Mechanical Ventilation: A Study Notes

I. Methods of Oxygen Delivery

There are 3 main methods to deliver oxygen:

A. Non-invasive

1. Face Mask + Oxygen Source

   • Patient breathes spontaneously.

   • Machine only delivers oxygen — patient does the work of breathing.

2. CPAP or BiPAP

   • Connected to a machine that provides positive airway pressure.

   • CPAP = Continuous Positive Airway Pressure → prevents alveolar collapse by keeping pressure > 0 (e.g., +5 cmH₂O).

   • BiPAP = Two pressure levels (inspiratory and expiratory).

B. Invasive

3. Endotracheal Intubation + Mechanical Ventilation

   • Used when patient cannot maintain airway or breathe adequately.

   • Common during surgery or critical illness (e.g., respiratory failure, anesthesia).

II. CPAP vs. PEEP

Feature	CPAP	PEEP
Invasiveness	Non-invasive (mask)	Invasive (intubated)
Function	Keeps airway pressure positive	Same effect but via ventilator
Key Concept	Maintains alveolar patency; prevents collapse

III. Indications for Mechanical Ventilation

• Respiratory failure (hypoventilation, apnea)

• Airway protection during anesthesia

• Severe hypoxia or acidosis

• Cardiac arrest

• Neuromuscular paralysis (e.g., coma, overdose)

ABC Approach:

• A – Airway: Intubate if not patent

• B – Breathing: Mechanical ventilation

• C – Circulation: Manage BP, HR, fluids, inotropes (dopamine, epinephrine, etc.)

IV. Purposes of Mechanical Ventilation

• Maintain oxygenation (PaO₂)

• Control CO₂ (PaCO₂)

• Maintain pH balance

• Deliver anesthetic gases during surgery

• Prevent aspiration

V. Modes of Ventilation

1. Continuous Ventilation

• Machine does all the work (no spontaneous breathing).

• Use if patient is sedated, paralyzed, or comatose.

• Example: Assist Control (AC/CMV) = Volume control mode.

2. Intermittent Ventilation

• Allows spontaneous breaths between machine breaths.

• Example: SIMV (Synchronized Intermittent Mandatory Ventilation)
  → safer for weaning if patient can trigger own breaths.

VI. Ventilator Settings You Control

Parameter	Definition	Notes
Tidal Volume (Vt)	Volume of air per breath	Normal ~500 mL
Rate (RR)	Breaths per minute	Usually 10–18/min
FiO₂	Fraction of inspired O₂	Room air = 21%; can raise to 40–100%
PEEP	End-expiratory pressure	Keeps alveoli open
Flow Rate	Volume/time of gas delivery	Affects inspiratory time
Pressure	Inspiratory pressure limit	Used in pressure-controlled modes

VII. Example Ventilator Order

Mode: Assist-Control
RR: 14
Vt: 500 mL
FiO₂: 40%
PEEP: 5 cmH₂O

VIII. Adjustments Based on ABG

Problem	Finding	Action
Respiratory Acidosis	↑ PaCO₂, ↓ pH	↑ RR and/or ↑ Vt
Hypoxemia	↓ PaO₂	↑ FiO₂ and/or ↑ PEEP

IX. Lung Compliance

• Definition: Ease of lung expansion
  →Compliance=ΔP/ΔV​

• Low compliance: Stiff lungs (e.g., ARDS, fibrosis)

• High compliance: Floppy lungs (e.g., emphysema)

If compliance ↓:

• In volume control, pressure ↑

• In pressure control, volume ↓

• Alarm triggers when extreme.

X. Boyle’s Law in Ventilation

X. Boyle’s Law in Ventilation

$P \propto \frac{1}{V}$ (at constant temperature)

• As volume ↑ → pressure ↓ (and vice versa).

• Explains negative-pressure inspiration and mechanical ventilation dynamics

• As volume ↑ → pressure ↓ (and vice versa).

• Explains negative-pressure inspiration and mechanical ventilation dynamics.

XI. Peak vs. Plateau Pressure

Pressure Type	Represents	Increased By	Indicates
Peak Pressure	Airway resistance	Bronchospasm, mucus, kinked tube	Airway problem
Plateau Pressure	Alveolar pressure (no flow)	Pulmonary edema, ARDS, pneumothorax	↓ Lung compliance

Mnemonic:

• ↑ Peak = Airway resistance problem

• ↑ Plateau = Lung compliance problem

XII. Pulmonary vs. Alveolar Ventilation

Concept	Formula	Significance
Minute (Pulmonary) Ventilation	RR × Vt	Total air moved per minute
Alveolar Ventilation	RR × (Vt – Dead Space)	Air that actually reaches alveoli

• Shallow rapid breathing: ↓ alveolar ventilation (wasted effort).

• Deep slow breathing: ↑ alveolar ventilation (more efficient).

XIII. Dangers of Excessive Settings

Parameter	Too High → Problem
RR	Auto-PEEP (air trapping) in COPD/asthma
Vt	Barotrauma, inflammation
FiO₂	Oxygen toxicity (retinopathy in neonates, lung injury)
PEEP	↓ Venous return → ↓ Cardiac output, hypotension

XIV. PEEP in CHF

• Beneficial: ↓ Venous return → ↓ cardiac workload

• Caution: Too much → hypotension, ↓ perfusion

• Always check vitals before adjusting PEEP.

XV. Clinical Terms

Term	Meaning
Triggering the Vent	Patient initiates a breath
Riding the Vent	Machine fully controls breathing
Spontaneous RR	Patient’s own breathing frequency

XVI. Special Cases

• ARDS / Restrictive disease: ↑ PEEP, ↓ Vt

• Emphysema / Obstructive: ↑ Flow rate to allow full exhalation, avoid auto-PEEP

XVII. Complications

1. Barotrauma – alveolar rupture due to overdistention

2. Ventilator-Induced Lung Injury (VILI)

3. Ventilator-Associated Pneumonia (VAP)

XVIII. Ventilator-Associated Pneumonia (VAP)

Definition: Pneumonia occurring 48–72 hrs after intubation

Common Pathogens:

Timing	Likely Organisms	Type
≤4 days (early)	Strep. pneumoniae, H. influenzae, Klebsiella	Community-type
≥5 days (late)	Pseudomonas, MRSA, Acinetobacter	Hospital-type (drug-resistant)

Predisposing Factor:

• Malnutrition in ICU patients.

Best Diagnostic Sample:

• Protected Specimen Brush (PSB) via bronchoscopy → deep, uncontaminated sample.

Empiric Treatment:

Timing	Therapy
Early (≤4 days)	Beta-lactam + Respiratory fluoroquinolone
Late (≥5 days)	Add anti-Pseudomonal + Vancomycin/Linezolid for MRSA