What are the recommendations for setting inspiratory flow and flow patterns in mechanical ventilation?

Setting Inspiratory Flow and Flow Patterns in Mechanical Ventilation

Primary Recommendation

Set inspiratory time and flow based on respiratory system mechanics using the time constant (compliance × resistance), monitor flow-time scalars to avoid end-inspiratory or expiratory flow interruption, and prioritize patient-ventilator synchrony over specific flow pattern selection. 1

Evidence-Based Approach to Flow Settings

Inspiratory Time and I:E Ratio

The 2017 Paediatric Mechanical Ventilation Consensus Conference provides the most comprehensive guidance on this topic:

• Inspiratory time and respiratory rate must be set in relation to respiratory system mechanics and disease trajectory—these parameters are closely correlated and cannot be judged independently. 1

• Use the time constant (τ = compliance × resistance) of the respiratory system as the primary guide for setting inspiratory time. 1

• At the bedside, avoid end-inspiratory flow interruption and especially avoid expiratory flow interruption to prevent air-trapping. 1

• Monitor pressure-time and flow-time scalars continuously to assess for air trapping and optimize patient-ventilator synchrony. 1, 2

Disease-Specific Considerations

For restrictive lung disease:

• Use higher respiratory rates to compensate for low tidal volumes and maintain minute ventilation 1, 2
• Shorter inspiratory times may be appropriate given reduced compliance 1

For obstructive airway disease:

• Ensure adequate expiratory time to prevent air-trapping 1
• Monitor for intrinsic PEEP development 1
• Lower respiratory rates with longer expiratory times are typically needed 1

Flow Pattern Selection: Square vs Decelerating

The Evidence is Mixed and Clinically Marginal

While there is debate about optimal flow patterns, the clinical differences between constant (square) and decelerating flow are modest and should not be prioritized over other ventilator settings that have clearer impact on outcomes.

Decelerating flow patterns may offer minor advantages in specific populations:

• In COPD patients, decelerating flow reduced peak inspiratory pressure, airway resistance, dead space ventilation (VD/VT), PaCO2, and ventilator work of breathing compared to constant flow 3
• Decelerating flow improved CO2 elimination in experimental acute respiratory failure without reducing inspiratory pressures or improving oxygenation 4
• However, in experimental ARDS, decelerating flow (pressure-controlled ventilation) increased overinflated lung volumes compared to constant flow, raising concerns about potential ventilator-induced lung injury 5

The most important factor is not the flow pattern itself, but rather limiting inspiratory flow rate in high-pressure ventilation:

• Limiting inspiratory flow rate (15 L/min) at high pressures (50 cmH2O) dramatically reduced lung injury, intrapulmonary shunt, intra-alveolar neutrophils, and lung edema compared to conventional flows in experimental models 6
• This suggests that excessively rapid flow delivery, regardless of pattern, may contribute to ventilator-induced lung injury 6

Practical Guideline Recommendation

The consensus guidelines do not make specific recommendations about flow pattern selection (square vs decelerating), emphasizing instead that patient-ventilator synchrony should be the primary target. 1

• Better patient-ventilator synchrony improves patient comfort, though effects on clinical outcomes remain unclear 1
• Flow pattern choice is less important than avoiding flow interruption and air-trapping 1

Monitoring and Adjustment Algorithm

Step 1: Set initial inspiratory time based on disease type

• Restrictive disease: Shorter inspiratory time, higher rate 1, 2
• Obstructive disease: Longer expiratory time, lower rate 1

Step 2: Monitor flow-time scalars continuously

• Ensure inspiratory flow reaches zero before end of inspiration 1
• Ensure expiratory flow reaches zero before next breath (critical in obstructive disease) 1

Step 3: Assess patient-ventilator synchrony

• Observe for patient effort and triggering 1, 7
• Adjust flow settings to minimize asynchrony 1, 7

Step 4: Measure and respond to physiologic parameters

• Monitor peak inspiratory pressure, plateau pressure, and mean airway pressure 1, 2
• Consider measuring intrinsic PEEP in obstructive disease 1
• Assess dynamic compliance 1, 2

Common Pitfalls to Avoid

• Do not use excessively high inspiratory flow rates, particularly when using high pressures, as this may increase ventilator-induced lung injury 6

• Do not allow expiratory flow interruption (incomplete exhalation), especially in obstructive airway disease, as this leads to air-trapping and auto-PEEP 1

• Do not focus excessively on flow pattern selection (square vs decelerating) at the expense of more important ventilator parameters like tidal volume, plateau pressure, and PEEP 1, 5

• Do not ignore flow-time scalars—these provide critical real-time information about respiratory mechanics and patient-ventilator interaction 1, 2, 7

• In pressure-controlled ventilation with decelerating flow, be aware of potential for increased overinflation compared to volume-controlled ventilation 5

Key Pressure and Volume Limits to Maintain

Regardless of flow pattern selected:

• Limit plateau pressure ≤28 cmH2O (or ≤29-32 cmH2O if chest wall elastance is increased) 1
• Limit plateau pressure ≤30 cmH2O in obstructive airway disease 1
• Keep tidal volume ≤10 mL/kg ideal body weight 1, 2
• Target physiologic tidal volumes 1