DNB Anaesthesia Exam Answers

4a) Weaning Criteria in ICU [5 Marks]

Definition

Step 1: Pre-Weaning Assessment Criteria (Readiness Criteria)

Step 2: Weaning Predictor Tests (Screening)

• f/VT = Respiratory frequency (breaths/min) / Tidal volume (L)
• Threshold: f/VT < 105 predicts successful weaning
• Sensitivity ~0.90 in multiple studies
• A patient breathing at 30 breaths/min with a tidal volume of 300 mL = f/VT of 100 (likely to succeed)
• Limitation: it is a screening test only - a confirmatory SBT is still required

• Negative Inspiratory Force (NIF) / MIP: more negative than -20 to -30 cmH₂O
• Minute ventilation < 10 L/min
• Vital capacity > 10-15 mL/kg

Step 3: Spontaneous Breathing Trial (SBT)

• No marked anxiety or dyspnea
• Respiratory rate < 35 breaths/min
• SpO₂ > 90%
• Systolic BP between 90 and 180 mmHg
• Heart rate change < 20% from baseline

Step 4: Weaning Methods

1. T-tube trials - Unassisted spontaneous breathing for 30-120 min, once daily. If successful, extubate. If failed, give 24h full ventilatory rest before retry. Evidence shows once-daily SBT is as effective as multiple daily trials.
2. Pressure Support Ventilation (PSV) weaning - Gradually reduce PSV in decrements of 3-6 cmH₂O until minimal level (typically 5-8 cmH₂O).
3. SIMV weaning - Reduce mandatory rate in steps; least preferred method as even 2-3 mandatory breaths/min can maintain acceptable ABGs while work of breathing remains excessive, risking respiratory muscle fatigue.
4. Automated weaning (SmartCare/ASV) - Closed-loop systems that automatically titrate support based on RR and minute ventilation; shown to reduce weaning time.

Step 5: Post-Extubation Risk Stratification

• Age > 65 years
• Congestive heart failure
• COPD
• APACHE-II score > 12
• BMI > 30
• Significant secretions

• 2 medical comorbidities

• 7 days on mechanical ventilation

4b) HACOR Scoring for Non-Invasive Ventilation [5 Marks]

Introduction

HACOR Score Variables and Points

Interpretation and Cut-off

• HACOR score > 5 at 1 hour of NIV predicts NIV failure in hypoxemic patients with accuracy > 80%
• If HACOR > 5 at 1 hour: consider early intubation - continuing NIV risks delayed intubation, which carries higher mortality
• Consciousness (GCS) carries the highest weight (up to 10 points) - reflecting that altered mentation is the most powerful single predictor of NIV failure

Clinical Context and Application

1. When to use: In hypoxemic ARF (not primarily hypercapnic COPD exacerbations - those are assessed differently). Especially applicable in ARDS, de novo hypoxemic failure, and immunocompromised patients.
2. Predictors of NIV failure incorporated in HACOR include:

• High APACHE II/SOFA score
• Shock
• Increased heart rate
• Acidosis
• Altered consciousness
• Tachypnea
• Severe hypoxemia (PaO₂/FiO₂ < 150 mmHg independently predicts failure)

1. Serial assessment: HACOR should be re-evaluated at 1h, 6h, and 12h. Persistent or worsening score mandates intubation.
2. Special populations where HACOR is particularly useful:

• ARDS - NIV failure rates are 22-47% based on severity; HACOR helps identify early failures
• Immunocompromised patients - NIV failure carries > 70% mortality in hematological malignancies; early prediction is essential
• Post-operative - helps triage early reintubation vs. continued NIV

Summary Table for Quick Recall

1. Harrison's Principles of Internal Medicine, 22e (2025), Chapter 313 - Liberation from Mechanical Ventilation
2. Murray & Nadel's Textbook of Respiratory Medicine, Table 136.3 - HACOR Score
3. Fishman's Pulmonary Diseases and Disorders, 5e - Chapters on Weaning and Weaning Trials
4. Duan J, Han X, Bai L, et al. Assessment of heart rate, acidosis, consciousness, oxygenation, and respiratory rate to predict noninvasive ventilation failure in hypoxemic patients. Intensive Care Med. 2017;43(2):192-199.
5. Paul L. Marino, The ICU Book, 4th ed. - Principles of ventilator weaning and liberation