Respiratory Distress (Dyspnea)

W. David Wilson • Jeanne Lofstedt • Jeffrey Lakritz

■ Definition Respiratory distress indicates an inappropriate degree of effort to breathe based on an assessment of respiratory rate, rhythm, and character.^11,67

Respiratory distress is a clinical sign that implies labored breathing, whereas dyspnea is a symptom that describes the subjective feeling of difficult, uncomfortable, or unpleasant breathing (shortness of breath) in human patients.^1,11,67 As such, the term dyspnea is not strictly applicable to animal patients, although it is widely used by veterinarians to describe respiratory distress.

Manifestations of respiratory distress include elevated respiratory rate (see the Tachypnea section earlier), extended head and neck position, mouth breathing (ruminants), nostril flaring (horses, sheep, and goats), abnormal respiratory noise (stridor or stertor), exercise intolerance, exaggerated intercostal or abdominal effort (or both), a double expiratory lift and a “heave line” (expiratory respiratory distress), abducted elbows, anxious expression, and inactivity. Animals in severe respiratory distress may show cyanosis (see the Cyanosis section later) and (especially those with severe RAO) may exert so much effort to breathe that the whole body rocks, the anus pumps in and out, and the animal does not move or eat because this diverts its energies from respiration.⁶⁷

■ Pathophysiology Normal respiratory rate and character are maintained by central and peripheral monitoring of blood gas and acid-base status, with resulting reflex adjustments that

■ BOX 5.12

Causes of Tachypnea in Horses^a

Common Respiratory Causes

Bacterial pneumonia

Pleuropneumonia, pleuritis

Pulmonary abscessation

Recurrent airway obstruction (RAO or chronic obstructive pul­monary disease)

Viral pneumonia (equine influenza, adenovirus, equine rhinitis A and B, equine viral arteritis, others)

Equine herpesvirus types 1 and 4 (EHV-1, EHV-4)

Aspiration pneumonia

Prematurity, dysmaturity, or immaturity (foals)

Common Nonrespiratory Causes

Hyperthermia (fever, postexhaustion syndrome, heatstroke, anhidrosis, other)

Pain (abdominal crisis, laminitis, exertional myopathy, other)

Acidosis (acute enterocolitis, urinary bladder rupture, renal tubular acidosis, other)

Anaphylaxis

Blood transfusion reaction

Shock (hypovolemic, cardiac, septic)

Anemia (neonatal isoerythrolysis, blood loss, hemolytic anemia, iron deficiency, bone marrow suppression, ruptured middle uterine artery, other)

Cardiac disease (ruptured mitral chordae tendineae, ventricular septal defect, endocarditis, other)

Gastric dilation

Less Common Respiratory Causes

Smoke inhalation pneumonia

Parasitic pneumonia (Dictyocaulus arnfieldi)

Stenotic nares, choanal atresia

Nasal septum abnormalities

Neoplasia (nose, paranasal sinuses)

Fungal granuloma

Nasopharyngeal cicatrix

Paranasal sinus infection

Dorsal displacement of the soft palate

Pharyngeal or retropharyngeal abscess or trauma

Epiglottic entrapment

Chondroma of arytenoid cartilage

Guttural pouch empyema

Tracheal stenosis, collapse, stricture

Foreign body (nasal, nasopharyngeal, laryngeal, tracheal, bronchial)

Diaphragmatic hernia Pneumothorax

Thoracic trauma

Less Common Nonrespiratory Causes

Anaphylaxis

Air embolism

Intracarotid injections

Fluid therapy complications

Tetanus (Clostridium tetani)

Malignant edema (Clostridium septicum)

Malignant hyperthermia

Meningoencephalitis

Anemia (piroplasmosis, iron deficiency, hemolytic blood loss, other) Cardiovascular anomalies (tetralogy of Fallot, tricuspid insuffi­ciency, atrial septal defect, hypoplastic left heart, transposition of great vessels, persistent right aortic arch, patent ductus arteriosus)

Cor pulmonale

Pericarditis

Atrial fibrillation

Ventricular tachycardia, fibrillation, flutter

Uncommon Respiratory Causes

Pulmonary lobar hypertrophy

Branchial or thyroglossal duct cyst

Tracheal rupture

Fracture of laryngeal cartilage

Fracture of hyoid bone

Cutaneous or nasal amyloidosis

Pulmonary tuberculosis

Pulmonary nocardiosis

Pneumocystis carinii (renamed PneumocystisJiroveci in humans) pneumonia

Uncommon Nonrespiratory Causes

Lactation tetany

Hydroallantois or hydrops

Nutritional myodegeneration (foals)

Methemoglobin reductase deficiency

Acute hepatic insufficiency

Electrocution

Cardiac neoplasia

Embryonal mediastinal cyst

Toxic Causes

α-Naphthyl thiourea (ANTU)

Arsenic

Bromide

Sodium fluoroacetate

Cantharidin (blister beetle)

