Physiological Basis of Mechanical Ventilation Wally Carlo, M.D. University of Alabama at Birmingham Department of Pediatrics Division of Neonatology wcarlo@peds.uab.edu
Fine Tuning Mechanical Ventilation (Outline) 1. Pulmonary mechanics 2. Gas exchange 3. Mechanoreceptor reflexes 4. Lung injury 5. Ventilatory strategies
COMPLIANCE 1. Is a measure of elasticity or distensibility of the lungs (and chest wall) 2. Can be calculated as: Compliance = Volume = V T Pressure PIP-PEEP
C C 20 Volume Above FRC (ml) 8 6 4 2 0 0 5 10 15 cmh 2 O P max 0.8 P max High inflection point Low inflection point 20
Tidal Volume (% Max) 100 Effect of Decreased Compliance and 80 60 40 20 0 Inspiratory Time on Tidal Volume 0 200 400 600 800 1000 Time (msec) Both Left Right
RESISTANCE 1. Is a measure of the friction generated by: - movement of gas in the airways (airway resistance) and - movement of lung tissue (tissue resistance) 2. Can be calculated as: Resistance = Pressure Flow
TIME CONSTANT 1. Is a measure of the time necessary for a 63% of a step change (e.g. airway pressure gradient) to equilibrate. 2. Calculated from the product of compliance and resistance. Time constant = Compliance x Resistance
Change in Pressure (%) 100 80 60 40 20 0 TIME CONSTANT 63% 86% 95% 98% 99% 0 1 2 3 4 5 Time Time Constants
Calculations of Pulmonary Mechanics 3 kg Term Normal PIP/PEEP 10/3 Tidal volume (cc) 14 Resistance (cmh 2 O/L/sec) 50 2 kg RDS 14/4 6 100 Compliance Δ V = 14 = 14 = 2 Δ P 10-3 7 Time constant = C x R Time constant = 2 x 50 = 100 msec 6 = 6 = 0.6 14-4 10 0.6 x 100 = 60 msec
Chest wall motion Short TI Optimal TI Long TI Inadequate tidal volume Short insp. plateau Long plateau Time Chest wall motion Short TE Optimal TE Long TE Gas trapping, inadvertent PEEP Expiratory plateau Long expiratory plateau Time
Effect of Incomplete Inspiration on Gas Exchange Incomplete Inspiration Tidal volume Mean airway pressure Hypercapnia Hypoxemia
Effect of Incomplete Expiration on Gas Exchange Incomplete expiration Gas trapping Compliance Tidal volume Mean airway pressure Tidal volume Cardiac output Hypercapnia Hyperoxemia
Detection of Gas Trapping Anticipate with high respiratory rates, normal C, and high R Decreased chest wall movement Inadvertent PEEP Overexpansion on chest radiograph CO 2 retention; or PaO 2 Increased lung volume Depressed cardiovascular function Increased CVP, decreased blood pressure Metabolic acidosis, peripheral edema Increased intrathoracic or esophageal pressure
Because the interaction of the ventilator and the lungs is strongly dependent on the mechanical properties of the respiratory system, the pathophysiology of the pulmonary disease deserves serious consideration when developing ventilatory strategies.
Birmingham Civil Rights Institute Vulcan Vulcan Park City of Birmingham McWane Center Historic Tutwiler Hotel
Fine Tuning Mechanical Ventilation (Outline) 1. Pulmonary mechanics 2. Gas exchange 3. Mechanoreceptor reflexes 4. Lung injury 5. Ventilatory strategies
Mechanisms of Hypoxemia 1. V/Q mismatch 2. Shunt 3. Hypoventilation 4. Diffusion limitation
Ventilation Perfusion (V/Q) Mismatch a. Is an important and frequent cause of hypoxemia in neonates (e.g. RDS) b. Normal ratio of ventilation to perfusion should approximate 1, but in V/Q mismatch it does not approximate 1 c. Supplemental oxygen can largely overcome the hypoxemia
Shunt a. Common cause of hypoxemia in neonates b. May be physiologic, intracardiac (e.g. PPHN, CHD), or pulmonary (e.g. atelectasis) c. Supplemental O 2 cannot reverse the hypoxemia
Hypoventilation a. Common cause of transient hypoxemia (e.g. apnea) b. Rate of oxygen uptake from the alveoli exceeds its replenishment c. Supplemental O 2 can easily overcome the hypoxemia
Diffusion Limitation a. Is an uncommon cause of severe hypoxemia, even in the presence of lung disease (e.g. pulmonary edema) b. Occurs when mixed venous blood does not equilibrate with alveolar gas c. Supplemental O 2 can easily overcome the hypoxemia
Summary of Mechanisms of Hypoxemia Common Responsive to O 2 V/Q mismatch Yes Somewhat Shunt Yes No Hypoventilation Transient Very Diffusion limitation No Very
