What is the best approach to measure resistance in a critically ill patient with a history of respiratory disease who requires mechanical ventilation?

Measuring Resistance in Mechanically Ventilated Patients

Measure airway resistance using the end-inspiratory hold maneuver to separate peak pressure from plateau pressure, which is the gold standard bedside technique for mechanically ventilated patients. 1, 2, 3

Primary Measurement Technique: End-Inspiratory Hold Maneuver

The difference between peak inspiratory pressure (Ppeak) and plateau pressure (Pplat) directly indicates airway resistance, allowing you to distinguish flow-dependent problems (bronchospasm, endotracheal tube obstruction) from compliance issues (stiff lungs, chest wall problems). 1, 2, 4

Technical Requirements for Accurate Measurement

• Perform the end-inspiratory hold during volume-controlled ventilation with an occlusion lasting >0.5 seconds to allow pressure equilibration throughout the respiratory system 1, 3
• The patient must be completely passive (not actively breathing) during measurement, as respiratory muscle activity creates artifacts that invalidate resistance calculations 1, 3
• Measure pressure at the proximal tip of the endotracheal tube (near the Y-piece in children <10 kg) to minimize measurement errors 1, 5

Calculating Resistance Values

The basic equation of motion provides the framework: Pappl = (1/C)V + RV̇, where R represents flow resistance. 1

Standard Calculation Methods

• Minimum resistance (Rrs,min) is calculated as (Ppeak - Pplat) divided by inspiratory flow rate, representing primarily inspiratory airway resistance 5, 6
• Maximum resistance (Rrs,max) incorporates expiratory resistance and can be obtained through end-expiratory occlusion maneuvers 5
• Six different calculation methods exist (Suter, Krieger, Neergard, Bergman, Comroe, Jonson), with methods evaluating expiratory resistance producing values approximately twice as high as inspiratory-only methods 6

Critical Correction for Endotracheal Tube Resistance

The endotracheal tube itself poses substantial, highly flow-dependent resistance that must be accounted for, particularly at high minute ventilation levels. 1

• The modified equation becomes: Pappl = (1/C)V + Rt + (k1V̇ + k2V̇²), where k1 and k2 represent laminar and turbulent flow constants 1
• Correct for endotracheal tube impedance by measuring tracheal pressure directly or by estimating tube impedance in vitro under similar flow conditions 1

Monitoring During Assisted Ventilation

Measuring resistance becomes substantially more challenging when patients have spontaneous breathing activity, requiring advanced techniques beyond simple airway occlusions. 7

Advanced Monitoring Options

• Esophageal manometry with balloon-catheter systems allows measurement of pleural pressure changes and calculation of true respiratory system resistance even during active breathing 1, 7
• Electrical Impedance Tomography (EIT) provides real-time regional ventilation distribution and can detect areas with abnormal time constants indicating increased regional resistance 2
• Flow-time scalar monitoring on the ventilator screen reveals incomplete exhalation patterns suggesting increased expiratory resistance and auto-PEEP development 2, 8

Clinical Interpretation and Pitfalls

Normal vs. Pathological Values

• Normal respiratory system resistance in mechanically ventilated adults ranges from 5-10 cmH₂O/L/s 5, 9
• COPD patients exhibit substantial frequency-dependence of resistance, with values reaching 25-27 cmH₂O/L/s using expiratory resistance methods 5, 6
• ARDS patients also demonstrate frequency-dependent resistance, contrary to earlier assumptions that this was specific to obstructive disease 5

Common Measurement Errors

• Intrinsic PEEP (auto-PEEP) must be measured and corrected for using end-expiratory hold maneuvers, as it can reach 10-15 cmH₂O in severe obstructive disease and dramatically affects resistance calculations 1, 2, 3, 5
• In patients with abnormal airway resistance breathing through intact upper airways, the long time constant retards pressure transmission from alveoli to mouth, underestimating true P0.1 values 1
• Conversely, in ventilator-dependent patients with rigid endotracheal tubes bypassing the compliant upper airway, P0.1 may more accurately reflect esophageal pressure changes 1

Integration with Overall Respiratory Mechanics Assessment

Always measure resistance alongside compliance and driving pressure to fully characterize respiratory system mechanics. 1, 2, 3

• Static compliance (Cst,rs) = Vt / (Pplat - PEEP) provides the elastic component 1, 2, 5
• Driving pressure (ΔP) = Pplat - PEEP represents total pressure needed to deliver tidal volume and may predict outcomes better than resistance or compliance alone 2, 3
• Dynamic compliance = Vt / (Ppeak - PEEP) provides real-time assessment but conflates resistance and compliance 2

Practical Clinical Algorithm

1. Perform end-inspiratory hold maneuver (0.5-1.5 seconds) in passive, volume-controlled patient 1, 3, 5
2. Record Ppeak and Pplat from ventilator display 2, 4
3. Calculate resistance as (Ppeak - Pplat) / inspiratory flow 5, 4, 6
4. Perform end-expiratory hold to measure auto-PEEP 1, 3, 5
5. Correct compliance calculations for auto-PEEP: use (Pplat - total PEEP) rather than (Pplat - set PEEP) 5
6. Monitor flow-time curves continuously to detect incomplete exhalation indicating dynamic hyperinflation 2, 8

Without correction for auto-PEEP, resistance and compliance values are systematically inaccurate, particularly in COPD patients where auto-PEEP is universal and in other critically ill patients where it occurs in up to 40% of cases. 5