Calculating Pediatric Inspiratory Flow Demand
Pediatric inspiratory flow demand is calculated as mean inspiratory flow (VT/TI), where VT is tidal volume and TI is inspiratory time, or can be estimated using the formula: Peak Tidal Inspiratory Flow (PTIF) ≈ 1.5-2.0 L/kg/minute in infants and young children. 1, 2
Primary Calculation Methods
Mean Inspiratory Flow Method
- Calculate mean inspiratory flow as VT/TI (tidal volume divided by inspiratory time), which represents the average flow rate during inspiration 3
- This measurement has been validated in children aged 4-16 years and shows significant correlation with occlusion pressure (Pmo,0.1) 3
- The calculation requires measurement of tidal volume in milliliters and inspiratory time in seconds 3
Peak Tidal Inspiratory Flow (PTIF) Method
- In infants up to 6 months with acute viral bronchiolitis, mean PTIF is 7.45 L/minute (range 2.40-16.00 L/minute) 1
- When indexed to weight, PTIF averages 1.68 L/kg/minute (range 0.67-3.00 L/kg/minute), with 89% of infants having PTIF <2.5 L/kg/minute 1
- PTIF correlates with patient weight (ρ=0.55, P<0.001) but not with disease severity markers 1
Age-Specific Considerations
Infants and Young Children (≤8 kg)
- Flow demand is approximately 1.5-2.0 L/kg/minute, with greater benefit from higher flows in this weight range 2
- Inspiratory time (TI) is shorter than in adults, affecting the duration of pressure measurements 3
- A period of at least 5 minutes of regular breathing is required before performing measurements 3
Older Children (4-16 years)
- Mean inspiratory flow (VT/TI) can be reliably measured in awake seated children who can breathe quietly with a noseclip and mouthpiece 3
- The ratio TI/Ttot (inspiratory time to total breath time) remains constant at approximately 0.45 across ages 4-16 years 3
Clinical Application Formula
For Ventilator Support
- Inspiratory pressure (PI) can be calculated as 0.5 × α × TI, where α = Pmo,0.1 × 10 3
- This allows estimation of inspiratory power for breathing at rest: PI × VT/TI × TI/Ttot 3
For High-Flow Nasal Cannula Settings
- Initial flow should be set at approximately 2.0 L/kg/minute for optimal reduction in effort of breathing 2
- Flow rates <2.5 L/kg/minute are appropriate in most situations for infants with bronchiolitis 1
Measurement Requirements
Equipment Specifications
- Flow must be measured at the airway opening using a pneumotachometer via mask or mouthpiece 3
- Data should be digitized at minimum 50 Hz (100 Hz recommended at high respiratory rates) 3
- Equipment dead space must be minimized to avoid altering respiratory control 3
Quality Control
- Measurements require stable, regular breathing pattern with visualization of both flow-volume and flow-time plots 3
- At least 5 acceptable measurements should be averaged for occlusion pressure determinations 3
- Posture should be standardized (preferably sitting) and clearly stated 3
Common Pitfalls to Avoid
- Do not use sedation in young children as it affects level of consciousness and respiratory drive 3
- Do not assume adult measurement durations (100 milliseconds) are appropriate for infants and children with shorter inspiratory times 3
- Do not ignore states of alertness, which significantly influence respiratory drive measurements in infants 3
- Do not apply flow rates uniformly across weight ranges - children ≤8 kg experience larger reductions in effort of breathing with increasing flow rates compared to children >8 kg 2
Advanced Calculations
Inspiratory Pressure Reserve
- Calculate as PI/PI,max at FRC to assess potential limitation of inspiratory muscles 3
- Normal values range from 13-26% depending on age and sex 3