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Mechanical Ventilation - Complete Reference Guide

Source: Pfenninger & Fowler's Procedures for Primary Care, Morgan & Mikhail's Clinical Anesthesiology, Fishman's Pulmonary Diseases, Roberts & Hedges' Clinical Procedures in Emergency Medicine

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1. What is Mechanical Ventilation?

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2. Indications for Mechanical Ventilation

A. Inadequate Oxygenation

B. Inadequate Ventilation

C. Need for Airway Protection

• Hypoxic respiratory failure
• Hypercapnic respiratory failure
• Cardiac or respiratory arrest
• Coma or altered consciousness
• Paradoxical breathing pattern
• Failure of non-invasive ventilation (NIV)

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3. Ventilator Phases and Terminology

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4. Key Ventilator Parameters

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5. Modes of Ventilation

5.1 Overview Table

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5.2 Controlled Mode

• Delivers a preset VT at a specified rate regardless of patient effort
• Requires heavy sedation or paralysis to prevent patient-ventilator dyssynchrony
• Used for general anesthesia in the OR
• Advantage: Clinician controls minute ventilation with certainty

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5.3 Assist-Control (AC) Mode

• Each breath (assisted or controlled) delivers a preset VT
• Guarantees minimum VT regardless of lung compliance
• Risk: Auto-PEEP if patient has rapid spontaneous RR (e.g., COPD, asthma)

• Each breath delivers gas until a preset peak pressure is reached
• VT varies breath-to-breath based on lung compliance - can drop dangerously low in stiff lungs
• Advantage: Controls airway pressure; no risk of excess barotrauma from preset VT

• Hybrid mode: guarantees preset VT but uses a decelerating flow like pressure control
• Delivers VT at lower peak and mean airway pressures than VC
• Ventilator auto-adjusts pressure each breath to achieve target volume
• Preferred in changing compliance (e.g., pediatrics, ARDS)

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5.4 Synchronized Intermittent Mandatory Ventilation (SIMV)

• Delivers a preset number of mandatory breaths synchronized to patient effort
• Between mandatory breaths, patient breathes spontaneously without full assistance
• Spontaneous breaths can be augmented by adding Pressure Support (PS)
• Used for weaning by progressively decreasing mandatory RR (by 1-2/min)
• Disadvantage: May increase work of breathing compared to AC

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5.5 Pressure Support Ventilation (PSV)

• For spontaneously breathing patients only - no mandatory rate
• Every patient-initiated breath is augmented by preset pressure
• Minimum PS of 6-10 cmH₂O to overcome ETT resistance
• Higher PS → larger VT
• Cycling: inspiration ends when flow falls to ~25% of peak flow
• Used for comfort and as primary weaning mode

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6. Initial Ventilator Settings

Standard Protocol (Box 207-1 from Pfenninger & Fowler's):

• AC (VC or PRVC) if limited patient effort or heavy sedation
• SIMV if some respiratory effort or patient-ventilator dyssynchrony on AC

• Continuous cardiopulmonary monitoring
• VT, minute volume, airway pressures
• Serial ABGs
• ETCO₂ monitoring (recommended for weaning)

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7. Lung-Protective Ventilation (LPV)

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8. ARDS - FiO₂/PEEP Titration Tables

Low PEEP/High FiO₂ Strategy (ARDSNet):

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9. Disease-Specific Ventilator Strategies

9.1 Asthma / COPD (Obstructive Pattern)

• Auto-PEEP, breath stacking, barotrauma
• Hypotension from increased intrathoracic pressure → decreased venous return
• Pneumothorax (sudden deterioration + rise in PIP)

9.2 ARDS

9.3 Raised Intracranial Pressure (Neurological)

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10. Auto-PEEP

• Obstructive lung disease (asthma, COPD)
• High respiratory rate
• High I:E ratio (short expiratory time)
• High VT

• Increased functional residual capacity (FRC)
• Raised intrathoracic pressure
• Barotrauma risk
• Decreased cardiac output (impaired venous return)
• Difficult patient-triggering

• Decrease respiratory rate
• Decrease VT
• Increase inspiratory flow rate
• Decrease I:E ratio (shorten IT, lengthen ET)
• Bronchodilators
• Disconnect patient briefly if severe (20-30 seconds to allow lung emptying)

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11. Ventilator Complications

• Inadequate sedation → patient-ventilator dyssynchrony
• Plateau pressure > 30 cmH₂O → barotrauma risk
• Inadequate HOB elevation → VAP
• High PEEP → decreased cardiac output, increased ICP
• VT > 10 mL/kg PBW → VILI, acute renal failure

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12. High Airway Pressures - Troubleshooting

Pplat - PEEP = Driving pressure. If driving pressure > 15 cmH₂O in ARDS, mortality increases.

