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 #
- 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×
- Prevalence:
- 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%
- Anatomical measurements:
- 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
- Tracheal diameter measurements:
- Everted Laryngeal Saccules
- Secondary change: Results from chronic high negative pressure
- Airway narrowing: Reduces laryngeal cross-sectional area by 25-50%
- 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%
- Staging system:
Physiologic Derangements in BOAS #
- 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%
- 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
- 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
- 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
- Maximum oxygen consumption:
- 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
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