated epithelial cells
    2. Few neutrophils, phagocytic cells
    3. Scant mucus
    4. Few pulmonary macrophages

    b. Bacterial diseases
    1. At least 50% cells neutrophils, many degenerate
    2. Bacteria, should be within some phagocytes
    3. Mucus abundant

    c. COPD (equine)
    1. Heavy mucus background
    2. Nondegenerate neutrophils, no bacteria
    3. ± eosinophils

    d. Intrapulmonary hemorrhage (Exercise induce pulmonary hemorrhage)
    1. Macrophages with intracytoplasmic hemosiderin (green inclusions) on Wright's stain.

    e. Parasitic bronchitis: to demonstrate larvae and/or eggs, centrifuge and make slides of sediment.

    f. Keratinized squamous epithelial cells: pharyngeal wash

    4. Cultures
    a. Correlate culture results with clinical signs. Growth of organisms normally found in the pharynx with decreased pulmonary defense mechanisms from any cause may not be significant.
    b. Large numbers of a single organism, especially if from more than one animal in a herd outbreak, indicative of pneumonic process.
    c. Pathogens, eg, S. equi horse
    d. Secondary invaders may be cultured in chronic pneumonias.
    e. If the animal was on previous antibiotic treatment, may not get any growth regardless of efficacy of therapy.
    f. Culture results improve with a decrease in the time of sampling to processing for culture.

    C. Bronchoalveolar Levage (BAL)
    1. Purpose: to collect cells from the distal airways and alveoli of a portion of the lung. Good to evaluate diffusely distributed small airway disease.
    2. Method: either a fiberoptic endoscope or a double lumen tube is wedged into a 4th or more distal bronchus. Aliquots (usually 100 ml) of sterile PSS are infused into the lung segment and aspirated by suction pump.
    3. Aliquots of recovered fluid are centrifuged and cytological evaluation performed on sediment smears. Total nucleated cell counts may also be done.
    4. Because only a small portion of the lung is sampled the disease process must be diffuse. Selective BAL of a portion of the lung is often not possible even with an endoscope. When determining the presence of microorganisms a properly performed TTW is indicated.

    D. Thoracentesis
    1. Method
    a. Seventh intercostal space on the left and 6th intercostal space on the right, just above the lateral thoracic vein.
    b. A bitch catheter or metal teat cannula with a 3-way stopcock to prevent air from entering the pleural space are the instruments of choice.
    c. Administer local anesthetic over the rib. Make a stab skin incision. Slide the skin over the intercostal space rostrally and insert the cannula on the cranial border of the rib through the skin incision and into the pleural space. Avoid the artery on the caudal border of the rib. Thus the skin incision will not be over puncture through pleura when procedure is terminated.

    2. Sample handling
    a. Cytology and protein analysis
  • 1. Transudate: protein <3.0 gm/dl or sp. gravity <1.016; ¾ 3,000 WBC/mm3
  • 2. Exudate: protein „ 3.0 gm/dl; 10,000 WBC/mm3
    b. Gram stain and culture, aerobic and anaerobic
    c. Interpretation

    1. Transudate, nonseptic occurs when the systemic factors of pleural fluid formation and absorption are altered.
    a. Neoplasia
    b. Purpura hemorrhagica
    c. EIA
    d. Herpes Virus I
    e. Heart failure, hypoproteinemia (rare)
    f. Idiopathic

    2. Chronic transudates may develop characteristics of exudate, eg protein, WBC increased, but not sepsis.

    3. Septic exudate. High protein > 3.0 gm/dl > 3,000 WBC/mm3 with degeneration and bacteria. Exudate results from disease of the pleural surface.
    a. Parapneumonia process most common cause of pleural effusion in the horse.
    b. Penetrating thoracic wound
    c. Pulmonary emboli
    d. Diaphragmatic hernia and bowel disease (rare)

    E. Sinuscentesis

    1. Method
    a. Local anesthesia of skin and periosteum over sinus
    b. Stab incision with scalpel blade
    c. Steinman pin and hand chuck to enter sinus

    2. Sample handing - same as above.

    F. Radiography

    1. Often not possible without special equipment, however, foals should be able to be radiographed with portable machines using rare earth screens. Portable machines adequate for radiographs of the UR structures of the head and neck.
    2. Radiographs of the chest are helpful in determining extent of disease, prognosis and evaluation of therapy in:
  • a. Pneumonia
  • b. Abscesses or intrathoracic masses
  • c. Pleural effusion

    3. Radiographs are of little benefit in horses with COPD.

    G. Ultrasound

    1. Useful for evaluation of the pleura and pleural space and for masses adjacent to the pleura. Because ultrasound does not penetrate air of little value for lesions deep in lung parenchyma

    H. Arterial blood gas determinations

    The measurement of arterial blood gas samples are extremely helpful in evaluating the severity of respiratory disease as it is a measure of the ability of the lung to accomplish gas exchange. Should the blood gas levels be abnormal then one must ask why. Defining the area of functional abnormality allows one to properly direct therapy and monitor disease progress.

    1. Collection
    a. Site: facial artery, submandibular artery, carotid artery, greater metatarsal artery (horse), tail artery (cow), ear artery (cow)
    b. 20-22 gauge needle with a heparinized small syringe (fill dead space with heparin). Cork sample after drawing.
    c. Store in ice water up to 3 hours after collection
    d. Record body temperature at collection time

    2. Normal
    a. Breathing room air at rest
    Pa0285-100 mmHg
    PaC0234-35 mmHg

    3. Decreased PaC02
    a. Excitation - Increased respiratory rate
    b. Increased ventilatory drive due to hypoxemia
    c. Metabolic acidosis

    4. Increased PaC02
    a. Alveolar hypoventilation
    b. Depression or injury to CNS - decreased ventilation
    c. Great increase in work of breathing
    d. Often animals with respiratory disease and an increase in the work of breathing will be normal at rest, but will have an increased PaC02 post exercise.

    5. Decreased Pa02
    a. Alveolar hypoventilation
    b. Diffusion impairment (rare)
    c. Right - left shunt
    d. Ventilation/perfusion inequalities (most common)
    e. To differentiate right - left shunts from V/P inequalities measure Pa02 after the animal has breathed 100% 02 for 7-10 min. The large amount of 02 in the airsacs will overcome most V/P inequalities and diffusion problems and Pa02 should be „ 300 mm Hg, however, if blood is bypassing the lung field Pa02 will remain substantially lower. A shunt of 20% of cardiac output will cause Pa02 to be 200-300 mm Hg while 50% shunt will be reflected by Pa02 < 100 mm Hg following inhalation of 100% 02.

    6. Exercise tolerance tests in the horse can be more accurately assessed by the use of Pa02. Within 5-10 min post exercise, Pa02 values should be equal or exceed normal resting values and PaC02 values may be decreased due to the increased respiratory rate.

    I. Pharyngeal and nasal swabs: questionable value except for culture of S. equi in horses or viral isolation.

    J. Pulmonary function tests
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