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13. Non-Invasive Ventilation (NIV)

• Cardiac or respiratory arrest
• Apnea
• Inability to protect airway
• Hemodynamic instability
• Uncooperative patient
• Copious secretions

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14. Weaning from Mechanical Ventilation

14.1 Readiness Criteria for Spontaneous Breathing Trial (SBT)

14.2 Conducting the SBT

• Perform: T-piece, or CPAP 5 cmH₂O + PS 6-8 cmH₂O, FiO₂ ≤0.4-0.5
• Duration: 30-120 minutes
• Daily sedation interruption before SBT

14.3 Terminate SBT (Failure Criteria)

14.4 Mechanical Criteria for Extubation

Patients who pass an SBT can be successfully extubated ~85% of the time. RSBI > 120 → retain ventilatory support.

14.5 Weaning Methods

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15. Post-Extubation / Post-operative Pulmonary Risk Reduction

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16. Equipment Required

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17. Summary of Common Ventilator Settings by Indication

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• Pfenninger & Fowler's Procedures for Primary Care, 3e - Chapter 207 (Mechanical Ventilation), pp. 1443-1446
• Morgan & Mikhail's Clinical Anesthesiology, 7e - Chapter 58, pp. 2519-2540
• Fishman's Pulmonary Diseases and Disorders - Chapters 141, pp. 2500-2502
• Rosen's Emergency Medicine - Chapter on Asthma, pp. 2071-2079
• ARDSNet ARMA Trial, NEJM 2000;342:1301-1308

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ABG Interpretation in Paediatrics: Beginner to Advanced

Sources: Current Surgical Therapy 14e, Barash Clinical Anesthesia 9e, Morgan & Mikhail 7e, Fishman's Pulmonary Diseases, Frameworks for Internal Medicine

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PART 1: FOUNDATIONS (Beginner)

1.1 What is an ABG?

• How well the lungs are oxygenating the blood
• How well the lungs/kidneys are regulating acid-base balance
• The body's ventilation status

1.2 Why ABG in Paediatrics?

• Higher metabolic rate - consume more O₂, produce more CO₂ per kg
• Smaller functional residual capacity - desaturate faster
• Immature kidneys - less reserve for bicarbonate regulation
• Normal values differ by age (especially neonates)
• Conditions like DKA, sepsis, bronchiolitis, asthma, congenital heart disease are common paediatric causes of ABG derangements

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1.3 Normal ABG Values

Adults and Children (>1 month)

Neonates (0-28 days) - Different Normal Values!

Key Paediatric Point: Neonates have lower HCO₃⁻ and slightly lower pH than adults because their kidneys are immature and cannot retain as much bicarbonate. PaO₂ targets are also lower to avoid oxygen toxicity (e.g., retinopathy of prematurity in premature infants).

Capillary vs Venous vs Arterial (Paediatric)

Capillary gas (commonly used in neonates) is useful for pH, PCO₂, and HCO₃⁻ but PO₂ is unreliable - use pulse oximetry for oxygenation.

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PART 2: STEP-BY-STEP INTERPRETATION SYSTEM

The 6-Step ABG Interpretation Approach

Step 1 → Is there acidaemia or alkalaemia?
Step 2 → What is the primary disorder (respiratory or metabolic)?
Step 3 → Is there appropriate compensation?
Step 4 → Calculate Anion Gap (if metabolic acidosis)
Step 5 → Delta-Delta / Delta Ratio (if high anion gap acidosis)
Step 6 → Assess Oxygenation

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STEP 1: Is the pH normal?

Use 7.40 as your baseline for all calculations - the body NEVER overcompensates, so if pH = 7.40 but both CO₂ and HCO₃ are abnormal, a mixed disorder exists.

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STEP 2: Identify the Primary Disorder

• CO₂ and pH move in opposite directions in respiratory disorders
• HCO₃⁻ and pH move in same direction in metabolic disorders

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STEP 3: Is Compensation Appropriate?

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STEP 4: Calculate the Anion Gap (if Metabolic Acidosis)

AG = Na⁺ - (Cl⁻ + HCO₃⁻)
Normal AG = 8-12 mEq/L (without albumin correction)

Always calculate AG even if no overt metabolic acidosis - it can unmask a mixed disorder.

Corrected AG = Measured AG + 2.5 × (4 - measured albumin in g/dL)

Anion Gap Metabolic Acidosis - MUDPILES Mnemonic

Non-Anion Gap (Hyperchloraemic) Metabolic Acidosis

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STEP 5: Delta-Delta Ratio (Advanced - Mixed Disorders)

Δ/Δ = (AG - 12) / (24 - HCO₃⁻)
       = Change in AG / Change in HCO₃⁻

Paediatric example: A child with DKA who has also been vomiting will have HAGMA + metabolic alkalosis → Δ/Δ > 2.0

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STEP 6: Assess Oxygenation

PaO₂/FiO₂ Ratio (P/F Ratio) - Used in Ventilated Children

P/F Ratio = PaO₂ ÷ FiO₂

Alveolar-Arterial (A-a) Gradient

A-a gradient = PAO₂ - PaO₂
PAO₂ = (FiO₂ × [Patm - PH₂O]) - (PaCO₂ / RQ)
      = (FiO₂ × 713) - (PaCO₂ / 0.8) [at sea level, on room air]
Normal A-a gradient: < 10-15 mmHg (increases with age and FiO₂)

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PART 3: THE 4 PRIMARY DISORDERS IN DETAIL

3.1 Respiratory Acidosis (↓pH, ↑CO₂)

Paediatric Causes:

• Increase minute ventilation (↑ RR or ↑ VT on ventilator)
• NIV (BiPAP) for COPD exacerbation / neuromuscular disease
• Treat underlying cause (bronchodilators, reversal agents, antibiotics)

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3.2 Respiratory Alkalosis (↑pH, ↓CO₂)

Paediatric Causes:

Salicylate toxicity pearls: ABG shows mixed respiratory alkalosis + high anion gap metabolic acidosis - a classic paediatric toxicology picture.