FiO 2 OXYGENATION Ventilation parameters that determine oxygenation MEAN AIRWAY PRESSURE PEAK INSP. PRESSURE FLOW END EXP. PRESSURE I:E RATIO
Mechanisms of Hypercapnia 1. Hypoventilation (decreased V T or f) 2. Increased dead space a. Gas exchange is inefficient because of wasted ventilation b. Includes ventilation to conducting airways and to alveolar spaces not perfused 3. V/Q mismatch 4. Shunt
CO 2 ELIMINATION Ventilation parameters that determine CO 2 elimination MINUTE VENTILATION FREQUENCY TIDAL VOLUME RESISTANCE EXP TIME INSP TIME TIME CONSTANT I:E RATIO PRESSURE GRADIENT COMPLIANCE END EXP PRESSURE PEAK INSP PRESSURE
UAB
Fine Tuning Mechanical Ventilation (Outline) 1. Pulmonary mechanics 2. Gas exchange 3. Mechanoreceptor reflexes 4. Lung injury 5. Ventilatory strategies
Respiratory Mechanoreflexes Hering-Breuer reflex Head s paradoxical reflex Intercostal phrenic inhibitory reflex
Hering-Breuer Reflex In response to lung inflation there is cessation of inspiration and prolongation of the expiratory time In response to an end expiratory occlusion there is a prolongation of the inspiratory time (because the lungs do not inflate and the reflex is not active)
Example of a Potent Hering-Breuer Reflex
Head s Paradoxical or the Augmenting Inspiratory Reflex 15 Volume (ml) 2 sec 15 +120 Oesophageal pressure (mmh 2 O) -120 30 Ventilator pressure (cmh 2 O) 0 Greenough et al. Neonatal Respiratory Disorders 1996
Fine Tuning Mechanical Ventilation (Outline) 1. Pulmonary mechanics 2. Gas exchange 3. Mechanoreceptor reflexes 4. Lung injury 5. Ventilatory strategies
LUNG INJURY DURING ASSISTED VENTILATION 1. Chest wall restriction limits pressure-induced lung injury (Hernandez, et al., 1988) 2. Overexpansion of the thorax with negative pressures causes lung injury (Dreyfus, et al. 1988)
VOLUME vs PRESSURE IN LUNG INJURY Pulm. Epith. Hyaline Lymph Filtr. Volume Pressure Edema Injury Memb. Flow Coef. IPPV High High Yes Yes Yes Yes Yes Iron Lung High Low Yes Yes Yes N/A N/A Strapping Low High No No No No No Dreyfus et al, 1988; Bshouty et al, 1988; Hernandez et al, 1989; Corbridge et al, 1990; Carlton et al 1990; Zhenxing et al, 1992
WHICH VOLUMES CAUSE LUNG INJURY? Volutrauma Zone Overdistention Time Volutrauma Zone A B C D Atelectasis A High V T low PEEP W. Carlo 2003 B Normal V T, high PEEP C Normal V T low PEEP D Optimal ventilation
EFFECT OF TIDAL VOLUME ON LUNG COMPLIANCE Compliance (cc/cmh 2 O kg) 3 2 1 0 0 60 120 180 240 Age (min) 8 cc/kg 16cc/kg 32 cc/kg Bjorklund et al. 39:326A, 1996; Bjorkland et al. Pediatr Res 42:348, 1997
EFFECT OF TIMING INFLATION ON LUNG VOLUTRAUMA Compliance (cc/cmh 2 O kg) 6 4 2 0 0 60 120 180 240 Age (min) Ingirmarsson et al. Pediatr Res 41:255A, 1997; Bjorkland et al. Pediatr Res 42:348, 1997. After Surfactant Before Surfactant
EARLY PEEP AND LUNG FUNCTION PEEP 4 PEEP 7 PEEP 0-Nat PEEP 0 Michna et al. Am J Respir Crit Care Med. 160:634, 1999
VENTILATOR-ASSOCIATED LUNG INJURY (VALI) Likely mechanisms Volume rather than pressures End expiratory volume rather than V T or FRC Transalveolar pressure and reopening of alveoli Repeated collapse and reopening of alveoli Very low positive end expiratory pressure Oxidant injury
REASONS FOR SUSCEPTIBLITY OF NEONATAL LUNG INJURY Poorly compliant alveoli but highly compliant airways Immature antioxidant defense systems Immature macrophages and leukocytes and altered airway clearance mechanisms Poorly developed antioxidants antiproteolytic, antielastolytic systems Increased permeability of alveolar-capillary membrane
CONCEPTS IN APPLICATION ASSISTED VENTILATION Assisted ventilation may lead to adverse consequences To minimize side effects, blood gas targets do not have to be in the normal ranges Gas trapping (dynamic hyperinflation) and alveolar overdistention may lead to lung damage and should be limited ACCP Conference. Chest 104:1833, 1993.
Fine Tuning Mechanical Ventilation (Outline) 1. Pulmonary mechanics 2. Gas exchange 3. Mechanoreceptor reflexes 4. Lung injury 5. Ventilatory strategies