Mechanical Ventilation and Modes of Ventilation

1. Introduction

• Roberts and Hedges' Clinical Procedures in Emergency (CASH), p. 222

2. Basic Physiology

Minute Volume and Alveolar Ventilation

• Minute volume (V̇E) = Tidal Volume (VT) × Respiratory Rate (f)
• Normal V̇E = 7-10 L/min
• VT = Alveolar Volume (VA) + Dead Space Volume (VDS)
• Anatomic dead space ≈ 2.2 mL/kg lean body weight (trachea and large airways)
• Pathologic dead space = ventilated but underperfused alveoli (added in disease states)
• Physiologic dead space = anatomic + pathologic dead space
• Alveolar minute ventilation: V̇A = (VT - VDS) × f
• PaCO2 is determined by V̇A and CO2 production rate

Volume-Pressure Relationship

Key Ventilator Parameters

PEEP Optimization

• As PEEP increases, plateau pressure initially rises proportionally
• At optimal PEEP, plateau pressure plateaus (no further rise) - lung is optimally recruited
• Beyond optimal PEEP, overdistension causes plateau pressure to rise again

3. Modes of Ventilation

A. Pressure Support Ventilation (PSV)

"PSV is the most widely used mode of mechanical ventilation." - Murray & Nadel, p. 3175

• All breaths are patient-triggered
• Ventilator delivers flow to meet a set inspiratory pressure
• Cycles off when inspiratory flow falls to a threshold (operator-set, typically 10-35% of peak inspiratory flow)
• VT is determined by pressure support level, patient effort, and respiratory system compliance
• FiO2 and PEEP set by clinician; patient controls respiratory rate and flow
• No set RR (most modern ventilators have a backup apnea rate)
• Best for: spontaneous breathing patients being weaned; good patient-ventilator synchrony
• Cycling adjustments: if the ventilator cycles prematurely (under-assistance) or late (delayed cycling/over-assistance), the flow threshold can be adjusted

B. Volume-Cycled Ventilation (VCV)

• Volume (VT) is the target - ventilator generates whatever pressure is needed to reach the set VT
• Clinician sets: VT, RR, FiO2, PEEP, flow rate, flow waveform (decelerating or square), I:E ratio, trigger sensitivity
• Most familiar and commonly used mode in adults
• Advantage: guarantees reliable tidal volume
• Disadvantage: does not account for dynamic changes in lung compliance; high airway pressures if compliance decreases
• Typical initial settings (VCV-AC): RR 10-14, VT 7-8 mL/kg IBW, PEEP 5 cm H2O, FiO2 100%, flow rate 60 L/min, decelerating waveform

C. Pressure-Cycled Ventilation (PCV)

• Pressure is the target - ventilator delivers to a set pressure level; VT varies with compliance and resistance
• Clinician sets: pressure high, PEEP, RR, FiO2, inspiratory time (Ti)
• VT not guaranteed - improves (rises) as compliance improves, decreases if compliance worsens
• Advantage: limits peak airway pressure, potentially safer for injured lungs
• Disadvantage: VT is not fixed; requires close monitoring of VT and minute ventilation alarms
• Typical initial settings (PCV-AC): RR 12-16, pressure high 20 cm H2O, PEEP 5, FiO2 100%; monitor VT to target 7-8 mL/kg IBW
• CASH, p. 227

D. Assist-Control (AC) Ventilation

• Ventilator delivers machine (control) breaths at a preset rate
• Every breath - whether patient-initiated or machine-initiated - is fully supported to the set target (VT in VCV, or pressure in PCV)
• If patient breathes faster than the set rate, additional assist breaths are triggered
• Advantage: guarantees minimum minute ventilation, reduces work of breathing completely
• Disadvantage: risk of hyperventilation (respiratory alkalosis); risk of auto-PEEP (breath stacking) in patients with high intrinsic RR, particularly in asthma where auto-PEEP can reduce cardiac output and cause cardiovascular collapse
• In VCV-AC: every breath (control + assist) receives the same set VT
• In PCV-AC: every breath receives the same set pressure

"Caution should be exercised to avoid auto-PEEP (also known as breath stacking) when using volume-targeted AC modes." - CASH, p. 228

E. Synchronized Intermittent Mandatory Ventilation (SIMV)

• Delivers a preset number of machine breaths (as VCV or PCV)
• Patient can trigger additional spontaneous breaths between the mandatory breaths - these are NOT fully supported
• Spontaneous breaths above the preset rate receive only patient-generated VT (high work of breathing)
• Typically combined with PSV to support spontaneous breaths and overcome circuit resistance
• SIMV has fallen out of favor - studies show it increases ventilator days and delays liberation compared to PSV or AC weaning protocols
• Murray & Nadel, p. 3175

