Assessing Respiratory Emergencies

Overview – Assessing Respiratory Emergencies

Assessing respiratory emergencies involves rapid clinical evaluation, supported by targeted investigations to identify airway compromise, impaired ventilation, or gas exchange failure. This article provides a structured approach combining primary survey (ABC), examination, and interpretation of key diagnostic tools like pulse oximetry and arterial blood gases (ABGs). Early intervention can be life-saving in respiratory failure.

Initial Assessment – ABC

• A: Airway: Check patency, obstruction, trauma, or swelling
• B: Breathing: Rate, depth, effort, oxygen saturation
• C: Circulation: BP, pulse, signs of perfusion

In-Depth Assessment

Appearance – “End-of-the-bed” Observations

• Level of consciousness
• Sweating
• Agitation
• Cyanosis or pallor
• Urticaria or angioedema

History

• Onset and progression of symptoms
• Nature of symptoms (e.g. dyspnoea, cough, wheeze)
• Associated features (e.g. fever, chest pain)
• Past episodes and treatment so far
• Relevant past medical history

Examination

• Respiratory rate (most sensitive vital sign in deterioration)
• Airway patency: Look for obstruction, stridor, swelling
• Work of breathing: Use of accessory muscles, nasal flaring
• Auscultation: Air entry, wheezes, crackles
• Percussion: Hyperresonance or dullness
• Tracheal deviation
• Jugular venous pressure (JVP)
• Pulse and blood pressure

Monitoring

• Oxygen saturation (SpO₂)
• Full set of vital signs

Investigations

• Arterial blood gas (ABG)
• Chest X-ray
• ECG
• Spirometry (when stable)
• Sputum culture

Pulse Oximetry – Key Concepts

• Higher O₂ saturation = higher arterial pO₂
• Based on the oxygen-haemoglobin dissociation curve:
  • Plateau phase favours O₂ loading
  • Steep phase favours O₂ unloading in tissues

Factors That Shift the Curve

• Right shift (favouring O₂ unloading):
  • Acidosis (↓pH)
  • Hypercapnia (↑pCO₂) – Bohr effect
  • Raised 2,3-BPG (seen in hypoxia)
  • Hyperthermia (e.g. exercise)

Clinical Implications of Arterial pO₂

• 90 mmHg: Normal
• ~55 mmHg: Euphoria, memory loss
• 30–55 mmHg: Loss of cognitive/motor function
• <30 mmHg: Unconsciousness

Arterial Blood Gas (ABG) Analysis

What ABGs Tell Us

• Oxygenation: PaO₂
• Ventilation: PaCO₂
• Acid–base status: pH, HCO₃⁻, base excess
• A-a gradient: Identifies gas exchange abnormalities

Normal Values

Interpreting Acid-Base Disturbance

Acidosis

• pH < 7.35
• Respiratory acidosis: Hypoventilation → ↑pCO₂
  • Compensation: Renal HCO₃⁻ retention
• Metabolic acidosis: Loss of HCO₃⁻ or gain of acid
  • Compensation: Hyperventilation (↓pCO₂)

Alkalosis

• pH > 7.45
• Respiratory alkalosis: Hyperventilation → ↓pCO₂
  • Compensation: Renal HCO₃⁻ excretion
• Metabolic alkalosis: H⁺ loss or excess base
  • Compensation: Hypoventilation (↑pCO₂)

Buffer System Recap

• Bicarbonate buffer:
  H⁺ + HCO₃⁻ ⇌ H₂CO₃ ⇌ CO₂ + H₂O
• Acidosis: Add HCO₃⁻ or reduce CO₂
• Alkalosis: Add H⁺ or raise CO₂

Anion Gap

• AG = [Na⁺] – ([Cl⁻] + [HCO₃⁻])
• High AG Metabolic Acidosis:
  • Lactic acidosis
  • Ketoacidosis
  • Renal failure
• Normal AG Metabolic Acidosis:
  • GI loss (diarrhoea)
  • Renal loss (renal tubular acidosis)

A-a Gradient

• A-a Gradient = Alveolar pO₂ – Arterial pO₂
• Normal value: <12 mmHg
• Elevated gradient → V/Q mismatch (e.g. PE, pneumonia, fibrosis)

6-Step ABG Interpretation Strategy

1. Assess pH: Acidosis or alkalosis?
2. Check pCO₂: Is it consistent with the pH?
3. Check HCO₃⁻: Metabolic contribution?
4. Identify primary disturbance
5. Evaluate compensation:
   • Base excess
   • Respiratory/metabolic signs of adjustment
6. Summarise your findings

Summary – Assessing Respiratory Emergencies

Assessing respiratory emergencies involves a structured approach to appearance, examination, and investigation. Tools like ABG analysis and pulse oximetry help differentiate between ventilation and oxygenation problems, enabling rapid, targeted management. For a broader context, see our Emergency Medicine Overview page.