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Comparative Physiologic Parameters

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Respiratory Physiology of Dogs and Cats: Detailed Physiologic Parameters #

Respiratory Volumes and Capacities #

Tidal Volume #

  • Dogs: 10-15 mL/kg in normal breeds; reduced by 15-30% in brachycephalic breeds
  • Cats: 8-12 mL/kg in normal breeds; reduced by 10-25% in brachycephalic breeds
  • Clinical significance: Brachycephalic animals compensate for reduced tidal volume with increased respiratory rate, creating higher minute ventilation demands

Minute Ventilation #

  • Dogs: 150-300 mL/kg/min at rest
  • Cats: 120-250 mL/kg/min at rest
  • Exercise adaptation: Dogs can increase minute ventilation up to 10-fold during exercise; cats typically achieve 4-5-fold increases

Functional Residual Capacity (FRC) #

  • Dogs: 30-40 mL/kg
  • Cats: 25-35 mL/kg
  • Brachycephalic impact: FRC may be reduced by 20-30% in severely affected animals

Dead Space #

  • Anatomical dead space:
    • Dogs: 2-3 mL/kg (approximately 30% of tidal volume)
    • Cats: 2-2.5 mL/kg (approximately 25-30% of tidal volume)
  • Physiologic dead space:
    • Normal dogs/cats: Approaches anatomical dead space in healthy animals
    • During disease: Can increase to 50-60% of tidal volume in pulmonary thromboembolism or ventilation-perfusion mismatch conditions

Oxygen Delivery and Transport #

Oxygen Delivery (DO₂) #

Calculated as: DO₂ = Cardiac Output × Arterial Oxygen Content

  • Dogs: 20-25 mL O₂/kg/min at rest
  • Cats: 15-20 mL O₂/kg/min at rest
  • Exercise capacity: Dogs can increase DO₂ 6-8 fold during peak exercise; cats typically achieve 3-4 fold increases

Arterial Blood Gas Parameters #

  • PaO₂ (arterial oxygen partial pressure):
    • Dogs: 85-100 mmHg (room air)
    • Cats: 80-100 mmHg (room air)
    • Brachycephalic animals: Often 5-15 mmHg lower than normal breeds
  • PaCO₂ (arterial carbon dioxide partial pressure):
    • Dogs: 35-45 mmHg
    • Cats: 30-42 mmHg
    • Brachycephalic tendency: Higher PaCO₂ (45-50 mmHg) due to hypoventilation

Hemoglobin Oxygen Affinity #

  • P₅₀ values (oxygen partial pressure at which hemoglobin is 50% saturated):
    • Dogs: 27-30 mmHg
    • Cats: 25-27 mmHg (indicating higher oxygen affinity)

Oxygen Extraction Ratio (O₂ER) #

  • Dogs: 25-30% at rest
  • Cats: 20-25% at rest
  • Exercise: Dogs can increase O₂ER to 70-80%; cats typically reach maximum of 55-65%

Respiratory Mechanics #

Airway Resistance #

  • Dogs: 0.5-2.0 cmH₂O/L/sec in normal breeds
  • Cats: 1.0-3.0 cmH₂O/L/sec in normal breeds
  • Brachycephalic breeds:
    • Mild BOAS: 2-4× normal resistance
    • Severe BOAS: 5-10× normal resistance

Lung Compliance #

  • Static compliance:
    • Dogs: 40-80 mL/cmH₂O
    • Cats: 15-30 mL/cmH₂O
  • Dynamic compliance (at normal breathing frequencies):
    • Dogs: 25-50 mL/cmH₂O
    • Cats: 10-20 mL/cmH₂O

Work of Breathing #

  • Normal dogs: 0.3-0.7 J/L
  • Normal cats: 0.5-0.9 J/L
  • Brachycephalic animals: May require 3-5× more energy for breathing compared to normal conformations