Ammonia

Amitraz

Propylene glycol

Dioctyl sodium sulfosuccinate

Iron

Selenium

Metaldehyde

Organophosphate

Organochlorine, chlorinated hydrocarbon

Phenothiazine

Potassium

Larkspur (Delphinium species)

Japanese yew (Taxus cuspidata)

White snakeroot (Eupatorium rugosum)

Water hemlock (Cicuta species)

Jimson weed (Datura stramonium)

Potato (Solanum tuberosum)

Cyanogenic plants

Onion (Allium species)

Red maple (Acer rubrum)

^aAll causes of respiratory distress (see Box 5.14) are also causes of tachypnea.

maintain carbon dioxide (CO₂), oxygen (O₂), and the blood hydrogen ion concentration (pH) within a narrow range.⁸ Respiratory distress may occur for the following reasons⁶⁶:

• Inadequate oxygenation of blood (i.e., a need for additional oxygen)

• Compensation for metabolic acidosis

• Excessive environmental heat

• Disorders that damage the central nervous system respiratory centers in the medulla (e.g., head trauma, inflammation, mass lesions)

• Disorders that cause dysfunction of motor nerves or weakness of respiratory muscles (e.g., botulism, hypocalcemia)

■ BOX 5.13

Causes of Tachypnea in Ruminants

Common Respiratory Causes

Mannheimia haemolytica or Pasteurella multocida pneumonia (includes shipping fever and enzootic calf pneumonia)

Histophilus somni pneumonia (B)

Visceral caseous lymphadenitis (Corynebacterium pseudotuber­culosis) (C, O)

Chronic bacterial pneumonia with consolidation or abscessation (Arcanobacterium pyogenes and other bacteria)

Necrotic laryngitis (Fusobacterium necrophorum) (B) Pulmonary embolus from posterior vena cava thrombosis (B) Respiratory syncytial virus

Parainfluenza type 3 virus

Adenovirus (B, O)

Infectious bovine rhinotracheitis virus (IBR; BHV-1) (B)

Ovine progressive pneumonia virus (O)

Mycoplasma species

Caprine Mycoplasma mycoides subsp.

Parasitic pneumonia (Dictyocaulus viviparus [B]; Dictyocaulus filaria [O, C]; Mullerius capillaris [O, C]; Protostrongylus rufe- scens [O, C])

Bovine atypical interstitial pneumonia (B)

Acute pulmonary edema and emphysema (B)

Farmer's lung (Faenia rectivirgula hypersensitivity pneumonitis) (B)

Aspiration or foreign body pneumonia

Common Nonrespiratory Causes

Hyperthermia (fever, heatstroke, rapid rise in ambient temperature, other)

Pain (abdominal crisis, urethral calculi, traumatic reticuloperitonitis, musculoskeletal injury, other)

Acidosis (ruminal lactic acidosis, pregnancy toxemia, other) Electrolyte aberrations (hypocalcemia, hypomagnesemia, other) Shock (hypovolemic, cardiac, septic)

Anemia (iron deficiency, postparturient hemoglobinuria, blood loss, other)

Distended abdominal viscus (ruminal bloat, other)

Anaphylaxis

Blood transfusion reaction

White muscle disease

Less Common Respiratory Causes

Nasal trauma

Tumors of the nose and paranasal sinuses

Nasal granuloma (fungal granuloma, atopic rhinitis)

Congenital cystic nasal conchae

Laryngeal trauma, abscess

Tracheal stenosis, collapse, stricture Bovine rhinovirus (B)

Bovine malignant catarrhal fever (B)

Ascaris suum migration (calves) (B)

Thoracic trauma, other causes of chest pain Pneumothorax

Diaphragmatic hernia

Pleuritis or pleural effusion

Caprine arthritis-encephalitis (CAE) pneumonia (C)

Sheep pulmonary adenomatosis virus pneumonia (O)

Less Common Nonrespiratory Causes

Cardiac anomalies (ventricular septal defect, tetralogy of Fallot, other)

Endocarditis

Pericarditis

Central nervous system disease (meningoencephalitis, polioen- cephalomalacia, other)

Esophageal obstruction or foreign body

Clostridial diseases (Black disease, enterotoxemia, tetanus) Anaphylaxis

Uncommon Respiratory Causes

Pleural mesothelioma

Pneumocystis carinii (renamed PneumocystisJiroveci in humans) pneumonia

Pulmonary aspergillosis

Pulmonary neoplasia

Cyst (branchial, cervical, thyroglossal duct) (B)

Bronchobiliary fistula

Contagious bovine pleuropneumonia (exotic) (B) Endemic ethmoid carcinoma (exotic) (B)

Uncommon Nonrespiratory Causes

Bluetongue (B, O)

Thymic lymphosarcoma (B)

Retrobulbar neoplasia (B)

Calf lymphosarcoma (B)

Adult multicentric lymphosarcoma (B)

Embryonal mediastinal cyst

Dwarfism (B)

Anthrax

Toxic Causes

Sodium fluoroacetate

Strychnine

Sulfur

Propylene glycol

Urea or nonprotein nitrogen

Water deprivation or salt toxicity

Potassium

Nitrates

Bromide

Iron

Selenium

Organophosphate

Organochlorine or chlorinated hydrocarbon

Larkspur (Delphinium species)

Japanese yew (Taxus cuspidata)

Sneezeweed (Helenium autumnale)

Hairy vetch (Vicia villosa)

Whitehead (Sphenosciadium capitellatum)

White snakeroot (Eupatorium rugosum)

B, Bovine; C, caprine; O, ovine.

