Mechanical Ventilation · Basics for Medical Students

⚙️ Mechanical Ventilation Basics A Medical Student's Guide to Modes, Settings & Management

1. Indications for Mechanical Ventilation

Hypoxemic Respiratory Failure (Type I)

PaO₂ <60 mmHg on high FiO₂
Refractory hypoxemia (shunt)
Examples: ARDS, pneumonia, pulmonary edema

Hypercapnic Respiratory Failure (Type II)

PaCO₂ >50 mmHg with pH <7.25
Impending respiratory arrest
Examples: COPD exacerbation, neuromuscular disease, overdose

Airway Protection: Altered mental status (GCS <8), inability to clear secretions.
Increased Work of Breathing: Severe asthma, post‑extubation stridor, flail chest.
Hemodynamic Instability: Shock states to reduce oxygen consumption of respiratory muscles.

2. Non‑Invasive Ventilation (NIV): CPAP & BiPAP

Mode	Mechanism	Indications	Contraindications
CPAP (Continuous Positive Airway Pressure)	Constant pressure throughout respiratory cycle	Cardiogenic pulmonary edema, obstructive sleep apnea	Altered mental status, inability to protect airway, facial trauma, excessive secretions, hemodynamic instability, untreated pneumothorax
BiPAP (Bilevel Positive Airway Pressure)	IPAP (inspiratory) + EPAP (expiratory). Pressure support = IPAP − EPAP	COPD exacerbation, acute hypercapnic respiratory failure, neuromuscular weakness

💡 NIV Titration Pearls: Start IPAP 8–10, EPAP 4–5 cm H₂O. Increase IPAP to improve ventilation (↓PaCO₂), increase EPAP to improve oxygenation (recruit alveoli). Avoid IPAP >20 (risk gastric insufflation).

3. Invasive Mechanical Ventilation: Core Modes

Volume‑Controlled vs. Pressure‑Controlled Ventilation

Feature	Volume Control (VC / AC‑VC)	Pressure Control (PC / AC‑PC)
Set parameter	Tidal Volume (Vᴛ)	Inspiratory Pressure (above PEEP)
Resulting variable	Pressure varies with compliance	Tidal volume varies with compliance
Advantage	Guaranteed minute ventilation	Limits barotrauma, improves synchrony
Disadvantage	Risk of high pressures (barotrauma)	Hypoventilation if compliance drops suddenly
Waveform	Square flow pattern	Decelerating flow pattern

Common Ventilator Modes Explained

Mode	Description	Clinical Use
Assist‑Control (AC)	Every breath (triggered or mandatory) delivers full set Vᴛ or pressure. Patient can trigger additional breaths.	Initial mode for most critically ill patients. Guarantees support but may cause hyperventilation.
Synchronized Intermittent Mandatory Ventilation (SIMV)	Set number of mandatory breaths; spontaneous breaths between mandatory breaths receive only pressure support (or none).	Weaning mode (though less favored now). Reduces respiratory muscle atrophy.
Pressure Support Ventilation (PSV)	Spontaneous breathing mode; patient triggers all breaths, ventilator provides set pressure boost.	Weaning (spontaneous breathing trials) and for comfortable spontaneous ventilation.
Airway Pressure Release Ventilation (APRV)	High continuous pressure (P‑high) with brief releases (T‑low) to allow CO₂ clearance. Inverse I:E ratio.	Severe ARDS with refractory hypoxemia; improves oxygenation and may reduce sedation needs.

4. Initial Ventilator Settings (AC‑VC Mode)

Setting	Typical Starting Value	Rationale / Adjustment
FiO₂	100% (then titrate down)	Target SpO₂ 88–95% (or 94–98% non‑COPD). Wean FiO₂ to ≤60% ASAP to avoid O₂ toxicity.
Tidal Volume (Vᴛ)	6–8 mL/kg ideal body weight (IBW)	ARDS: 6 mL/kg IBW. IBW male: 50 + 2.3 × (height in inches − 60); female: 45.5 + 2.3 × (height in inches − 60).
Respiratory Rate (RR)	12–16 breaths/min	Adjust to achieve desired PaCO₂ and pH. Avoid auto‑PEEP with high rates.
PEEP	5 cm H₂O	Prevents atelectasis. Increase for hypoxemia (ARDS). Caution: high PEEP can ↓ venous return.
Inspiratory Flow Rate	60 L/min	Higher flow → shorter inspiratory time, longer expiration (helps in obstructive disease).
I:E Ratio	1:2 to 1:3	Normal inspiration ~1 sec, expiration ~2 sec. Inverse ratio (2:1) used in APRV for oxygenation.

⚠️ Ideal Body Weight is CRITICAL: Using actual body weight leads to volutrauma (excessive tidal volumes). Always calculate IBW.

5. ARDSNet Protocol: Lung‑Protective Ventilation

Target Vᴛ: 6 mL/kg IBW (may reduce to 4 mL/kg if plateau pressure >30).
Plateau Pressure (Pplat) Goal: ≤30 cm H₂O.
Permissive Hypercapnia: Allow PaCO₂ to rise (pH ≥7.15–7.20) to avoid high pressures.
PEEP / FiO₂ Table: Use ARDSNet PEEP‑FiO₂ ladder to optimize oxygenation while minimizing FiO₂.

FiO₂	0.3	0.4	0.5	0.6	0.7	0.8	0.9	1.0
PEEP (Lower)	5	5–8	8–10	10	10–14	14	14–18	18–24

Prone Positioning: Recommended for moderate‑severe ARDS (PaO₂/FiO₂ <150) for ≥16 hours/day. Improves V/Q matching and reduces mortality.

