Advanced Pulmonary Mechanics during Mechanical Ventilation

Points of Discussion Basics Abnormalities Equation of motion Dynamic Compliance Pressure-volume loop Flow-volume loop Work of breathing Lower and upper inflection points Hysterexis Intrathoracic Pressures Air-leak Air trapping Increased airway resistance Inadequate flow support Inadequate sensitivity Atelectasis Inadequate PEEP Airway obstruction Over-distension

Tube + Spring Model Resistive Forces Elastic Forces

Static and Dynamic Pressures PIP Flow-Resistive Pressure difference (Pres) Pplat Alveolar Distending (recoil) Pressure difference (Pdis) PEEP time

Pressure difference = Flow Rate x Resistance Airway Resistance Pressure difference = Flow Rate x Resistance dP = Q x R R = 8 L (visc.) p r 4

Resistive and Elastive Forces DYNAMIC CHARACTERISTICS: dP = dV / Cdyn RESISTANCE: dPresistive = R x Flow STATIC COMPLIANCE: dPdistensive = dV / Cst dP = dPresist. + dP dist. dP = R x Flow + dV / C st

Basic Calculations dP = R x Flow + dV / C st Cst = dV / (Pplat-PEEP) Pressure Cst = dV / (Pplat-PEEP) R = (PIP-Pplat) / Flow Pplat PEEP time

Assessment of static P-V curve Super-syringe method: Stepwise inflation from a big syringe with multiply occlusions at each volumes to record recoil pressure Time consuming Cumbersome to perform Difficult to standardize Patient must be paralysed, connected to a special equipment Great risk of oxygen desaturation Volume Pressure

Assessment of static P-V curve Slow Flow Single Inflation Method Slow (5-10 lpm) inspiratory flow with large Vt and ZEEP The inspiratory curve of the dynamic P-V loop closely approximates the static curve The flow-resistive pressure component could be subtracted Easy to perform, fast and relatively comfortable Servillo: AJRCCM 1997 Lu: AJRCCM 1999 Volume Static curve UPIflex inspiration LPIflex Pressure

FRC and PV Loop Normal Compliance TLC FRC FRC VOLUME Negative Positive DISTENDING PRESSURE Normal Compliance FRC FRC

Components of Pressure-Volume Loop VT Expiration Volume (mL) Inspiration PIP Paw (cm H2O)

Pressure-Volume Loop (Type of Breath) Vol (ml) I E I I Paw (cm H2O) Controlled Assisted Spontaneous I: Inspiration E: Expiration

PEEP and P-V Loop VT PIP Volume (mL) PEEP Paw (cm H2O)

Inflection Points Upper Inflection Point: Represents pressure resulting in regional overdistension Lower Inflection Point: Represents minimal pressure for adequate alveolar recruitment Upper Inflection Point Volume (mL) Lower Inflection Point Pressure (cm H2O)

Decreased Compliance Normal Patient Volume(ml) Pressure (cm H2O)

Lung Compliance Changes and the P-V Loop Volume Targeted Ventilation Preset VT Increased Normal Decreased Volume (mL) Paw (cm H2O) PIP levels

Lung Compliance Changes and the P-V Loop VT levels Increased Normal Decreased Pressure Targeted Ventilation Volume (mL) Paw (cm H2O) Preset PIP

Hysteresis Normal Hysteresis Abnormal Hysteresis Volume (ml) Pressure (cm H2O)

Flow-Volume Loop Inspiration PIFR FRC VT PEFR Expiration Volume (ml) Flow (L/min) PEFR Expiration

Positive Pressure Ventilation: The Equation of Motion In a passive subject, airway pressure represents the entire pressure (P) applied across the respiratory system. The work required to deliver a tidal breath (Wb) = tidal volume (VT) x airway pressure The pressure (P) associated with the delivery of a tidal breath is defined by the simplified equation of motion of the respiratory system (lungs & chest wall): P = VT/CR+ VT/Ti x RR + PEEP total P elastic P resistive P elastic Where CR = compliance of the respiratory system, Ti = inspiratory time and VT/Ti = Flow, RR = resistance of the respiratory system and PEEP total = the alveolar pressure at the end of expiration = external PEEP + auto (or intrinsic) PEEP, if any. Auto PEEP = PEEP total – P extrinsic (PEEP dialed in the ventilator) adds to the inspiratory pressure one needs to generate a tidal breath.

Work of Breathing B A: Resistive Work B: Elastic Work A Volume (ml) Pressure (cm H2O)

Work of Breathing WOB is a major source of caloric expenditure and oxygen consumption Appr. 70% to overcome elastic forces, 30% flow-resistive work Patient work is a one of the most sensitive indicator of ventilator dependency Comparison of Ventilator and Patient work is a useful indicator during weaning process WOB may be altered by changes in compliance, resistance, patient effort, level of support, PEEP, improper Ti, demand system sensitivity, mode setting Elevated WOB may contraindicate the weaning process

WOB Measurements WOB = ∫0 ti P x Vdt B C E D A Elasic work: ABCA Resistive work Inspiratory: ADCA Expiratory: ACEA B C E D A P

Work of Breathing Measurements WOB = ∫0 ti P x Vdt Paw: Ventilator Work: The physical force required to move gas into the lung, represents the total work of the resp. system (patient + ventilator) Peso: Patient Work: done by respiratory muscles, represents the pulmonary work of breathing Paw-Ptr: Imposed Work by the Endotracheal tube

P-V Loop and WOB V P V V P P Normal Compliance Decreased Compliance Increased Resistance Decreased Compliance Normal Resistance P V Normal Compliance Normal Resistance V P P

Work of Breathing Work per breath is depicted as a pressure-volume area Work per breath (Wbreath) = P x tidal volume (VT) Wmin = wbreath x respiratory rate WEL = elastic work Volume Volume Volume WR = resistive work VT Pressure Pressure Pressure The total work of breathing can be partitioned between an elastic and resistive work. By analogy, the pressure needed to inflate a balloon through a straw varies; one needs to overcome the resistance of the straw and the elasticity of the balloon.