Note: Tachypnea is a rather nonspecific sign and is associated with toxicity caused by a large number of plant species; only a few are listed in this box. See also Box 5.15.

Painful conditions involving the respiratory sensory nerves, muscles, pleura, and ribs (e.g., chest trauma, pleural infection, neoplasia)

Inadequate oxygenation of blood leads to arterial hypoxemia (low partial pressure of oxygen in arterial blood [PaO2]). This can be caused by a low partial pressure of inspired oxygen (PiO₂), such as occurs at high altitude; by disorders that interrupt the transfer of oxygen from the environment to the blood (e.g., upper and lower airway obstruction, pulmonary disease associated with alveolar flooding or collapse, pulmonary or intracardiac right-to-left shunting of blood); or by a decrease in the oxygen-carrying capacity of the blood, such as occurs in anemia, methemoglobinemia, and carboxyhemoglobinemia.⁶6 Primary or secondary disease conditions that affect the respira­tory and cardiovascular systems induce arterial hypoxemia by causing alveolar hypoventilation, ventilation-perfusion mis­match, diffusion limitation, right-to-left shunting of blood, or combinations of these abnormalities (see the Cyanosis section later).^11,66

Compensation for metabolic acidosis involves “blowing off” carbon dioxide, which may increase both the rate and the depth of respiration.⁶⁶ The resulting hyperventilation causes a decline in the partial pressure of arterial carbon dioxide (PaCO₂) in the face of clinical signs of respiratory distress.

Animals dissipate a considerable amount of heat through the respiratory tract. In ruminants, but not in horses, heat dissipation is further aided by the animals' ability to mouth breathe and pant to increase evaporative cooling of blood passing through the tongue and other structures in the oral cavity and oropharynx. The need to dissipate heat when exposed to high environmental temperatures induces labored breathing (Boxes 5.14 and 5.15).

■ BOX 5.14

Causes of Respiratory Distress in Horses

Common Respiratory Causes

Bacterial pneumonia

Pleuropneumonia or pleuritis

Pulmonary abscessation

Recurrent airway obstruction (housing or pasture associated) Strangles (Streptococcus equi infection)

Viral pneumonia (influenza, equine viral arteritis, other viruses) Equine herpesvirus types 1 and 4 (EHV-1, EHV-4)

Aspiration pneumonia

Prematurity, dysmaturity, immaturity (foals)

Neonatal septicemia (foals)

Pharyngeal, retropharyngeal abscess or trauma

Common Nonrespiratory Causes

Cardiac disease (e.g., congestive cardiac failure, mitral insufficiency, other cardiac diseases)

Shock (septic, cardiogenic, hypovolemic, acute blood loss) Endotoxemia

Anemia (e.g., neonatal isoerythrolysis, autoimmune hemolytic anemia, blood loss, other causes of acute anemia)

Pain (e.g., abdominal crisis, laminitis, myopathy, fracture, other lameness)

Hyperthermia (e.g., fever, postexhaustion syndrome, anhidrosis, heatstroke, erythromycin associated)

Less Common Respiratory Causes

Epiglottic entrapment with secondary infection or granulation Arytenoid chondritis

Guttural pouch empyema, tympany, mycosis, neoplasia Progressive ethmoid hematoma

Nasal polyps

Pharyngeal, subepiglottic cysts

Fungal rhinitis, cryptococcal rhinitis, equine nasal granuloma, nasal aspergillosis, rhinosporidiosis, rhinophycomycosis, maduromy­cosis, mycetoma

Cleft palate

Laryngeal or hyoid trauma, fractured laryngeal cartilages, laryngeal granuloma or scar

Paranasal sinus infection, cyst, trauma, tumor

Nasal trauma, nasal neoplasia

Foreign body (nasal, pharyngeal, laryngeal, tracheal, bronchial) Exercise-induced pulmonary hemorrhage (EIPH)

Parasitic pneumonia (Dictyocaulus arnfieldi) Coccidioidomycosis, cryptococcosis, mycotic pneumonia Inhalation pneumonia, smoke inhalation, drowning, water inhalation Hyaline membrane disease (foals) Acute bronchointerstitial pneumonia

Peripartum asphyxia syndrome Fractured ribs or sternum, thoracic trauma Pneumothorax

Diaphragmatic hernia Mediastinal abscess

Less Common Nonrespiratory Causes

Purpura hemorrhagica

Blood or plasma transfusion reaction

Complications of fluid therapy

Anaphylaxis

Intracarotid injection Acidosis

Gastric distention (e.g., as in small intestinal obstruction)

