4.1 Upper Airway Obstruction: BOAS, Laryngeal Paralysis & Tracheal Collapse

Upper Airway Obstruction: BOAS, Laryngeal Paralysis & Tracheal Collapse

Core Principle: Upper airway obstruction (UAO) is one of the most rapidly fatal emergencies encountered in veterinary critical care. Dynamic airflow limitation generates massive negative inspiratory intrathoracic pressures, leading to severe laryngeal and pharyngeal edema, progressive hyperthermia from the mechanical work of breathing, dynamic collapse of cartilaginous structures, and fatal asphyxiation or non-cardiogenic pulmonary edema if not relieved immediately.

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1. Anatomy & Pathophysiology of Brachycephalic Obstructive Airway Syndrome (BOAS)

Brachycephalic Obstructive Airway Syndrome (BOAS) is a complex congenital and acquired disorder resulting from selective breeding for chondrodysplastic shortening of the facial skeleton without a proportional reduction in upper airway soft tissue mass.

[Selective Craniofacial Shortening]
               │
 ┌─────────────┴─────────────┐
 ▼                           ▼
Primary Anatomic Defects   Excess Soft Tissue Resistance
• Stenotic Nares            • Elongated Soft Palate
• Hypoplastic Trachea       • Aberrant Nasal Turbinates
 └─────────────┬─────────────┘
               ▼
 [Massive Negative Inspiratory Airway Pressure (Poiseuille's Law)]
               ▼
 [Secondary Soft Tissue Edema & Cartilage Fatigue]
               │
 ┌─────────────┼─────────────┐
 ▼             ▼             ▼
Grade I       Grade II      Grade III
Everted       Cuneiform     Corniculate Collapse
Saccules      Collapse      (Total Airway Closure)

Primary Anatomic Abnormalities

• Stenotic Nares: Severely narrowed, pinched, or collapsed external nostrils that markedly increase upper airway resistance during nasal inspiration. According to Poiseuille's Law ($R \propto 1/r^4$), reducing airway radius by half increases resistance to flow by 16-fold.
• Elongated Soft Palate: The soft palate extends caudally past the tip of the epiglottis, hanging into the rima glottidis and creating high-velocity turbulent airflow, mucosal vibration, and stertorous breathing.
• Hypoplastic Trachea: Abnormally small tracheal diameter characterized by rigid, overlapping cartilage rings and a narrow tracheal lumen (tracheal-to-thoracic inlet ratio $<0.16$ in non-Bulldog brachycephalics, $<0.11$ in English Bulldogs).
• Aberrant / Confluent Nasal Turbinates (Rostral/Caudal): Turbinate bone and mucosal tissue protruding into the nasopharynx, obstructing the ventral nasal meatus.

Secondary Acquired Pathophysiology: Laryngeal Collapse

Chronic, severe negative intraluminal pressure generated during inspiration acts as a powerful suction force that stretches, inflames, and weakens laryngeal soft tissues and cartilages over time. This leads to Laryngeal Collapse, staged in three progressive grades:

Associated Gastrointestinal Pathology

Chronic high negative intrathoracic pressure pulls the stomach cranially, predisposing brachycephalic dogs to sliding hiatal hernia, gastroesophageal reflux disease (GERD), chronic esophagitis, delayed gastric emptying, and severe aspiration pneumonia.

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2. Laryngeal Paralysis & GOLPP

Laryngeal paralysis is characterized by the failure of the arytenoid cartilages and vocal folds to abduct during inspiration, resulting in fixed or paradoxical upper airway obstruction.

Etiology & Neuropathology

• Geriatric Onset Laryngeal Paralysis Polyneuropathy (GOLPP): The most common form in dogs, typically affecting geriatric, large-to-giant breeds (Labrador Retrievers, Golden Retrievers, Newfoundlands, Saint Bernards). GOLPP is a slowly progressive, generalized peripheral axonopathy affecting the longest nerves first.
• Recurrent Laryngeal Nerve Dysfunction: The recurrent laryngeal nerve innervates the cricoarytenoideus dorsalis (CAD) muscle, which is the sole abductor of the arytenoid cartilage.
• Paradoxical Vocal Fold Movement: On inspiration, instead of abducting widely to open the rima glottidis, the denervated arytenoid cartilages are passively sucked inward toward the midline by negative inspiratory pressure, severely occluding the airway.

