What is the proper technique and interpretation of forced oscillation testing (FOT, impulse oscillometry) in children?

Forced Oscillation Technique in Children

FOT is a noninvasive, effort-independent lung function test that requires only tidal breathing, making it ideal for children aged 2-6 years who cannot perform spirometry maneuvers. The technique applies small-amplitude pressure oscillations at the mouth during normal breathing to measure respiratory impedance, which includes both resistance (Rrs) and reactance (Xrs) 1, 2.

Proper Technique

Equipment Requirements

• System must measure reference impedance of at least 1.5 kPa·L⁻¹·s within 10% or 0.1 kPa·L⁻¹·s, whichever is greater 2
• Optimal excitation frequencies should include the 4-8 Hz range 2
• Equipment dead space and resistance must be appropriate for small children, as adult-designed equipment may impact measurements 2

Patient Positioning and Setup

• Child seated upright, breathing through a mouthpiece with noseclip applied 2
• Cheeks and mouth floor must be firmly supported to prevent upper airway shunting 2
• Child should breathe quietly at tidal volume without forced maneuvers 1

Measurement Protocol

• Each acquisition period should cover several breathing cycles, typically 8-16 seconds 2
• Perform 3-5 measurements per session 2
• Report results as the mean of these measurements with coefficient of variation (CV) calculated from SD to mean ratio 2

Feasibility by Age

Success rates vary significantly with age. In emergency room settings, feasibility ranges from 0-20% at age 2 years, increasing to 83% by age 5 years 2. In controlled laboratory or field settings with trained staff, success rates of 80-100% can be achieved in healthy preschool children or stable patients 2.

Interpretation

Key Parameters

Resistance (Rrs) - measured at low frequencies (typically 5-6 Hz):

• Reflects frictional losses in airways and lung parenchyma 2
• Rrs at lower frequencies (Rrs5) is most sensitive to airway obstruction 1
• Increased Rrs indicates airway narrowing or obstruction

Reactance (Xrs) - measured at 5 Hz:

• Related to elastic and inertial properties of the respiratory system 2
• More negative Xrs values indicate increased stiffness or reduced compliance
• Particularly useful in chronic lung disease of prematurity 1

Clinical Applications in Children

Asthma Detection:

• Rrs measured at lower frequencies (5-6 Hz) is significantly different between healthy and asthmatic children 1
• Distinguishes asthmatics with and without abnormal spirometry 1
• In acute asthma, Rrs8 correlates with clinical severity, though the relationship is only marginal in preschoolers 1

Bronchodilator Response:

• Respiratory resistance values (Rrs5) have proved sensitive to bronchodilation in children 1
• Response should be expressed as number of standard deviations to improve sensitivity 2
• FOT shows larger bronchodilator responses than the interrupter technique and agrees well with FEV1 and plethysmographic airway resistance 2

Bronchial Hyperresponsiveness:

• FOT is a reliable method for assessing bronchial hyperresponsiveness in children 1
• Critical advantage: Unlike spirometry, FOT does not require deep inspiration and therefore does not modify airway smooth muscle tone 1
• This makes it more physiologically relevant for assessing true airway reactivity

Important Limitations and Pitfalls

Cystic Fibrosis:

• FOT shows generally poor correlation with spirometry in CF patients 1
• This discrepancy likely reflects alterations in bronchial wall elastic properties 1
• Caution: Routine assessment in CF children should not rely on FOT alone - use in conjunction with spirometry when possible 1

Restrictive Physiology:

• FOT may not identify lung function abnormalities in children with restrictive physiology 3
• Correlates poorly with spirometry and plethysmography in diffuse parenchymal lung disease 3

Preschool-Specific Challenges:

• Poor tolerance of equipment dead space 1
• Poor signal-to-noise ratios due to rapid respiratory rate 1
• Increased influence of upper airway wall compliance reduces discriminative power 1
• Lack of cooperation remains the primary limitation in children under 3 years 1

Diagnostic Cutoffs

Airway Obstruction:

• Rrs at lower frequencies significantly elevated compared to predicted values
• Further research needed to establish practical FOT index for defining airway obstruction on routine basis 1
• Current evidence shows Rrs distinguishes between asthmatics with and without abnormal spirometry, but specific cutoff values require validation 1

Bronchodilator Reversibility:

• Significant decrease in Rrs5 post-bronchodilator indicates reversible obstruction
• Specific cutoff levels for meaningful reversibility in children remain to be confirmed in future studies 1

Key Advantages Over Spirometry

• Requires minimal cooperation - no forced respiratory maneuvers needed 1, 4
• Can be performed in children as young as 2-3 years 2
• Does not alter airway smooth muscle tone through deep inspiration 1
• Suitable for emergency room assessment of acute illness 2
• Particularly valuable when spirometry cannot be performed or results appear unreliable 4

The FOT represents a practical, noninvasive alternative for assessing lung function in young children, with particular utility in asthma diagnosis, bronchodilator response testing, and bronchial challenge testing. However, clinicians must recognize its limitations in CF and restrictive lung disease, where complementary testing is essential.