Mechanical Ventilation - Quick Reference

Full notes: Mechanical Ventilation

Ventilator Modes at a Glance

Mode	Trigger	Control	Cycle	Key Feature	Use When
AC	Time/P/Flow	Volume	Time	All breaths fully supported	Full support needed, no hyperventilation
SIMV	Time + P/Flow	Volume + Pressure	Time + Flow	Mandatory + partial spontaneous	Hyperventilating patients, auto-PEEP risk
PCV	Time only	Pressure	Time	No patient triggering	High barotrauma risk
PSV	P/Flow	Pressure	Flow	All breaths patient-triggered	Conscious, pre-extubation
PRVC	Time	Adaptive P→V	Time	Guarantees VT, minimises P	Best of both worlds

AC vs SIMV: In excess of set RR — AC gives full support; SIMV gives partial support

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Initial Ventilator Settings

Parameter	Normal	COPD	ARDS
Mode	AC or SIMV	SIMV	AC or SIMV
FiO₂	Start 100%, wean to ≤60%	Start 100%, wean	Start 100%, wean
VT	10 mL/kg IBW	8 mL/kg IBW	6 mL/kg IBW
RR	10–20	Avoid ↑ RR	10–20
PEEP	5–8 cm $\mathrm{H}{}_2\mathrm{O}$	Low/ZEEP	≥8–12 cm $\mathrm{H}{}_2\mathrm{O}$
PS	5–20 cm $\mathrm{H}{}_2\mathrm{O}$	—	—
I:E	1:2	1:3 or 1:4	1:2

FiO₂ >60% → add strategies: ↑ PEEP, recruitment manoeuvres, change mode
Pplateau >30 cm $\mathrm{H}{}_2\mathrm{O}$ → ↓ VT in 1 mL/kg steps

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Pressure Interpretation (Volume Control Modes)

P_peak ↑, P_plateau normal  →  Airway resistance ↑ (bronchospasm, secretions, kink)
P_peak ↑, P_plateau ↑       →  Lung compliance ↓ (pulmonary oedema, pneumothorax, ARDS)

$P_{\text{Peak}}\propto \frac{\text{Airway Resistance}}{\text{Compliance}}{P}_{\text{Plateau}}\propto \frac{1}{\text{Compliance}}$

$\text{Compliance}=\frac{V_{\text{T}}}{P_{\text{plateau}}-\text{PEEP}}\text{Driving Pressure}=P_{\text{plateau}}-\text{PEEP}$

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Phase Variables Summary

Phase	Variable	Types	Key Point
Trigger	What starts a breath	Time / Flow / Pressure	Flow trigger most comfortable; pressure trigger for auto-triggering
Limit	Max value during inspiration (does not end breath)	Pressure / Flow / Volume	—
Cycle	What ends a breath	Time / Flow / Pressure / Volume	Flow cycling most comfortable; time cycling best for CO₂ control

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PEEP Effects

Effect	Low-Mid PEEP	High PEEP
Oxygenation	↑ (alveolar recruitment)	↓ (overdistension of healthy alveoli → ↑ dead space)
Preload	↓ (↑ CVP, ↑ RV afterload)	↓↓
LV afterload	↓ (↓ Ptransmural)	↓
ICP	Possible ↑ (↓ venous return)	—

Optimal PEEP = best compliance = highest O₂ delivery (sweet spot)

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Auto-PEEP

Cause: Incomplete exhalation → gas trapping

Detect: End-expiratory hold → rise in pressure above set PEEP

Consequences: Barotrauma, ↓ CO/hypotension, V/Q mismatch, dyssynchrony, ↑ WOB

Mechanism	Fix
High minute ventilation	↓ VT, ↓ RR, ↓ I:E ratio
Expiratory flow limitation	Bronchodilators, secretion management, ↑ applied PEEP
Expiratory resistance	Upsize ETT, ↑ sedation/paralytics

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Patient-Ventilator Dyssynchrony

Type	Sign	Fix
Trigger delay / Missed trigger	Effort without breath	↓ trigger threshold, ↓ sedation, fix auto-PEEP
Auto-triggering	Breaths without effort	↑ trigger threshold, address leak/noise
Flow dyssynchrony	Scalloped/concave pressure curve	↑ flow rate, change pattern/mode
Delayed termination	Pressure spike end of inspiration	Cycle earlier, use time cycling
Premature termination	Pressure drops below baseline → double triggering	Cycle later

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Crashing Patient — DOPES

Letter	Problem	Action
D	Dislodged ETT	Confirm position, reintubate
O	Obstructed ETT	Suction, bougie, bronchoscope
P	Pneumothorax	USS → finger thoracostomy
E	Equipment failure	Disconnect, BMV 100% $\mathrm{O}{}_2$
S	Stacked breathing (auto-PEEP)	Disconnect, allow exhalation

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SBT Readiness Criteria

• Respiratory: FiO₂ <50%, PEEP ≤10 cm $\mathrm{H}{}_2\mathrm{O}$, $\mathrm{CO}{}_2$ at baseline
• CVS: No ischaemia, HR <140, low vasopressors
• Neuro: Arousable, following commands
• Renal: No uncontrolled acid-base disturbance

SBT Settings: PSV 5/5 (PS 5 cm $\mathrm{H}{}_2\mathrm{O}$ + PEEP 5 cm $\mathrm{H}{}_2\mathrm{O}$) for 30 minutes

Pass criteria: $S_{\text{p}}\mathrm{O}{}_2$ >88%, no ↑ Et$\mathrm{CO}{}_2$ >10 mmHg, RSBI <105, no arrhythmia/haemodynamic instability

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High Pressure Alarm Troubleshooting

SpO₂ falling? → Crashing patient protocol (DOPES)
      ↓
Assess patient → Check ETT → Inspect circuit → Suction
      ↓
Inspiratory hold → measure P_plateau
      ↓
P_peak - P_plateau < 5 cm H₂O (↑ plateau)?
  → Rule out auto-PEEP → USS/CXR → ↓ VT until Pplateau <30
P_peak - P_plateau > 5 cm H₂O (↑ peak only)?
  → Bronchodilators → Check circuit → OK if Pplateau <30

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Ventilator Alarm Quick Reference

Alarm	Common Causes	First Step
High pressure	↑ resistance, ↓ compliance, coughing	Assess patient, suction, inspiratory hold
Low pressure	Circuit disconnect, cuff leak	Check circuit connections
Low VT	Leak, malpositioned ETT	Check circuit, cuff, position
High frequency	Distress, auto-triggering, secretions	Assess patient, check airway
Apnoea	Disconnect, ↓ respiratory drive	Manual ventilation, check circuit
High PEEP	Auto-PEEP, air trapping, bronchospasm	↑ expiratory time, ↓ RR, bronchodilators
Low PEEP	Circuit leak, cuff leak	Check circuit and cuff