Clinical Presentation & Risk Factors

• Classic inspiratory stridor (high-pitched wheeze/whistle generated at the larynx) and voice change (loss of bark / hoarse bark).
• Exercise intolerance, panting, anxiety, and progressive hyperthermia.
• High risk of acute decompensation during warm, humid weather or stressful events.
• Concomitant subclinical esophageal dysfunction (megaesophagus) and generalized hindlimb weakness/proprioceptive deficits as part of GOLPP, predisposing to aspiration pneumonia.

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3. Tracheal Collapse: Mechanics & Grading

Tracheal collapse is a dynamic, progressive disease characterized by hypocellularity of tracheal cartilage rings (loss of glycosaminoglycans and chondroitin sulfate), replacement of hyaline cartilage with fibrous tissue, and laxity of the dorsal tracheal membrane (trachealis muscle).

               DYNAMIC TRACHEAL MECHANICS

     INSPIRATION                   ACTIVE EXPIRATION
  (Negative Airway)               (Positive Pleural)
         │                                │
         ▼                                ▼
┌─────────────────┐              ┌─────────────────┐
│ Cervical Rings  │              │ Intrathoracic   │
│ COLLAPSE        │              │ Rings COLLAPSE  │
└─────────────────┘              └─────────────────┘
(Intraluminal < Ambient)         (Pleural > Intraluminal)

Dynamic Respiratory Mechanics

1. Cervical Tracheal Collapse (Extrathoracic):
   • During inspiration, atmospheric pressure exceeds the subatmospheric intraluminal pressure inside the cervical trachea. The weakened cervical rings and redundant dorsal membrane collapse inward.
   • Causes inspiratory dyspnea and inspiratory stridor.
2. Intrathoracic Tracheal & Carinal Collapse:
   • During active expiration, intrathoracic pleural pressure becomes highly positive to force air out of the lungs. This external positive pleural pressure exceeds the intraluminal airway pressure, causing dynamic collapse of the intrathoracic trachea and mainstem bronchi.
   • Causes expiratory dyspnea, prolonged expiratory phase, and expiratory "clicks" or wheezes.

Tangner & Hobson Tracheal Collapse Grading Scheme

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4. Emergency Management of the Upper Airway Crisis

When an animal presents in acute upper airway distress, immediate physical restraint for IV catheters or radiographs can trigger catastrophic respiratory and cardiac arrest. The "hands-off" stabilization protocol must take precedence.

[Acute Upper Airway Crisis Presentation]
                   │
         Hands-Off Triage & Flow-By O2
                   │
       Sedation & Anti-Inflammatory
• Butorphanol (0.2–0.4 mg/kg IV/IM)
• Acepromazine (0.005–0.02 mg/kg) OR Midazolam (0.2–0.3 mg/kg)
• Dexamethasone SP (0.1–0.2 mg/kg IV)
                   │
     Assess Temperature (>104°F / 40°C)
                   │
         Active Cooling Protocols
   (Stop at 103.0°F / 39.4°C immediately)
                   │
        ┌──────────┴──────────┐
    Stabilized            Refractory /
 (Quiet ICU Cage)        Arrest Impending
                              │
                     Emergency Intubation
                     or Tracheostomy

Step 1: Hands-Off Triage & Flow-By Oxygen

• Place the patient in a cool, quiet, climate-controlled oxygen cage or provide gentle flow-by oxygen (2–3 L/min held 2–4 cm from the nares).
• Avoid tight face masks, neck leads, muzzle wraps, or aggressive physical restraint.

Step 2: Emergency Sedation Protocols

Breaking the cycle of anxiety, air hunger, turbulent airflow, and progressive laryngeal edema is essential:

• Butorphanol: $0.2\text{ to }0.4\text{ mg/kg IV/IM}$ — Provides mild sedation, potent antitussive action, and minimal cardiovascular or respiratory depression.
• Combined with Micro-Dose Acepromazine: $0.005\text{ to }0.02\text{ mg/kg IV/IM/SC}$ — Potent phenothiazine anxiolytic and muscle relaxant that relieves laryngeal spasm. Caution: Use lowest dose and avoid in hypovolemic, hypotensive, or dehydrated patients.
• Alternative (Midazolam): $0.2\text{ to }0.3\text{ mg/kg IV/IM}$ — Preferred in geriatric, hemodynamically unstable, or cardiovascularly compromised patients, though it may cause paradoxical excitation in some dogs.

Step 3: Anti-Inflammatory Glucocorticoids

• Dexamethasone Sodium Phosphate (Dex SP): $0.1\text{ to }0.2\text{ mg/kg IV}$ (or IM) — Rapidly reduces acute mucosal edema and laryngeal swelling without mineralocorticoid fluid retention. Avoid long-acting or repository steroids.