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3.3 Metabolic Acidosis (↓pH, ↓HCO₃⁻)

Severity Classification:

Bicarbonate Therapy Formula:

HCO₃⁻ deficit (mEq) = 1/3 × body weight (kg) × base deficit
Give half to correct, reassess

Caution: Do not correct metabolic acidosis with NaHCO₃ unless pH < 7.10-7.15 (in DKA, this is generally avoided as it worsens cerebral oedema risk)

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3.4 Metabolic Alkalosis (↑pH, ↑HCO₃⁻)

Paediatric Causes:

Projectile vomiting → loss of HCl
→ Hypochloraemic, hypokalaemic metabolic alkalosis
→ pH ↑, HCO₃⁻ ↑, Cl⁻ ↓, K⁺ ↓, PaCO₂ slightly ↑ (compensation)
→ Urine is paradoxically acidic (kidney retains Na⁺, excretes H⁺)
Treatment: Fluid resuscitation with 0.9% NaCl + KCl replacement BEFORE surgery

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PART 4: COMPENSATION STATES

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PART 5: BASE EXCESS / DEFICIT

HCO₃⁻ deficit = 1/3 × body weight (kg) × base deficit

Base deficit is particularly useful in paediatric trauma, sepsis, and post-resuscitation monitoring as a marker of illness severity and treatment response.

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PART 6: MIXED ACID-BASE DISORDERS

Clues to Mixed Disorders:

1. pH is normal but CO₂ and HCO₃⁻ are both abnormal
2. Compensation does not match the expected formula
3. Delta-delta ratio outside 1-2

Common Mixed Disorders in Paediatrics:

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PART 7: HENDERSON-HASSELBALCH & pH-[H⁺] RELATIONSHIP

pH = 6.1 + log ([HCO₃⁻] / 0.03 × PaCO₂)

[H⁺] = 24 × PaCO₂ / [HCO₃⁻]

• For every 1 mmHg rise in PaCO₂ from baseline → pH falls by 0.008
• For every 1 mmHg fall in PaCO₂ from baseline → pH rises by 0.008

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PART 8: WORKED PAEDIATRIC EXAMPLES

Example 1 (Beginner) - Acute Asthma

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Example 2 (Intermediate) - Diabetic Ketoacidosis

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Example 3 (Advanced) - Septic Shock + Respiratory Failure

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Example 4 (Paediatric Neonatal) - Preterm with RDS

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PART 9: QUICK REFERENCE SUMMARY CHART

ABG Quick Cheat Sheet

Paediatric-Specific Normal Ranges at a Glance

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PART 10: KEY PAEDIATRIC CLINICAL PATTERNS

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PART 11: QUICK INTERPRETATION FLOWCHART

ABG Result
    │
    ├── Step 1: pH?
    │     ├── <7.35 → ACIDAEMIA
    │     ├── >7.45 → ALKALAEMIA
    │     └── 7.35-7.45 → Normal (check if mixed!)
    │
    ├── Step 2: CO₂ and HCO₃⁻?
    │     ├── pH↓ + CO₂↑ → Respiratory Acidosis
    │     ├── pH↓ + HCO₃⁻↓ → Metabolic Acidosis
    │     ├── pH↑ + CO₂↓ → Respiratory Alkalosis
    │     └── pH↑ + HCO₃⁻↑ → Metabolic Alkalosis
    │
    ├── Step 3: Does compensation match formula?
    │     ├── Yes → Simple disorder
    │     └── No → MIXED DISORDER
    │
    ├── Step 4: If Metabolic Acidosis → Calculate AG
    │     ├── AG > 12 → HAGMA (MUDPILES)
    │     └── AG normal → Non-AG acidosis (HARDUPS/diarrhoea)
    │
    ├── Step 5: If HAGMA → Calculate Δ/Δ
    │     ├── <1.0 → HAGMA + non-AG acidosis
    │     ├── 1-2 → Pure HAGMA
    │     └── >2.0 → HAGMA + metabolic alkalosis
    │
    └── Step 6: Oxygenation
          ├── PaO₂ < 60 → Significant hypoxaemia
          └── P/F ratio < 300 → ARDS spectrum

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• Current Surgical Therapy 14e - ABG Interpretation section, pp. 1608-1609
• Barash Clinical Anesthesia 9e - Chapter 16, Table 16-8/16-9, pp. 1158-1162
• Morgan & Mikhail's Clinical Anesthesiology 7e - Chapter 58, Table 58-4
• Fishman's Pulmonary Diseases - Chapter 141
• ATS ABG Interpretation Guidelines