---
title: Comparative Physiologic Parameters
description: "### 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…"
url: "https://anesthesiabraintrust.org/docs/comparative-physiologic-parameters/"
updated: "2025-02-27"
category: Knowledge Home / Case Studies / Respiratory
---

# Comparative Physiologic Parameters

### 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

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