Acute severe asthma

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Quick Overview

Acute severe asthma is a life-threatening emergency requiring immediate recognition and treatment. Misclassification of severity or delayed intervention significantly increases morbidity and mortality. NICE NG80 emphasizes structured assessment using objective criteria (PEF, oxygen saturation, respiratory rate) and escalating therapy based on severity markers.

Core Facts & Concepts

Severity Classification (NICE NG80)

SeverityPEFSpO₂Clinical Features
Moderate>50-75% best/predicted≥92%Increasing symptoms, no severe features
Acute Severe33-50% best/predicted≥92%RR ≥25/min, HR ≥110/min, inability to complete sentences
Life-Threatening<33% best/predicted<92%Silent chest, cyanosis, exhaustion, arrhythmia, hypotension, altered consciousness
Near-FatalAny severityAnyRaised PaCO₂ and/or requiring mechanical ventilation

Figure 1: Chest X-ray showing hyperinflated lungs with flattened hemidiaphragms

📊 Critical Thresholds

  • PEF <33% = life-threatening
  • SpO₂ <92% = life-threatening
  • Normal PaCO₂ (4.6-6.0 kPa) in acute asthma = exhaustion warning (should be low due to hyperventilation)
  • RR ≥25/min or HR ≥110/min = acute severe

Problem-Solving Approach

Immediate Management Algorithm (NICE NG80)

  1. Oxygen therapy: Target SpO₂ 94-98% via high-flow or reservoir mask
  2. Bronchodilators (within 15 minutes):
    • Salbutamol 5mg nebulized (back-to-back if life-threatening)
    • Ipratropium bromide 0.5mg nebulized (add in acute severe/life-threatening)
  3. Steroids (within 1 hour):
    • Prednisolone 40-50mg PO OR hydrocortisone 100mg IV
    • Continue 5-7 days (no taper needed)
  4. Magnesium sulfate 2g IV over 20 minutes if:
    • Life-threatening features OR
    • Acute severe not responding to initial bronchodilators

Figure 2: Arterial blood gas showing respiratory acidosis with elevated CO2

🚩 Red Flags for ICU Referral

  • Deteriorating PEF despite treatment
  • Persisting/worsening hypoxia (SpO₂ <92%)
  • Hypercapnia (PaCO₂ >6.0 kPa)
  • Exhaustion, drowsiness, altered consciousness
  • Respiratory arrest

Analysis Framework

Decision Points for Escalation

Key Discriminators: Severe vs Life-Threatening

FeatureAcute SevereLife-Threatening
Can speakIn wordsUnable/monosyllabic
Chest soundsWheeze presentSilent chest
Conscious levelAgitatedConfused/drowsy
PaCO₂Low (hyperventilation)Normal or high

Key Points Summary

PEF <33% or SpO₂ <92% = life-threatening asthma requiring immediate full treatment

Normal PaCO₂ in acute asthma = near-fatal (should be hypocapnic from hyperventilation)

First-line treatment: Oxygen (target 94-98%) + salbutamol 5mg nebulized + prednisolone 40-50mg

Magnesium sulfate 2g IV for life-threatening or poor response to initial bronchodilators (within 1 hour)

Add ipratropium 0.5mg to nebulizers in acute severe/life-threatening cases

ICU referral criteria: Deteriorating PEF, hypercapnia (PaCO₂ >6.0 kPa), exhaustion, altered consciousness

Silent chest = life-threatening (insufficient airflow to generate wheeze), NOT improvement

⚠️ Warning: Patients can deteriorate rapidly despite initial response-continuous monitoring essential for first 24 hours

Practice Questions: Acute severe asthma

Test your understanding with these related questions

A 56-year-old man presents with progressive dyspnea and chest tightness. He works in coal mining. Chest X-ray shows bilateral upper lobe nodules. What is the most likely diagnosis?

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Flashcards: Acute severe asthma

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Long-term oxygen therapy should not be offered to COPD patients who are _____

TAP TO REVEAL ANSWER

Long-term oxygen therapy should not be offered to COPD patients who are _____

active smokers

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