Time Constants in the PICU | PICU Doc on Call Shorts

In this episode of *PICU Doc on Call Shorts*, pediatric ICU physicians Dr. Pradip Kamat and Dr. Rahul Damania discuss respiratory time constants and their clinical relevance in pediatric critical care. Using a case of a six-year-old with near-fatal status asthmaticus on mechanical ventilation, they explain how prolonged time constants from high airway resistance cause air trapping, dynamic hyperinflation, and intrinsic PEEP. They emphasize recognizing these issues through ventilator waveforms and highlight that increasing respiratory rate can worsen hypercapnia in obstructive disease. Key management strategies include reducing respiratory rate, extending expiratory time, and accepting permissive hypercapnia to ensure hemodynamic stability.Show HighlightsRespiratory time constants and their clinical significance in pediatric patientsCase study of a six-year-old boy with near-fatal status asthmaticusManagement of severe obstructive respiratory failure in pediatric patientsUnderstanding airway resistance and lung compliance in relation to time constantsImpact of ventilator settings on patient outcomes, including air trapping and intrinsic PEEPImportance of adequate expiratory time to prevent dynamic hyperinflationRecognizing signs of inadequate expiratory time in mechanically ventilated patientsStrategies for managing hypercapnia and optimizing ventilator settingsDifferences in time constants related to various pediatric respiratory conditionsKey takeaways for pediatric critical care practice and ventilator managementReferencesDepta F, Kallet RH, Gentile MA, Kassis EN. Expiratory time constants in mechanically ventilated patients: rethinking the old concept — a narrative review. Intensive Care Medicine Experimental. 2025;13:40. The review summarizes the definition of expiratory time constant, the relationship to resistance and compliance, the 63/86/95/98/99% rule, and clinical applications in obstructive and acute lung injury states.Depta F, et al. Six methods to determine expiratory time constants in mechanically ventilated patients: a prospective observational physiology study. Intensive Care Medicine Experimental. 2024. This study describes expiratory time constant as a parameter that can guide respiratory rate and I:E adjustment to support complete exhalation.Alibrahim O, Rehder KJ, Miller AG, Rotta AT. Mechanical Ventilation and Respiratory Support in the Pediatric Intensive Care Unit. Pediatric Clinics of North America. 2022;69(3):587–605. This pediatric review specifically discusses passive exhalation, the expiratory time constant, and why asthma and bronchiolitis require longer expiratory times to avoid gas trapping.Arnal JM. Monitoring respiratory mechanics in mechanically ventilated patients. Hamilton Medical Knowledge Base. This source provides a practical bedside description of time constants, waveform-based respiratory mechanics, and typical RCexp ranges, while emphasizing dependence on resistance and compliance.Emeriaud G, López-Fernández YM, Iyer NP, et al.; PALICC-2 Group; PALISI Network. Executive summary of the second international guidelines for the diagnosis and management of pediatric ARDS. Pediatric Critical Care Medicine. 2023;24(2):143–168. The PALICC-2 guideline framework supports lung-protective ventilation in PARDS, including attention to tidal volume, PEEP, plateau pressure, and driving pressure.