Pulmonary edema

Malignant hyperthermia

Cardiovascular anomalies (ventricular septal defect, patent ductus arteriosus, tetralogy of Fallot, common ventricle, other anomalies)

Endocarditis

Pericarditis

Cardiac dysrhythmias (atrial fibrillation, heart block, ventricular premature beats, ventricular tachycardia, ventricular fibrillation) Ruptured mitral chordae tendineae

Mitral insufficiency or stenosis

Clostridial infections (e.g., tetanus, malignant edema, injection abscess)

Procaine penicillin G reaction or intravascular administration Hyperkalemic periodic paralysis (HYPP)

Uncommon Respiratory Causes

Stenotic external nares

Cutaneous, nasal amyloidosis

Cutaneous, nasal habronemiasis (summer sore)

Failure of closure of the false nostril

Abnormalities of the nasal septum

Choanal (posterior nares) atresia or stenosis (foals)

Nasopharyngeal cicatrix

Laryngopalatal dislocation (dorsal displacement of soft palate), soft palate hypoplasia

Rostral displacement of the palatopharyngeal folds

Pharyngeal hematoma

Laryngeal paralysis

Fistula (pharyngeal, esophageal, esophagobronchial, esophago- tracheal)

Chondroma of the arytenoid cartilage

Laryngeal spasm

Hypertrophic ossification of the laryngeal cartilages, laryngeal chondropathy

Neoplasia of the upper airway

Tracheal stenosis, stricture, collapse, rupture

Phycomycosis, pythiosis

Pneumoconiosis (e.g., silicosis)

Interstitial pneumonia (restrictive pulmonary disease)

Pulmonary thromboembolism

Pneumocystis carinii (renamed PneumocystisJiroveci in humans) pneumonia

Pulmonary lobar hypertrophy (foals)

Continued

■ BOX 5.15

Causes of Respiratory Distress in Ruminants

Common Respiratory Causes

Mannheimia haemolytica or Pasteurella multocida pneumonia (includes shipping fever and enzootic calf pneumonia)

Bacterial pneumonia with consolidation or abscessation (Arcano- bacterium and other bacteria)

Histophilus somni pneumonia (B)

Visceral caseous lymphadenitis (Corynebacterium pseudotuber­culosis) (O, C)

Aspiration or foreign body pneumonia (especially after hypocalcemia) Necrotic laryngitis (Fusobacterium necrophorum) (B, O) Infectious bovine rhinotracheitis virus (IBR; BHV-1) (B) Respiratory syncytial virus

Ovine progressive pneumonia virus (O)

Mycoplasma mycoides subsp.

Mycoplasma bovis (B)

Mycoplasma ovipneumoniae (O)

Parasitic pneumonia (Dictyocaulus viviparus [B], Dictyocaulus filaria [O, C], Mullerius capillaris [O, C], Protostrongylus rufe- scens [O, C])

Bovine atypical interstitial pneumonia (B)

Acute bovine pulmonary edema and emphysema (B)

Farmer's lung disease (Faenia rectivirgula hypersensitivity pneumonitis) (B)

Common Nonrespiratory Causes

Hyperthermia (fever, heatstroke, rapid rise in ambient temperature, other)

Pain (abdominal crisis, urethral calculi, traumatic reticuloperitonitis, other)

Distended abdominal viscus

Acidosis (ruminal lactic acidosis, pregnancy toxemia, other) Electrolyte aberrations (hypocalcemia, hypomagnesemia, other) Hypovolemic, cardiac, or septic shock

Fluid or electrolyte loss (acute diarrhea, gastrointestinal obstruc­tion, other)

Endotoxemia (coliform mastitis, metritis, enteritis, salmonellosis, septicemia, other)

Neonatal septicemia

Anemia (iron deficiency, postparturient hemoglobinuria, hemolytic, anaplasmosis, eperythrozoonosis, other)

White muscle disease (nutritional myodegeneration) Anaphylaxis or allergy, milk allergy

Less Common Respiratory Causes

Pulmonary embolus from posterior vena cava thrombosis (B) Parainfluenza virus type 3

Adenovirus (B, O)

Nasal trauma

Tumors of the nose, paranasal sinuses, oral cavity

Nasal granulomas (fungal granuloma, atopic rhinitis) Congenital cystic nasal conchae

Nose bots (Oestrus ovis)

Sinusitis (maxillary, frontal, postdehorning)

Laryngeal trauma or abscess

Trauma (oral, pharyngeal, retropharyngeal), abscess, hematoma Tracheal stenosis, collapse, stricture

Bovine rhinovirus (B)

Bovine malignant catarrhal fever (B)

Bovine herpesvirus DN-599 (B)

Ascaris suum migration (calves) (B)

Thoracic trauma, rib fracture

■ BOX 5.15

Causes of Respiratory Distress in Ruminants—cont'd

Pneumothorax

Pleuritis or pleural effusion

Caprine arthritis-encephalitis (CAE) pneumonia (C)

