Dynamic Lung Mechanics PDF

VENTILATION Tamethia Perkins MS, RRT-NPS, RRT-ACCS RT 3005/6005 Dynamic Lung Mechanics ◼ Study of forces in action. ◼ Movement of gas in and out of the lungs and P changes required to move the gas. ◼ Explained by ◼ Poiseuille’s law ◼ Airway resistance equation ...

VENTILATION Tamethia Perkins MS, RRT-NPS, RRT-ACCS RT 3005/6005 Dynamic Lung Mechanics ◼ Study of forces in action. ◼ Movement of gas in and out of the lungs and P changes required to move the gas. ◼ Explained by ◼ Poiseuille’s law ◼ Airway resistance equation  Poiseuille’s Law ◼ During inspiration: ◼ Ppl decreases ◼ Bronchial airways lengthen and increase diameter (passive dilation). ◼ During exhalation: opposite phenomenon. ◼ In abnormal situations: ◼ Bronchial gas flow and Ppl may change … affecting gas flow. Normal Inspiration Poiseuille’s Law for Flow ◼ °V= ΔPr4  /8lŋ ◼ ŋ= viscosity of gas or fluid ◼ ΔP= change in P ◼ l= length of the tube ◼ °V= gas flowing through the tube ◼  /8= constants ◼ Flow will increase if P and radius increase, and will decrease if viscosity and length are increased. ◼ If P remains constant and r is decreased by half = flow decreases to 1/16th of initial flow. Poiseuille’s Law for Flow  Poiseuille’s Law for P ◼ P= °V 8lŋ /r4  ◼ P is directly proportional to flow, length and viscosity of the gas. ◼ P decreases in response to increased tube radius. ◼ Combined: P= °V/r4 … °V=Pr4 Poiseuille’s Law for P Poiseuille’s Law for P Poiseuille’s Combined Airway Resistance (Raw) ◼ Raw= P difference between the mouth and the alveoli (Pta). ◼ Resistance to gas flow created by the airways. ◼ Raw= P(cmH2O)/ °V(L/sec) ◼ Normal Raw=1.0-2.0 cm H2O/L/sec ◼ Two types of flow: ◼ Laminar flow ◼ Turbulent flow  Types of Flow ◼ Laminar Flow ◼ Streamlined. ◼ Gas molecules move parallel to the sides of the tube. ◼ At low flow rates and low P gradients. ◼ Turbulent Flow ◼ Random movement of gas. ◼ Gas flow encounters R from both sides of the tube and collide with each other. ◼ At high flow rates and high P gradients. Types of Flow  Time Constants ◼ Product of Raw and CL. ◼ Time in seconds necessary to inflate a lung region to 60% of its potential filling capacity. ◼ Lung regions with high Raw and high CL require more time to inflate (Long Time Constant). ◼ TC(sec)=(P/°V) Raw x (V/ P) CL Time Constants  Dynamic Compliance ◼ How readily a lung region fills during a period of gas flow. ◼ It is a product of time constants. ◼ CL(dyn) is measured during a period of gas flow. ◼ In the healthy lung the CL dyn is about equal to CL stat. ◼ In the obstructed airways CL dyn/ CL stat falls at high respiratory rates (frequency dependent alveoli). Breathing Frequencies