Brachycephalic Airway Syndrome: Detailed Physiology #

Anatomical Components and Physiologic Consequences #

  1. Stenotic Nares
    • Prevalence:
      • 58-85% of brachycephalic dogs
      • 25-40% of brachycephalic cats
    • Airflow reduction:
      • Severe stenosis may reduce nasal airflow by 60-80%
      • Increases negative pressure in the pharyngeal region by 2-3×
  2. Elongated Soft Palate
    • Anatomical measurements:
      • Normal dogs: Soft palate extends to middle of epiglottis
      • Brachycephalic dogs: Extends 3-5 mm beyond epiglottis
      • Normal cats: Soft palate extends to rostral edge of epiglottis
      • Brachycephalic cats: Extends 1-3 mm beyond epiglottis
    • Airflow dynamics: Creates a flutter valve effect during inspiration, reducing airflow by 40-60%
  3. Hypoplastic Trachea
    • Tracheal diameter measurements:
      • Normal dogs: Tracheal diameter to thoracic inlet ratio of 0.16-0.20
      • Bulldogs: Often 0.09-0.13
      • Normal cats: Tracheal diameter to thoracic inlet ratio of 0.13-0.17
      • Brachycephalic cats: Often 0.09-0.12
    • Airflow limitation: Poiseuille’s law dictates that resistance increases inversely with the fourth power of radius, making even slight narrowing physiologically significant
  4. Everted Laryngeal Saccules
    • Secondary change: Results from chronic high negative pressure
    • Airway narrowing: Reduces laryngeal cross-sectional area by 25-50%
  5. Laryngeal Collapse
    • Staging system:
      • Stage I: Eversion of laryngeal saccules
      • Stage II: Loss of rigidity and medial displacement of cuneiform processes
      • Stage III: Collapse of corniculate processes and glottic stenosis
    • Physiologic impact: Stage III can reduce glottic area by 70-90%

Physiologic Derangements in BOAS #

  1. Ventilation-Perfusion Mismatch
    • Increased negative intrathoracic pressure: -8 to -12 cmH₂O (vs. normal -2 to -5 cmH₂O)
    • Results in peripheral airway collapse and atelectasis
    • V/Q mismatch increases physiologic shunt from 2-3% (normal) to 8-15%
  2. Blood Gas Abnormalities
    • Hypoxemia: PaO₂ often 65-80 mmHg on room air
    • Hypercapnia: PaCO₂ often 45-60 mmHg in moderate-severe cases
    • Compensatory metabolic alkalosis: HCO₃⁻ often 24-28 mEq/L
  3. Pulmonary Hypertension
    • Chronic hypoxic vasoconstriction leads to pulmonary arterial pressures of 30-45 mmHg (normal: 15-25 mmHg)
    • Right ventricular hypertrophy develops in 30-40% of severely affected animals
  4. Exercise Capacity
    • Maximum oxygen consumption:
      • Normal dogs: 130-140 mL/kg/min
      • Severe BOAS: 60-80 mL/kg/min
      • Normal cats: 90-110 mL/kg/min
      • Brachycephalic cats: 50-70 mL/kg/min
  5. Thermoregulation
    • Panting efficiency reduced by 40-60% in brachycephalic dogs
    • Critical temperature threshold lowered by 3-5°C in severe cases
    • Heat stress occurs at ambient temperatures of 26-28°C vs. 32-35°C in normal dogs

Quantitative Assessment Methods for Respiratory Function #

Plethysmography #

  • Barometric whole-body plethysmography values:
    • Enhanced pause (PENH) in normal dogs: 0.4-0.7
    • PENH in brachycephalic dogs: 0.8-2.5
    • PENH in normal cats: 0.3-0.6
    • PENH in brachycephalic cats: 0.7-1.8

Functional Grading Systems #

  • Functional Grading System for BOAS:
    • Grade 0: No respiratory noise/effort at rest or exercise
    • Grade I: No noise/effort at rest, mild noise/effort with exercise
    • Grade II: Mild noise/effort at rest, moderate with exercise
    • Grade III: Severe noise/effort at rest, severe respiratory distress with exercise or inability to exercise

Pulse Oximetry Parameters #

  • Oxygen saturation:
    • Normal dogs/cats: 95-100% at rest
    • Mild BOAS: 92-95% at rest
    • Moderate-severe BOAS: 85-92% at rest
    • Exercise desaturation: 5-15% decrease with moderate exercise in severe cases

References #

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Updated on February 27, 2025

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