Sheep pulmonary adenomatosis virus (Jaagsiekte) (O)

Smoke inhalation

Foreign body (nasal, oral, pharyngeal, laryngeal, tracheal, bronchial) Caprine herpesvirus (C)

Bluetongue

Peste des petits ruminants (C, O) (exotic)

Congenital cardiac anomalies (ventricular septal defect, tetralogy of Fallot, patent ductus arteriosus, transposition of the great vessels, other anomalies)

Acquired cardiac failure (bacterial endocarditis, valvular incom­petence, valvular stenosis, cardiomyopathy, pericarditis, other causes)

Central nervous system disease (trauma, meningoencephalitis, encephalomalacia, abscess, louping ill, pseudorabies, other causes)

Esophageal obstruction, foreign body, laceration, rupture, mega­esophagus

Clostridial diseases (Black disease, enterotoxemia, tetanus, blackleg, bacillary hemoglobinuria, others)

Anthrax

Complications of fluid therapy (pulmonary edema)

Water deprivation (salt poisoning) (B)

Blood transfusion reaction

Anaphylaxis

Burn, thermal injury, electrocution

Systemic toxoplasmosis (C, O)

Tick paralysis (C, O)

Bladder rupture

Bee or wasp sting, snakebite

Photosensitization

Uncommon Respiratory Causes

Actinobacillosis (wooden tongue)

Actinomycosis (lumpy jaw)

Diaphragmatic hernia

Pleural mesothelioma

Pneumocystis carinii (renamed PneumocystisJiroveci in humans) pneumonia

Pulmonary aspergillosis

Pulmonary neoplasia

Pulmonary tuberculosis

Chlamydia psittaci pneumonia

Cyst (branchial, cervical, salivary, thyroglossal duct)

Bronchobiliary fistula

Contagious bovine pleuropneumonia (exotic) (B)

Endemic ethmoid carcinoma (exotic) (B)

Theileria annulata and Theileria hirci (exotic) (B)

Heartwater (Cowdria ruminantium) (exotic)

Uncommon Nonrespiratory Causes

Retrobulbar neoplasia (B)

Enzootic bovine leukosis (B)

Thymic lymphosarcoma or thymoma

Calf lymphosarcoma (B)

Adult multicentric lymphosarcoma (B)

Embryonal mediastinal cyst

Dwarfism (B)

Botulism

Vesicular stomatitis

High altitude (brisket) disease, cor pulmonale (B, C)

Procaine penicillin G reaction or intravascular administration Liver disease (infectious, toxic, parasitic, other)

Border disease (hairy shaker) (C, O)

Toxic Causes

Sodium fluoroacetate

Strychnine

Sulfur (B, O)

Propylene glycol (B) Oxalate, ethylene glycol (B, O) Urea or nonprotein nitrogen Potassium (B)

Nitrates Bromide (B, C) Lead

Mercury (C) Iron (B, C)

Selenium

Arsenic

Organophosphate or carbamate Organochlorine or chlorinated hydrocarbon Permethrin

Gossypol

Ergot (Claviceps purpurea) Phosphate fertilizer (B, O) Vitamin D₃ (B)

Warfarin, dicumarol, diphacinone Metaldehyde

Formaldehyde

Ammonia

Hydrogen sulfide Monensin, salinomycin Dinitrophenol (B, O) Copper (acute oral toxicity) Insect fogger pneumonitis (B)

Aflatoxicosis (C)

Levamisole (C, O) Polychlorinated biphenyl (PCB) (B) Xylazine-induced pulmonary edema (O) Carbolic dips (O)

Cyanogenic plants (arrow grass, Johnson grass, common sorghum, Sudan grass, chokecherry, acacia, other plants)

Avocado (Persea americana)

Moldy sweet clover (Melilotus species)

Moldy sweet potato (Ipomoea batatas) (B)

Brassica species Ryegrass (Lolium species) (B, O) Larkspur (Delphinium species) Japanese yew (Taxus cuspidata) Hairy vetch (Vicia villosa) (B) Whitehead (Sphenosciadium capitellatum) (B) White snakeroot (Eupatorium rugosum) (B, O) Chinese tallow (Sapium sebiferum) (B) Purple mint (Perilla frutescens) (B) Oleander (Nerium oleander) Nightshade (Solanum species) (B) Cocklebur (Xanthium species) (B, O)

Locoweeds (Astragalus species, Oxytropis species) (B, O) Foxglove (Digitalis purpurea) (B) Water hemlock (Cicuta species)

Milkweed (Asclepias species) (B, O)

Hepatotoxic plants (Senecio species, Amsinckia species, others containing pyrrolizidine alkaloid)

Fescue summer poisoning (B) Rubberweed (Hymenoxys species) Sneezeweed (Helenium autumnale) False hellebore (Veratrum species) (B, O) Rhododendron (Andromeda species) (B, O) Algae

Lupines (Lupinus species) (B, O) Prickly paddy melon (Cucumis myriocarpus) has risen rapidly, such as occurs when cattle in cold climates are brought indoors in the winter. When the environmental temperature has been consistently low, cattle in feedlots may also experience respiratory distress if the temperature suddenly rises from 4.4° to 10° C (40° to 50° F).

