Mechanical Ventilation: Modes, Mechanics, and Dyssynchrony

Foundational Concepts

Understanding ventilator modes requires a grasp of four fundamental variables that define any breath:

• Trigger: What initiates inspiration (time, patient effort via pressure or flow)
• Limit: What constrains the breath during inspiration (pressure, volume, flow)
• Cycle: What terminates inspiration (volume, time, flow, pressure)
• Baseline: The expiratory pressure level (PEEP)

Every breath is classified as either mandatory (ventilator-initiated, full support) or spontaneous (patient-initiated). This taxonomy underpins all mode selection decisions in the ICU.

Modes of Mechanical Ventilation

Volume-Control Assist-Control (VC-AC)

In VC-AC, each breath - whether triggered by the patient or the ventilator timer - delivers a fixed tidal volume at a constant (square-wave) inspiratory flow. The pressure required varies breath to breath depending on respiratory system compliance and resistance.

Advantages: Guaranteed minute ventilation; easily measurable respiratory mechanics (plateau pressure, driving pressure); predictable $V_T$ regardless of effort.

Disadvantages: Fixed flow can cause flow starvation in high-demand patients; risk of volutrauma if patient generates large efforts above the set $V_T$ (double triggering); alkalosis if patient rate exceeds set rate substantially.

Monitoring: Inspiratory pause (0.5-2 s) to measure plateau pressure. Driving pressure:

$$\Delta P = P_{plat} - PEEP$$

Target $\Delta P < 15 \text{ cmH}_2\text{O}$; this is an independent predictor of outcome in ARDS.

Pressure-Control Assist-Control (PC-AC)

Each breath delivers a fixed inspiratory pressure above PEEP for a set inspiratory time. Flow decelerates exponentially as alveolar pressure equilibrates with the set pressure.

Advantages: Limits peak airway pressure (barotrauma protection); decelerating flow is more physiological and may improve $\dot{V}/\dot{Q}$ matching; compensates partially for small circuit leaks.

Disadvantages: $V_T$ varies with compliance/resistance changes - dangerous in ARDS where compliance fluctuates; if patient effort increases, $V_T$ can increase silently, causing occult volutrauma (P-SILI: patient self-inflicted lung injury).

Pressure-Regulated Volume Control (PRVC)

PRVC is a dual-control mode that targets a set $V_T$ by automatically adjusting the inspiratory pressure breath-to-breath, using a decelerating flow pattern. It uses the previous breath's compliance to calculate the pressure required to deliver the target volume.

$$P_{applied(n+1)} = P_{applied(n)} \pm 3 \text{ cmH}_2\text{O} \text{ (typical step change)}$$

Advantages: Combines the $V_T$ guarantee of VC with the pressure-limited, decelerating-flow delivery of PC; potentially lower peak pressures than VC-AC for equivalent $V_T$.

Disadvantages: With increased patient effort, PRVC reads higher $V_T$ and decreases applied pressure - paradoxically allowing larger patient-generated volumes if intrinsic effort is high (the "runaway $V_T$" phenomenon); requires careful monitoring in transitioning/weaning patients.

Synchronized Intermittent Mandatory Ventilation (SIMV)

SIMV delivers a set number of mandatory breaths per minute (VC or PC), synchronized to patient effort within a trigger window preceding each scheduled mandatory breath. Breaths outside this window are spontaneous - unsupported, or supported by an adjunct pressure support (SIMV+PS).

Clinical role: Historically used for weaning. Most contemporary evidence suggests SIMV alone prolongs weaning compared to PSV alone or T-piece trials, due to unsupported spontaneous breaths increasing work of breathing. SIMV+PS is more physiologically rational.

When still useful: Apnoea backup in unreliable patients; as a bridge mode; post-cardiac surgery protocols.

Pressure Support Ventilation (PSV)

Every breath is patient-triggered, and the ventilator augments each effort with a fixed pressure above PEEP. Cycling occurs when inspiratory flow drops to a set percentage of peak flow (typically 25%, adjustable 5-45% on most platforms).

$$\text{Work of breathing} \propto \frac{1}{\text{PS level}}$$

Advantages: Preserves respiratory muscle activity; promotes patient-ventilator synchrony; allows natural rate and $V_T$ variability; standard weaning mode.

Disadvantages: No guaranteed backup rate (apnoea alarm must be active); $V_T$ highly variable; cycle criterion mismatch can cause dyssynchrony (see below).

Weaning: Reduce PS by 2-4 cmH$_2$O steps; if tolerated at PS 5-8 cmH$_2$O (above circuit resistance compensation), SBT readiness is supported.

