Asthma

Approaches for Asthma Jon N. Meliones, MD, MS, FCCM Professor of Pediatrics and Anesthesia Duke University

Outline • What is the question? • Pressure Control vs Volume Control • Decelerating Flow vs Constant Flow • Ventilation Physiology • Asthma Pathology • Data on Decelerating Flow • If you have to ventilate…the preferred approach

Modes of Ventilation Limit Pressure Volume Pressure PRVC Control Decelerating Flow Pressure Volume Constant Control Control

Outline • What is the question? • Ventilation Physiology • Asthma Pathology • Data on Decelerating Flow • If you have to ventilate…the preferred approach

Ventilation Myths • Increasingly Complex (Marketing directors) • Host of New “Toys” • New Modes: Do Not Describe Functionality • Different Ventilator Manufacturers – Similar modes = different functions – Cute names that mean nothing – Don’t say what they do – Measures are inaccurate

Key Functionality of Ventilators • Flow Pattern – Gas Flow Delivered & Distribution During Inspiration – Decelerating or Constant • Limit – Safety: Prevents the Ventilator from Exceeding Preset limit – Volume or Pressure • Cycle – When Inspiration Ends • Trigger – How the Breath is Initiated • Breath Type – Single or Mixed

Effects of Flow Pattern on Airway Pressures Decelerating Square Flow (l/sec) MAP = Area Under Curve PIP PIP Airway Pressure (cmH20) MAP

Square Wave Flow v. Decel Flow • Randomized Cross Over Controlled Study of VCV v PCV • Saline Lavage Decel Square Wave pValue PIP 38.2 + 5.5 46.0 + 4.4 <0.0001 MAP 12.0 + 2.2 10.0 + 2.1 <0.02 Cdyn 15.7 + 3.0 14.2 + 3.0 <0.0001 E. Williams: CCM 2000

Peak Inspiratory Pressure P<0.05 PIP (cm H2O) Cheifetz: CCM 1995

Dynamic Compliance P<0.05

Benefits of Limiting Peak Plateau Pressure • There are no prospective randomized controlled studies demonstrate ANY significant benefit based on the method of ventilation except for limiting Pplat: • Benefits of Limiting Pplat: Amato; ARDS network etc. – Pplat<20 above PEEP – VT< 6ml/kg – Dec mortality 22% • Who cares what the PaCO2 is: – The heart still works, you can always buffer! – As long as ICP not increased! – Limiting Pplat determines outcome!

Pediatric Asthma Data • Heliox: Pediatrics 2005 Nov Kim IK – Reduces the risk of admission for greater than 12 hours by 60% • Iipratropium bromide – No Data to support • Magnesium – No good data • NIBP – Transient improvements

Limit • Safety Check • Prevents The Ventilator From Exceeding a Set Variable – Pressure Limit • Controlled or Support Modes – Volume Limit • Controlled Modes – Minute Volume - Support Modes

Limits • Pressure Limits – Dependent Variable = VT:(compliance/resistance) – Theoretical Advantage: Limit PIP (barotrauma) – Theoretical Disadvantage: Hypo/Hyper Ventilation • Volume Limits – Dependent Variable = Pressure :(compliance/resistance) – Theoretical Advantage: Stable Min Vent (PaCO2) – Theoretical Disadvantage: PIP (barotrauma) • Minute Volume – Advantage: Auto weaning – Disadvantage: Fast / slow breathing rates

Modes of Ventilation Limit Pressure Volume Pressure PRVC Control Decelerating Flow Pressure Volume Constant Control Control

Outline • What is the question? • Ventilation Physiology • Asthma Pathopsiology • Data on Decelerating Flow • If you have to ventilate…the preferred approach

Pathophysiology of Asthma • Marked increased airways resistance • Prolonged Time Constant • TC = Resistance x Compliance • Degree of obstruction non-uniform resulting in varying TC • Expiratory TC worse: • Narrowing of airway during expiration. • Upstream displacement of equal pressure point, • Usually, minimal alveolar disease

Pathophysiology of Asthma

Intrinsic PEEP: Dynamic Hyperinflation Beginning Premature initiation of of Inspiration Inspiration End of Inspiration Retained Gas Results in PEEPi Termination Beginning Premature of of Termination of Exhalation Exhalation Exhalation