B, Bovine; C, caprine; O, ovine.

Note: Respiratory distress is a sign associated with toxicity caused by a large number of plant species. Not all have been listed here.

Observation of the nature of the respiratory distress may give important clues as to the functional characterization, and perhaps cause, of the underlying disease process. Obstructive diseases involving the intrathoracic airways (e.g., RAO in horses and farmer's lung in cattle) are more likely to cause flow limita­tion during expiration because of dynamic airway narrowing or collapse (see the Cough section earlier).^11,68,69 This results in expiratory respiratory distress and a pattern of respiration in which the expiratory phase occupies an increased proportion of the respiratory cycle as the patient attempts to expel air from the lungs.⁶⁸ In the extrathoracic airways, dynamic collapse occurs during inspiration because intraluminal pressures are subatmospheric at this time.^11,68,69 Therefore patients with upper airway obstructions, especially of the nonfixed type (e.g., laryngeal hemiplegia), generally show inspiratory respiratory distress and may have a prolonged inspiratory phase.^11,68 Fixed airway obstructions of either the upper or the lower airway (e.g., intraluminal mass, bronchoconstriction) are present during both phases of respiration and may lead to both inspiratory and expiratory distress.¹¹ However, the distress is likely to be accentuated during a particular phase of respiration, depending on the anatomic site of the obstruction (i.e., fixed upper airway obstructions cause more distress during inspiration, and fixed lower airway obstructions cause more distress during expiration). Restrictive diseases (e.g., pleural effusion, pneumoconiosis) inhibit expansion of the lungs and therefore generally lead to inspiratory respiratory distress.¹¹ Because an animal with restrictive disease has reduced compliance and must perform more respiratory work than normal to expand its lungs, a common strategy for maintaining adequate ventilation is to increase the respiratory rate and lower the tidal volume (i.e., rapid, shallow breathing).¹¹ Animals with obstructive diseases generally have a normal or even increased tidal volume.¹¹

In many instances, respiratory distress is not apparent at rest but occurs in association with exercise. Under these cir­cumstances the animal's capacity to exercise is impaired, and the owner may complain of exercise intolerance (see the Exercise Intolerance and Poor Performance in Horses section later).

Approach to Diagnosis of Respiratory Distress

HISTORY. After ruling out environmental causes of respira­tory distress (e.g., heat stress, high humidity, moving from outside into a heated barn in the winter, handling stress, relocation to high altitude) and attending to the immediate needs of the patient, a careful history should be taken that includes the following factors: time and speed of onset of the clinical signs of respiratory distress; progression of clinical signs; whether this is the first episode of respiratory distress or the animal is subject to recurrent attacks; whether signs are present at rest or only after exercise; the relationship of signs to environmental conditions and the response to environmental change; recent administration of pharmacologic or biological agents; the presence of an audible respiratory noise; or other signs of respiratory tract, oropharyngeal, or neurologic disease (e.g., nasal discharge, cough, dysphagia, facial paralysis, ret­ropharyngeal swelling). A history of recent trauma or exposure to potentially toxic substances such as lead-containing paints, nitrate-accumulating plants or urea (ruminants), or carbon monoxide should be elicited. The animal's appetite and attitude and signs of disease in other systems (e.g., diarrhea) should be determined. In neonates the circumstances surrounding gestation and parturition should be ascertained because pre­maturity, dysmaturity, congenital infection, birth trauma (e.g., rib fracture) related to dystocia, prolonged parturition, and aspiration of amniotic fluid and meconium are all important causes of respiratory distress. However, it should be noted that tachypnea, with respiratory rates of 60 to 80 breaths/min, is normal in foals during the first 30 minutes after birth as they “blow off” carbon dioxide.

PHYSICAL EXAMINATION. The physical examination should follow the same general approach as that described for cough. In particular, the following should be determined: rectal temperature, pulse rate, respiratory rate and character; regularity and pattern of breathing; presence of excessive intercostal or abdominal respiratory effort; synchrony and symmetry of chest excursion; presence of a “heave line”; presence of stridor at rest (the examiner listens at the nostrils); symmetry of airflow from each nostril; effect of occlusion of each nostril indepen­dently; odor from the nose or mouth (or both); presence and character of nasal discharge; swelling around the external nares, inside the nasal passages, or inside the false nostril; facial symmetry and swelling; ocular discharge; resonance or painful response on percussion of the maxillary and frontal sinuses; palpable abnormalities of the mandibles and hyoid apparatus; enlargement of submandibular, parotid, retropharyngeal, and other regional lymph nodes; enlargement of the parotid salivary glands or thyroid gland; swelling, pain, or palpable abnormalities in the retropharyngeal region; a palpable, left-sided pit on the dorsal surface of the larynx; accentuation of stridor, induction of a cough, or evidence of pain on application of pressure to the larynx and trachea; palpable swelling or flattening of the cervical trachea; masses at the thoracic inlet; and palpable turbulence in the extrathoracic airway.