Intrinsic PEEP and Work of Breathing When present, intrinsic PEEP contributes to the work of breaking and can be offset by applying external PEEP. Volume VT VT Dynamic Hyperinflation FRC Pressure PEEPi PEEPi = intrinsic or auto PEEP; green triangle = tidal elastic work; red loop = flow resistive work; blue rectangle = work expended in offsetting intrinsic PEEP (an expiratory driver) during inflation

The Pressure and Work of Breathing can be Entirely Provided by the Ventilator (Passive Patient) + + ₊ ₊ + +

Paw = Airway pressure, Pes= esophageal pressure The Work of Breathing can be Shared Between the Ventilator and the Patient The ventilator generates positive pressure within the airway and the patient’s inspiratory muscles generate negative pressure in the pleural space. AC mode PAW patient machine PES time Paw = Airway pressure, Pes= esophageal pressure

Work of breath Paw Pes Pressure Work to inflate the chest wall Volume Resistive Work Elastic Work of Lung Pressure Elastic Work of Chest Paw Pes Work to inflate the chest wall Inflation Deflation Volume

Relationship Between the Set Pressure Support Level and the Patient’s Breathing Effort The changes in Pes (esophageal pressure) and in the diaphragmatic activity (EMG) associated with the increase in the level of mask pressure (Pmask = pressure support) indicate transfer of the work of breathing from the patient to the ventilator. Carrey et al. Chest. 1990;97:150.

Partitioning of the Workload Between the Ventilator and the Patient How the work of breathing partitions between the patient and the ventilator depends on: Mode of ventilation (e.g., in assist control most of the work is usually done by the ventilator) Patient effort and synchrony with the mode of ventilation Specific settings of a given mode (e.g., level of pressure in PS and set rate in SIMV)

Intrathoracic pressures PROX. AIRWAY PRESSURE TRACHEAL PRESSURE PLEURAL PRESSURE ALVEOLAR PRESSURE

3 Levels of Lung Mechanics DYNAMIC CHARACTERISTICS: dP = dV / Cdyn RESISTANCE: dP = R x Flow STATIC COMPLIANCE: dP = dV / Cst AIRWAY RESISTANCE: dP = Raw x Flow LUNG COMPLIANCE: dP = dV / CL IMPOSED RESISTANCE: dP = Rimp x Flow CHEST WALL COMPLIANCE: dP = dV / Ccw

A closer look at lung mechanics Crs = Vt / dPdist (aw) Ccw = Vt / dPdist (pl) CL = Vt / Pdist (aw - pl) or (tr - pl) Rrs = Presist (aw) /Flow RL = Presist (tr) / Flow Rimp = Presist (aw-tr)/ Flow Paw Ptr Ppl P dP resist dPdist t

Respiratory Mechanics in ARF* Volume Reduced range of volume excursion: Low compliance Flattering at low and high volumes: Lower and upper inflection points *Bigatello: Br J Anaest 1996 NORMAL ARDS Pressure

Lung Protective Strategy Set PEEP above the lower Pflex to keep the lung open and avoid alveolar collapse Apply small Vt to minimize stretching forces Set Pplat below the upper Pflex to avoid regional overdistension Volume Pressure

Abnormalities Air-leak Air trapping Increased airway resistance Inadequate flow support Inadequate sensitivity Atelectasis Inadequate PEEP Airway obstruction Over-distension

Air Leak Volume (ml) Air Leak Pressure (cm H2O)

Air Leak Inspiration Expiration Air Leak in mL Normal Abnormal Flow (L/min) Volume (ml) Air Leak in mL Normal Abnormal Expiration

Air Leak Volume (mL) Time (sec)

Air Trapping Inspiration Normal Patient Time (sec) Flow (L/min) } Auto-PEEP Expiration

Air Trapping Inspiration Expiration Normal Abnormal Flow (L/min) Does not return to baseline Volume (ml) Normal Abnormal Expiration

Response to Bronchodilator Before After Time (sec) Flow (L/min) Long TE PEFR Shorter TE Higher PEFR

Increased Airway Resistance Inspiration Flow (L/min) Volume (ml) Normal Abnormal “Scooped out” pattern Decreased PEFR Expiration

Increased Raw Normal Slope Lower Slope Higher PTA Pressure (cm H2O) Vol (mL) Normal Slope Lower Slope Pressure (cm H2O)

Inadequate Inspiratory Flow Volume (ml) Active Inspiration Inappropriate Flow Paw (cm H2O)

Airway Secretions/Water in the Circuit Inspiration Flow (L/min) Volume (ml) Normal Abnormal Expiration

Airway Obstruction F F V V Before Suction After Suction

Optimising PEEP V V P P PEEP: 3 cmH2O PEEP: 8 cmH2O

Inadequate Sensitivity Volume (mL) Paw (cm H2O) Increased WOB

Atelectasis Lost FRC Replaced FRC V V P P

With little or no change in VT Overdistension With little or no change in VT Normal Abnormal Volume (ml) Pressure (cm H2O) Paw rises

Overdistension Overdistension occurs when the volume limit of some components of the lung has been exceeded Abrupt decrease in compliance at the termination of inspiration Results in a terminal “Beaking” of the P/V Loop Volume Pressure

Overdistension Index Volume C20 Cdyn Pressure 0.8 Pmax Pmax

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