Adjusting Ventilator Settings Based on Respiratory Mechanics

ARDS-Net / Lung-Protective Strategy

Obstructive Lung Disease (Asthma/COPD)

• Reduce respiratory rate (10-14/min) and increase expiratory time (I:E 1:3 to 1:5)
• Allow moderate hypercapnia; avoid auto-PEEP
• Monitor for auto-PEEP via expiratory hold

$$\text{auto-PEEP} = P_{exp-hold} - \text{set PEEP}$$

• If auto-PEEP $> 10$ cmH$_2$O, reduce $\dot{V}_E$, increase expiratory time, consider sedation/paralysis

Setting PEEP

• Titration by compliance: Incremental PEEP trial - select PEEP at best $C_{RS}$

$$C_{RS} = \frac{V_T}{P_{plat} - PEEP}$$

Normal $C_{RS}$: 60-100 mL/cmH$_2$O; ARDS typically $< 40$ mL/cmH$_2$O

• Oesophageal manometry: Targets transpulmonary pressure $P_L = P_{aw} - P_{es} > 0$ cmH$_2$O at end-expiration

Patient-Ventilator Dyssynchrony

Dyssynchrony occurs when the timing or magnitude of ventilator support mismatches the patient's neural respiratory drive. It affects 25-50% of mechanically ventilated patients, is associated with increased sedation requirements, prolonged ventilation, and worsened outcomes.

Double Triggering

Definition: Two ventilator breaths delivered within a single neural inspiratory effort - the patient's effort outlasts the first breath's inspiratory phase, triggering a second breath.

Mechanism: Short ventilator $T_i$ relative to neural $T_i$; commonly seen in VC-AC with short fixed $T_i$ and high respiratory drive. The second breath stacks on the first, delivering $2 \times V_T$ - a major cause of occult volutrauma.

Reverse Triggering

Definition: A mandatory (ventilator-initiated) breath entrains the diaphragm to contract reflexively, via a mechano-neural coupling reflex. The diaphragm activation follows, rather than precedes, the machine breath.

Mechanism: Stretch receptors activated by lung inflation during mandatory breath trigger efferent diaphragmatic activity, generating active effort during or at end of machine inspiration - can cause breath stacking and high $V_T$ if effort occurs during expiration.

Flow Starvation

Definition: In VC-AC, the fixed peak inspiratory flow rate is insufficient for the patient's inspiratory demand, causing the patient to actively "pull" against the fixed flow.

Mechanism: Pressure-time waveform shows a concave dip (scooped appearance) in airway pressure during inspiration, rather than the normal convex rise - the patient's effort reduces circuit pressure below what the fixed flow delivers.

$$P_{airway} < P_{expected} \text{ during inspiration (scooped waveform)}$$

Other Dyssynchrony Patterns

CICM Final Implications

Hot Case / Bedside Approach

When reviewing a ventilated patient, systematically interrogate:

1. Mode appropriateness: Is this patient controlled, partially supported, or weaning? Does the mode match clinical intent?
2. Waveform interrogation: Pull up pressure-time, flow-time, and volume-time traces. Identify the breath type, flow pattern, and any dyssynchrony signatures.
3. Mechanics assessment:
4. Perform inspiratory hold → $P_{plat}$ → calculate $\Delta P$ and $C_{RS}$
5. Perform expiratory hold → measure auto-PEEP

6. Note: these manoeuvres are unreliable in actively breathing patients

7. Dyssynchrony assessment:

8. Double triggering → check $V_T$ delivered (alarm for $> 8$ mL/kg PBW)
9. Flow starvation → waveform morphology
10. Reverse triggering → often missed; consider in deeply sedated patients on control modes with unexplained $V_T$ variability

Viva Key Points

• PRVC is not inherently safer than VC - in spontaneously breathing patients with high drive, it may reduce support as effort increases, paradoxically increasing delivered $V_T$
• Driving pressure ($\Delta P = P_{plat} - PEEP$) reflects stress on the functional lung units and is more discriminating than $V_T$ or $P_{plat}$ alone in predicting ARDS mortality
• SIMV alone should not be used for weaning - it prolongs duration of mechanical ventilation compared with PSV-based or T-piece SBT approaches
• Reverse triggering occurs disproportionately in deeply sedated or paralysed patients - reducing neuromuscular blockade or sedation may worsen or reveal it
• In PSV, the cycle threshold is the key adjustment for premature vs. delayed cycling - this is often overlooked but highly examinable
• Oesophageal pressure monitoring ($P_{es}$) allows quantification of patient effort ($\Delta P_{es}$) and transpulmonary pressure, and is the gold standard for distinguishing patient-generated from ventilator-generated pressures - particularly important when assessing P-SILI risk

Patient-Centred Mode Selection Summary