The mucous membranes should be examined carefully for cyanosis, pallor, cherry red color, hemorrhages, congestion, or injection. The capillary refill time should be determined, and the peripheral pulse rate, rhythm, and character should be assessed. Other signs of heart failure (e.g., jugular distention or pulsation and peripheral edema) and signs of dehydration (e.g., delayed jugular filling, dry mucous membranes, altered skin turgor) should be noted. The larynx, trachea, and lungs should be carefully auscultated at rest and, after the rate and depth of respiration have been increased, if it is safe to do so, by application of a rebreathing bag, by occlusion of the nostrils, or by exercising the animal, so that turbulent airflow and abnormal lung sounds may be detected. The heart should be auscultated to detect murmurs, cardiac dysrhythmias, muffling of heart sounds, or other abnormalities. The chest wall should be carefully palpated to detect rib fractures and other lesions, and both sides of the chest should be percussed to detect large mass lesions, lung consolidation, pleural effusion, hyperinflation, pneumothorax, or a painful response, which may indicate pleuritis. A thorough oral examination is important, and in cattle this should include palpation of the base of the tongue, the oropharynx, and, if possible, the larynx (see the Nasal Discharge section earlier).

A general physical examination should be completed so that diseases in systems other than the cardiovascular and respiratory systems (e.g., respiratory distress in cattle secondary to ruminal bloat and respiratory distress secondary to central nervous system trauma or severe metabolic acidosis) can be detected. Attempts should also be made to identify conditions that may cause severe acid-base disturbances and hemo concentration (e.g., diarrhea, renal disease), pain (e.g., laminitis, trauma), or hyperthermia (e.g., infectious conditions, heatstroke).

Further diagnostic evaluation of respiratory distress may include a complete blood count with fibrinogen concentration; blood gas analysis; serum biochemistry determinations; endos­copy of the upper and lower airways and esophagus; nasal or nasopharyngeal swabbing or scraping; virus identification, isolation, and serologic testing; bronchodilator response testing; tracheal aspiration; BAL; ultrasound examination of the chest and of suspected mass lesions that impinge on the upper airway; thoracocentesis; radiography of the nasal passages, paranasal sinuses, pharynx, guttural pouches, larynx, trachea, and chest; fecal examination for lungworms and other parasites; CT scanning of the upper airway; nuclear scintigraphy; pulmonary function testing; and biopsy of externally visible or palpable lesions or those identified by ultrasound, endoscopy, or radiography as described for the evaluation of cough and nasal discharge (see the Cough and Nasal Discharge sections earlier). The examiner should exercise discretion in the selection of diagnostic tests for animals with respiratory distress because procedures such as BAL or endoscopy may prove to be exces­sively stressful and exacerbate the distress, with potentially fatal consequences.

COMPLETE BLOOD COUNT. A complete blood count includ­ing fibrinogen and plasma protein concentrations helps evaluate the role of hemoconcentration, anemia, or leukocytosis and hyperfibrinogenemia, which may accompany pneumonia and other inflammatory conditions.

ENDOSCOPIC EXAMINATION. Endoscopic examination of the upper and lower airways is particularly helpful in evaluating patients suspected of having obstructive disease⁷⁰ (see the Cough and Nasal Discharge sections earlier and the Abnormal Respira­tory Noise [Stridor] section later). Endoscopic examination of the esophagus is indicated in patients with a history of bloat, dysphagia, or return of ingesta through the nose in addition to respiratory distress (see the Nasal Discharge section earlier and the Abnormal Respiratory Noise [Stridor] section later).

BLOOD GAS ANALYSIS. Blood gas analysis and acid-base determinations should be performed on arterial blood to determine O₂ and CO₂ tensions so that the contribution of hypoxemia or acidosis to the signs of respiratory distress can be ascertained. In foals and calves, arterial samples are usually obtained from the great metatarsal artery or the brachial artery as it crosses the medial aspect of the foreleg.⁷¹ The auricular artery is also convenient in calves. The femoral artery can be used in neonates but is less convenient because it tends to roll. The facial artery can be used in adult horses, and the auricular or coccygeal artery can be used in mature ruminants.⁷² During the sampling process, the patient should be quiet and not struggling, which can decrease the PaCO₂.⁷³ Local subcutaneous infiltration of 2% lidocaine without epinephrine over the artery being sampled minimizes needless struggling and facilitates sample collection. A heparinized syringe and a 22- to 26-gauge needle can be used. Any air bubbles should be removed after sample collection and the needle properly sealed. If the sample is kept on ice, the pH will remain unchanged for 3.5 hours and the blood gases for 6 hours.⁷⁴ The animal's temperature should be recorded at the time of sample collection so that it can be used in the calculation of actual blood gas concentrations.