6. Ventilator Mechanics: Key Pressures

Peak Inspiratory Pressure (PIP): Sum of airway resistance + elastic recoil.
Plateau Pressure (Pplat): Measured during inspiratory hold (no flow). Reflects alveolar pressure / compliance.
Driving Pressure (ΔP) = Pplat − PEEP. Lower driving pressure (<15 cm H₂O) associated with improved survival in ARDS.

↑ PIP, Normal Pplat

→ Increased airway resistance
Examples: bronchospasm, mucus plug, kinked ETT, biting tube

↑ PIP, ↑ Pplat

→ Decreased lung compliance
Examples: ARDS, pulmonary edema, pneumothorax, abdominal distension

7. Auto‑PEEP (Intrinsic PEEP)

Incomplete exhalation before next breath → progressive air trapping.
Risk factors: High RR, high Vᴛ, obstructive lung disease (COPD, asthma).
Recognition: Expiratory flow does not return to zero on waveform. Measured by expiratory hold.
Consequences: Hypotension (↓ venous return), barotrauma, increased work of breathing.
Management: Decrease RR, decrease Vᴛ, increase inspiratory flow (prolong expiration), treat bronchospasm.

8. Weaning from Mechanical Ventilation

Readiness Criteria (Daily Spontaneous Breathing Trial — SBT)

Reversal of underlying cause of respiratory failure.
Adequate oxygenation: PaO₂/FiO₂ >150–200, PEEP ≤8, FiO₂ ≤0.4–0.5.
Hemodynamic stability (off or low‑dose vasopressors).
Intact respiratory drive and ability to protect airway (cough, minimal secretions).

Spontaneous Breathing Trial (SBT) Methods

Pressure Support (PS) 5–8 + PEEP 5: Overcomes ETT resistance; most common.
T‑Piece Trial: Disconnect from vent, provide humidified O₂. Most challenging.
Duration: 30–120 minutes.

SBT Failure Criteria

RR >35 or <8 breaths/min
SpO₂ <90% on FiO₂ ≥0.5
Tachycardia >140 or sustained change >20%
Systolic BP >180 or <90 mmHg
Anxiety, diaphoresis, accessory muscle use
pH <7.32 with ↑ PaCO₂

Rapid Shallow Breathing Index (RSBI)

    RSBI = Respiratory Rate (breaths/min) / Tidal Volume (liters)

    RSBI <105 predicts weaning success.

9. Complications of Mechanical Ventilation

Complication	Mechanism	Prevention / Management
Ventilator‑Associated Pneumonia (VAP)	Aspiration of colonized secretions around ETT cuff	Head of bed >30°, subglottic suction, daily SBT, oral care with chlorhexidine
Barotrauma / Volutrauma	Excessive pressure (Pplat >30) or volume	Low Vᴛ strategy, limit Pplat, monitor driving pressure
Ventilator‑Induced Diaphragm Dysfunction	Disuse atrophy from controlled ventilation	Daily SBT, maintain spontaneous breathing efforts
Hemodynamic Compromise	↑ intrathoracic pressure ↓ venous return (preload)	Optimize PEEP, consider fluids, lower Vᴛ if auto‑PEEP
Oxygen Toxicity	Free radical injury from high FiO₂	Wean FiO₂ to ≤0.6 as soon as possible
Auto‑PEEP / Air Trapping	Incomplete exhalation	Decrease RR, increase flow, treat bronchospasm

10. Ventilator Alarms & Troubleshooting

Alarm	Common Causes	Initial Action
High Pressure Alarm	Secretions, biting tube, bronchospasm, pneumothorax, coughing, kinked circuit	Listen to breath sounds, suction, check ETT position, assess for pneumothorax
Low Pressure / Low Volume Alarm	Disconnection, leak in circuit, cuff leak, ETT displacement	Check connections, assess cuff pressure, confirm ETT depth
Low Minute Ventilation Alarm	Apnea, patient not triggering, sedation	Check patient effort, ensure backup rate set
High Respiratory Rate Alarm	Pain, anxiety, metabolic acidosis, hypoxemia	Treat underlying cause; adjust ventilator if dyssynchrony
Apnea Alarm	No spontaneous breaths in spontaneous mode	Switch to AC mode, assess sedation, check for ETT obstruction

11. Quick Reference: Adjusting the Ventilator

Problem	Parameter to Adjust	Direction	Notes
Hypoxemia (low PaO₂)	FiO₂, PEEP	↑ FiO₂ first, then ↑ PEEP	Consider recruitment maneuver, prone positioning
Hypercapnia (high PaCO₂)	Respiratory Rate, Tidal Volume	↑ RR or ↑ Vᴛ (if Pplat safe)	In ARDS, tolerate permissive hypercapnia to keep Vᴛ 6 mL/kg
Hypocapnia (low PaCO₂)	Respiratory Rate	↓ RR	If on AC, may need sedation to reduce patient triggering
High Plateau Pressure (>30)	Tidal Volume	↓ Vᴛ (even if PaCO₂ rises)	Use IBW; consider switching to PC mode
Auto‑PEEP / Air Trapping	RR, Inspiratory Flow	↓ RR, ↑ Flow	Increase expiratory time (lower I:E ratio)

🔄 DOPES Mnemonic for Acute Desaturation/Hypotension:
Displacement of ETT · Obstruction of ETT · Pneumothorax · Equipment failure · Stack (auto‑PEEP)

⚙️ Mechanical Ventilation Basics · High‑yield reference for medical students, ICU rotations, and exams.
Covers modes, initial settings, ARDSNet protocol, NIV, weaning, complications, and troubleshooting.