Intrinsic PEEP/Dynamic Hyperinflation • Expiratory gas flow continues at the end of the time allotted for exhalation. • PEEPi may lead to excessive MAP. – Pulmonary effects: • Barotrauma – Cardiac effects: • Impedance of venous return • Decreased cardiac output

Systemic Venous Return (RV Preload) PSV RAP = mean systemic venous pressure PPV increases right atrial pressure Right Atrial spontaneous Pressure breathing 0 0 Max Systemic Venous Return

Effect of Lung Volume on PVR Overexpansion DHI Atelectasis PVR Total PVR Small Vessels Large Vessels FRC Lung Volume

Overdistention and C.O. 1000 950 PEEP 5 PEEP 10 900 Cardiac 850 Output 800 750 (mL/min) 700 650 600 550 500 10 15 20 Tidal Volume (mL/kg) Cheifetz: CCM 1998

Decelerating Flow in Asthma • Pressure controlled ventilation in severe asthma. Lopez Pediatr Pulmonol 1996;21:401 • Pressure-support ventilation in children with severe asthma. Wetzel Crit Care Med 1996;24:1603-1605. • Refractory asthma, part 2: airway interventions and management. Jagoda A. Ann Emerg Med. 1997;29:275- 281 • Mechanical ventilation for children with status asthmaticus. Sabato K, Hanson JH. Respir Care Clin North Am. 2000;6:171-188. • Decelerating Flow in 51 Pediatric Asthma Patients

Decelerating Flow in Asthma • Hypothesis: • VCV with constant flow distributes more volume to the less obstructed airways with shorter TC and less volume to longer TC. • Uneven Ventilation, Hyperexpansion of “normal lung” under-ventilation of obstructed units • Elevated PIP and higher airways resistance • Decreased Compliance • High resistance, short IT = Premature termination of breath and set VT not achieved

Decelerating Flow in Asthma • Decelerating flow • Flow varies; • High at first (overcomes high resistance) to achieve set pressure early in inspiration • Lower later in inspiration to maintain this pressure through the inspiratory time.

Decelerating Flow in Asthma • Decelerating flow • Provides a relatively constant inflation pressure: • Large airways fill with peak flow, smaller airways with slower flow • Lung units with short TC attain final volume early • Lung units with long TC continue to receive volume later in inspiration • Pressure equilibrium more even ventilation • Lower Pplat or better ventilation for same PIP • Increased Compliance

Decelerating Flow in 51 Pediatric Asthma Patients Sarnaik, PCCM 2004 pH VCV PCV Mode of Ventilation

Decelerating Flow in 51 Pediatric Asthma Patients PaCO2 VCV PCV Mode of Ventilation

Decelerating Flow in 51 Pediatric Asthma Patients • In Pts with PCO2>45, median time to reversal was 5 hrs • SaO2 >95% in all patients • 2 pts with Pneumos pre PCV • 1 pts developed pneumothorax, 1 pt subq emphesema; all well tolerated and resolved • 100% survival • 100% neuro intact • Median ventilation 4-107 hrs.

Adults Agree! Decelerating flow not just for kids! • Measurement of air trapping, PEEPI and DHI in mechanically ventilated patients. Blanch Respir Care. 2005;50:110-124. • Clinical Review: Severe Asthma Papiris Critical Care 2002;6:30-44. • Lung Protective Strategies for Acute Severe Asthma. Brown. J of Resp Care Pract. 2002;2 • Refractory asthma, part 2: airway interventions and management. Jagoda Ann Emerg Med. 1997;29:275-281. • Mechanical ventilation for children with status

Ventilation Approach • Get the gas out…Limit lung injury! • Avoid DHI / Auto PEEP: • Prolong exhalation times & Low rates • Graphics to ensure complete exhalation • Minimize Pplat: Assure adequate oxygenation & ventilation but allow hypercapnea if Pplat elevated

Ventilation Approach • PEEP controversial; low but not zero, usual <5 • Pick a mode you know…but…Decelerating flow appears to be the best choice! • A volume/minute ventilation “measurement / guarantee” during decelerating flow is preferred

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Asthma

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Asthma — Presentation Transcript