The normal PaO₂ for the horse is 83.6 ± 1.7 mm Hg, and the normal PaCO₂ is 42.2 ± 0.8 mm Hg.⁷⁵ Hypoxemia is defined as a PaO₂ below 80 mm Hg.⁷⁵ Cyanosis is usually not evident until the PaO₂ is much lower than this (usually below 40 mm Hg).⁷⁵ Hypercarbia, or hypercapnia, is a condition of increased PaCO₂ (above 44 mm Hg).⁷⁵ Because CO₂ diffuses readily, considerable ventilatory dysfunction can occur before the PaCO₂ rises; therefore severe hypoxemia can occur with a normal PaCO₂. An elevated PaCO₂ generally indicates that hypoventilation or a severe pulmonic pathologic condition is present. Because venous blood samples reflect tissue metabolism, they are not considered adequate for evaluating pulmonary function. However, a partial pressure of carbon dioxide in venous blood (PvCO₂) above 60 mm Hg usually reflects arterial hypercapnia, and a partial pressure of oxygen in venous blood (PvO₂) below 20 mm Hg usually indicates arterial hypoxemia.⁷⁴

Venous admixture is the term used to denote the ways that blood can pass from the right side to the left side of the circula­tion without being properly oxygenated and represents the efficiency with which the lung oxygenates blood. The magnitude of venous admixture (efficiency of oxygenation) can be assessed by calculating the PaO₂TFiO₂ ratio or the alveolar-arterial PO₂ gradient. The PaO₂∕FiO₂ ratio is normally 500. PaO₂∕FiO₂ ratios between 300 and 400 reflect significant lung injury, values below 300 reflect profoundly inefficient lung function such as occurs with acute lung injury, and values below 200 reflect severe life-threatening lung disease typical of acute respiratory distress syndrome (ARDS). The alveolar-arterial (A-a) PO₂ gradient difference is the difference between calculated alveolar PO₂ and the measured arterial PO₂ Alveolar PO₂ = Inspired PO₂ - PaCO₂ (1.1), where inspired PO₂ = barometric pressure ? 21% and 1.1 = 1∕RQ, where RQ is the respiratory quotient and has an assumed value of 0.9. The A-a PO₂ difference is typically about 10 mm Hg in a normal horse breathing 21% oxygen (room air) at sea level and about 100 mm Hg when breathing 100% oxygen. The “120 rule” is a simplified version of the alveolar gas equation that can be applied to a patient breathing 21% oxygen at near sea level. The measured PaO₂ and PaCO₂ should add up to 140 ± 10. If the sum is less than 120, the patient has venous admixture; as well, the lower the value, the worse the admixture.

INSUFFLATION OF 100% OXYGEN. Insufflation of 100% oxygen causes a significant increase in PaO₂ if hypoxemia is caused by hypoventilation or ventilation-perfusion mismatch, whereas little or no improvement in PaO₂ occurs if hypoxemia is caused by anatomic or physiologic right-to-left shunting.¹¹ Therefore measuring PaO₂ 5 minutes after insufflation of 100% oxygen can help in determining the pathophysiologic processes contributing to arterial hypoxemia. In full-term neonates and adults, the PaO₂ should exceed 200 mm Hg after 5 minutes of oxygen administration.⁷³ Continued cyanosis or inability to raise the PaO₂ above 100 mm Hg is highly suggestive of a right-to-left shunt.

DETECTION OF ABNORMAL HEMOGLOBIN. Blood samples should be evaluated for abnormal hemoglobin (e.g., methe­moglobin) if exposure to nitrates or other intoxicants is suspected (see the Cyanosis section later).

RADIOGRAPHIC EXAMINATION. Radiographic examination of the nasal passages, pharynx, larynx, and trachea permits detection and evaluation of obstructive lesions in the upper airway, especially if tachypnea is accompanied by inspiratory stridor, asymmetric nasal airflow, or other evidence of upper airway obstruction.

Radiographic examination of the trachea and thorax should include end-inspiratory phase radiographs to facilitate identifica­tion of pulmonary lesions and disorders such as dynamic collapse of the trachea that are visible only on inspiratory-phase radiographs. Chest radiographs help detect evidence of pneu­monia, pleural effusion, pneumothorax, cardiomegaly, and mediastinal lesions. The cardiac silhouette and the pattern and caliber of the aorta, vena cava, pulmonary artery, pulmonary veins, and other vessels should also be evaluated for evidence of cardiac failure and pulmonary vascular disease. Contrast angiography and other diagnostic procedures such as echocar­diography, electrocardiography, and hemodynamic (pressure and flow) studies may be indicated in patients with respiratory distress that has occurred secondary to cardiovascular disease.

ULTRASOUND EXAMINATION. Ultrasound examination of the thorax is performed to detect pleural inflammation and effusion, pulmonary consolidation, pulmonary abscessation, and cardiac anomalies if the clinical examination suggests that pulmonary, pleural, or cardiac disease is the cause of the signs of respiratory distress.