F. Pressure-Regulated Volume Control (PRVC) / Volume Support (VS)

• PRVC (pressure-regulated volume control): A type of pressure-control (PACV) mode using VT as feedback to continuously adjust the pressure target
  • If respiratory mechanics improve → applied pressure decreases
  • If respiratory mechanics worsen → applied pressure increases
  • Guarantees volume while limiting pressure
• Volume Support (VS): Patient-triggered, pressure-targeted, flow-cycled PSV where pressure is auto-adjusted to achieve a target volume
  • Increasing patient effort → lower applied pressure (automatic unloading)
  • Decreasing patient effort → higher applied pressure
• Murray & Nadel, p. 3175

G. Adaptive Support Ventilation (ASV)

• Assist-control, pressure-targeted, time-cycled mode
• Clinician sets only: desired minute ventilation and patient height (for dead space estimation)
• Ventilator uses respiratory system mechanics to automatically select VT-frequency pattern
• Murray & Nadel, p. 3175

H. Airway Pressure Release Ventilation (APRV) / Bi-Level Ventilation

• Patient spends most time at P-high → establishes oxygenation via sustained recruitment
• Brief release to P-low → clears CO2 (elastic recoil drives large-volume gas flow)
• Patient can spontaneously breathe in ALL phases of the cycle (unique "floating valve" system)
• No traditional set respiratory rate; T-high + T-low = one "phase cycle"

• IRV requires paralysis/heavy sedation; APRV/Bi-Level does not
• Patient can exhale 50-200 mL during P-high phase (excess gas release)
• More comfortable; reduces sedation requirements

• CASH, p. 229

I. Advanced Modes: PAV+ and NAVA

• Proportional Assist Ventilation with load-adjustable gain factors (PAV+):
  • Uses transient end-inspiratory occlusions to measure elastance and resistance
  • Delivers a fixed proportion of the total inflation pressure needed
  • As patient effort increases → ventilator pressure increases proportionally (true unloading)
• Neurally Adjusted Ventilatory Assist (NAVA):
  • Delivers pressure proportional to diaphragmatic electrical activity (Edi signal)
  • Edi acquired from EMG electrodes on an esophageal catheter detecting diaphragmatic crura activation
  • Highest synchrony mode available
• Murray & Nadel, p. 3176

4. Modes Commonly Used in the ED (CASH Summary)

"Assist/control (AC) and SIMV are the ventilation modes most commonly used in the ED. Both are acceptable, and no data have demonstrated a better outcome with either mode." - CASH, p. 228

Initial Ventilator Settings Quick Reference (from CASH)

• FiO2: titrate based on SpO2 or PaO2
• PEEP: increase for persistent hypoxia
• Minute ventilation: adjust based on blood pH (note: on AC, decreasing RR does NOT change minute ventilation if patient breathes above set rate)

5. Ventilator Components and Circuit Considerations (Murray & Nadel)

• Gas blenders: Mix air + O2 to produce FiO2 0.21-1.0; newer systems also blend heliox, nitric oxide, anesthetic agents
• PEEP valve: Maintains positive airway pressure throughout expiration to keep alveoli patent and improve V/Q matching
• Humidification: Required because upper airway is bypassed
  • Active humidifiers: external water source, heat to ≥35°C, water content ≥40 mg/L
  • Passive (HME) humidifiers: capture heat/moisture from expired gas; adequate for short-term MV (≥30-33°C, ≥28-32 mg/L)
• Aerosol delivery: Bronchodilators, steroids, vasodilators, antibiotics via in-line nebulizers or MDI adapters; optimal position is inspiratory limb, several cm proximal to Y-connector
• Circuit compliance: Flexible tubing has compliance of 2-4 mL/cm H2O - applied pressure may distend circuit rather than reach lungs when airway opening pressure is high

Summary Table: Modes at a Glance

1. Plateau pressure must stay ≤30 cm H2O to avoid ventilator-induced lung injury (VILI)
2. Auto-PEEP in asthma on AC-VCV can cause cardiovascular collapse - monitor flow-time curves for incomplete exhalation
3. SIMV delays liberation from MV - most guidelines now favor AC + spontaneous breathing trials
4. Lung-protective strategy: VT 6 mL/kg IBW, Pplat ≤30, adequate PEEP
5. On AC mode, decreasing RR will NOT reduce minute ventilation in a patient who is breathing above the set rate