Step 4: Active Cooling for Severe Hyperthermia

Extreme muscular exertion from breathing and loss of panting heat dissipation cause severe hyperthermia ($>104^{\circ}\text{F} / 40^{\circ}\text{C}$ to $>107^{\circ}\text{F} / 41.7^{\circ}\text{C}$):

• Wet the fur with tepid-to-cool water and place ambient fans over the body to maximize evaporative cooling.
• Apply cold packs wrapped in towels to the axillary and inguinal regions.
• Critical Endpoint: Halt active cooling once core temperature reaches $103.0^{\circ}\text{F}$ ($39.4^{\circ}\text{C}$) to prevent rapid hypothermic undershoot and severe rebound vasoconstriction.

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5. Emergency Intubation & Temporary Tracheostomy

If sedation, cooling, and oxygen fail to relieve cyanosis, or if the patient becomes obtunded or develops agonal respirations, definitive airway control must be established immediately.

Emergency Orotracheal Intubation

• Perform rapid sequence induction using propofol (2–4 mg/kg IV to effect) or alfaxalone (1–2 mg/kg IV to effect).
• Have an assortment of endotracheal tubes available, including tubes 1 to 2 sizes smaller than expected due to severe laryngeal edema or tracheal hypoplasia.
• Use a rigid stylet and direct laryngoscopy with a Miller or Macintosh blade to visualize the vocal folds.

Emergency Temporary Tracheostomy Technique

Indicated when oral intubation cannot bypass an anatomical obstruction (e.g., Grade III laryngeal collapse, massive pharyngeal neoplasm, severe trauma, foreign body):

1. Patient Positioning: Sternal recumbency with the neck extended over a rolled towel, or dorsal recumbency with ventral neck clipped and prepared with rapid chlorhexidine scrub.
2. Incision: Ventral midline cervical incision from the cricoid cartilage extending 3–5 cm caudally. Separate the sternohyoid muscles along the median raphe.
3. Tracheal Entry: Expose the trachea and perform a transverse incision between the 3rd and 4th or 4th and 5th tracheal rings. Incise no more than 50% of the tracheal circumference to avoid ring transection and subsequent tracheal stenosis.
4. Stay Sutures: Place two sturdy non-absorbable monofilament stay sutures (2-0 or 0 nylon/polypropylene):
   • Cranial stay suture: Looped around the tracheal ring immediately cranial to the incision.
   • Caudal stay suture: Looped around the tracheal ring immediately caudal to the incision.
   • Label and tape stay sutures to the neck; pulling the cranial suture dorsally and the caudal suture ventrally opens the tracheostomy stoma during tube changes.
5. Tube Selection: Use a silicone or PVC cuffed or uncuffed tracheostomy tube with a removable inner cannula sized to approximately 50–75% of the tracheal lumen diameter.

        TEMPORARY TRACHEOSTOMY LANDMARKS & STAY SUTURES

                   [Cricoid Cartilage]
                           │
               Ring 1      │
               Ring 2      │
         ═══════════════════════════════
         Ring 3  ─────[Cranial Stay Suture]───── (Pull UP)
     ─── Transverse Incision (Max 50% Circumference) ───
         Ring 4  ─────[Caudal Stay Suture]────── (Pull DOWN)
         ═══════════════════════════════
               Ring 5      │
               Ring 6      │

Tracheostomy Tube Nursing Care & Maintenance

Patients with tracheostomy tubes have lost the natural warming, humidification, and filtration of the upper airway, requiring 24-hour continuous 1-on-1 monitoring:

• Sterile Suctioning: Perform sterile endotracheal suctioning every 2 to 4 hours (and PRN). Pre-oxygenate for 2–3 minutes with 100% O2, instill 0.5 to 2.0 mL of sterile 0.9% saline, insert a sterile suction catheter to the end of the tube, and apply suction for $<10\text{ seconds}$ while rotating the catheter on withdrawal.
• Humidification & Nebulization: Administer saline nebulization every 2 to 4 hours to prevent mucus desiccating into a rock-hard lumen-occluding plug.
• Cuff Pressure Management: If using a cuffed tube, maintain cuff pressure $<20\text{ cmH}_2\text{O}$ (using a cuff manometer) to prevent tracheal mucosal ischemia, pressure necrosis, and tracheal perforation.
• Emergency Preparedness: Always keep a spare tracheostomy tube (same size and one size smaller), sterile obturator, sterile water-soluble lubricant, suction unit, and suture scissors taped directly to the patient's ICU kennel.