mechanical ventilation Arjun Srinivasan

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1 Respiratory mechanics in mechanical ventilation Arjun Srinivasan

2 Introduction Mechanics during ventilation PV curves Application in health & disease Difficulties & pitfalls The future.

3 Monitoring Mechanics Pressure, flow, and volume Time based graphics (waveforms) Pressure Flow Volume Derived measures Compliance Resistance Loops Pressure volume Flow volume

4 Pressures Peak inspiratory pressure (PIP) Pressure generated to drive gas into the lungs to overcome resistance & elastic property of RS Dependant on flow, resistance, TV, compliance & PEEP Slope of the curve depends on flow pattern Plateau pressure( P plat ) Measured after E ins pause of ~2 secs Represents alveolar l pressure True inflation pressure

5 PEEP i Presence of positive i P aw at E exp in the absence of PEEP E Measured by application of E exp pause Etiology Dynamic hyperinflation with airflow obstruction Dynamic hyperinflation without airflow obstruction Without either Leads to Hyperinflation volutrauma Decreases ti trigger sensitivity fti fatigue Hemodynamic compromise

6 Flow with PEEP i w flo inhalation i 0 time auto PEEP exhalation

7 PEEP E Positive end expiratory pressure set by the clinician Ashbaugh & Petty in 1967 as useful tool in Mx of ARDS Our understanding continues to evolve Is being applied in wide variety of pulmonary disorders So how does it work?

8 Edema & /or atelectasis PEEP E FRC compliance Shunt Work of breathing P a O 2 Principles & practice of mechanical ventilation Martin J. Tobin

9 Obstructive airway disease PEEP E Inspiratory threshold load (PEEP i ) Work of breathing Triggering of ventilator P i i l & ti f h i l til ti Principles & practice of mechanical ventilation Martin J. Tobin

10 Lung inflation & mechanics P rs = P AO + P mus = V x R + V/C V = flow R = resistance V = volume C = compliance of sytem During mechanical ventilation P rs = P AO = V x R + V/C

11 Dynamic mechanics Relation between Pressure, flow & volume without flow interruption Ventilators use linear regression analysis to compute C & R from constantly changing variables from 100 or more equations per breath Static mechanics Done with flow interruption ti ( zero flow)

12 Compliance (C) Change in lung volume per unit change in pressure gradient. V/ P C rs = C l + C cw 1 C Total = C 1 Chestwall + 1 C Lung

13 Static compliance Measured after an end inspiratory pause of few seconds ( to attain P 2 = P plat ) Pause ensures P aw = P avl Is not influenced by R aw Dynamic compliance Measured between E exp and E ins without pause (PIP) Affected by R aw

14 PIP P rs = P AO = V x R + V/C Lucangelo, Respir Care 2005; 50:55 Pressure (cm H2O) Volume (ml) Flow (L/mi in)

15 Need to know about compliance.. Volume dependentd Its relation with volume is non linear ( sigmoid) Specific compliance is useful in overcoming this problem( C/TLC) Hysteresis Unrecoverable energy, or delayed recovery of energy, that is applied to a system Attributed mainly to airfluid interface Intrinsic property also contribute

16 hysteresis

17 Regional variation Compliance measured is a function of many different regional compliances In health, the difference is not significant In disease, its may contribute significantly to distribution of ventilation

18 Resistance Airflow & lung visco elastic property Natural & artificial airways contribute IPPV flow is considered to be laminar

19 R rs = R aw + R l R aw = PIP P 1 / V R l = P 1 P 2 / V R rs = PIP P 2 / V

20 Resistance affected by Volume Larger the volume, lower the resistance Changeswithinspiration inspiration & expiration Regional ldifferences in health h& disease Natural & artificial airway in series

21 PIP Ppl (Peso) tidal volume Crs = Pplat - PEEP Ccw = tidal volume ΔPeso Palv CL = tidal volume (Pplat) (Pplat PEEP) - ΔPeso R i = PIP - Pplat flow

22 Work of breathing (Wob) Performed by the ventilator on a paralyzed patient on full support Calculated during passive constant flow W = (PIP 0.5 x P plat )/ 100 x V T Increases with increase in R, V T or decrease in C PEEP i increases Wob Increase fatigue & weaning fil failure

23 PV curves The quasi static (P V) relationship Lungs deform during breathing in health and disease Devised to diagnose & stage ARDS half a centuary back Plotted with the hope of Diagnosing lungdisease Customize ventilator setting Prognostication i Improve standard of care

24 How to measure? Static methods Supersyringemethod Multiple occlusion method ( gold standard) Dynamic method Constant low flow technique(< 10 L/min) ( easy & fast )

25 Normal PV curve Sigmoid in shape upward concavity at low pressures & downward concavity at higher pressures Balance of forces between chest wall (diaphragm and rib cage) and lung Forces are equal & opposite at FRC Below FRC, chest wall contributes to curvature Lung contributes to curvature above FRC

26 Classics concepts Zone 1 Low compliance Due to collapsed alveoli LIP Proposed magicpoint of recruitment Z 2 Zone 2 Area of linear compliance

27 Lower & upper pflex Tangent from slope UIP Startof over distension Zone 3 Decreased compliance due to over distension

28 Practical difficulties,trials, controversies & changing concepts Identification of LIP Inter observer variation O Keefe OKeefe et al 5 to 9cm variability Harris et al 13cms variability Inflation vs. deflation limb Optimal PEEP? How does recruitment take place? A pubmed search revealed over 600 articles

33 Concept of baby lung Opening Pressure Inflated 0 Small Airway Collapse 10-20cmH 2 O Alveolar l Collapse (Reabsorption) 40-60cmH 2 O Consolidation (modified from Gattinoni)

34 Current understanding Heterogeneity of lung injury wide range of recruitment pressures Injury evolves over time so does the curve LIP beginning i of significant ifi recuitment UIP end of significant recruitment vs. beginning of significant overdistension PEEP is definitely it helpful l but what is the optimum PEEP?

35 Emphysema Early studies of P V relationship in COPD hoped to diagnose and establish the severity of emphysema CT has supplanted the P V curve for diagnosis of emphysema Done on spontaneously breathing patients Increased concavity towards pressure axis irrespective of volume (increased compliance)

36 Asthma Again in spontaneously breathing patients In one study, P V curves (via plethysmography) during exacerbations showed a reduction in lung volume and an increase in elasticity with salbutamol It was due measurement error Thorax1978;33(3): No data describing changes in the P V curve during status asthmaticus

37 ILD Alveoli become fibrotic, reducing lung gas volume, which shifts the P V curve downward on the volume axis Concavity towards pressure axis is reduced Shape of the P V curve may not be a sensitive means of assessing alveolar fibrosis

38 PV curves in emphysema & ILD

39 In CCF Alveoli progressively fill with fluid, which impairs surfactant function and reduces gas volume Pathophysiology similar to early ARDS Marked increase in hysteresis due to surfactant loss Paucity of data depicting PV curves in CCF

40 Obesity Markedly reduced FRC Decreased compliance Lung Chest wall

RESPIRATORY PHYSIOLOGY, PHYSICS AND PATHOLOGY IN RELATION TO ANAESTHESIA AND INTENSIVE CARE

Course n : Course 3 Title: RESPIRATORY PHYSIOLOGY, PHYSICS AND PATHOLOGY IN RELATION TO ANAESTHESIA AND INTENSIVE CARE Sub-category: Intensive Care for Respiratory Distress Topic: Pulmonary Function and