Principles of Biomedical Systems & DevicesPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Principles of Biomedical Systems & DevicesLecture 18The Respiratory System MeasurementsPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Measurements of the Pulmonary Function Assessment of the pulmonary function is primarily based on the measurements of the ª lung volumes, ª lung capacities, ª air flow, ª airway resistance, ª lung compliance and elasticity, ª Intrathoracic and intraalveolar pressures These measurements make use of one or more of four main gas lawsPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Gas Laws Boyle’s Lawª Volume of gas varies inversely with the pressure, if temperature is held constant Charles’ Lawª Volume of a gas is directly proportional to its absolute temperature, if pressure is held constant2112PPVV=1212TTVV=PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Gas Laws Dalton’s Lawª Total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of various gasses. Furthermore, the partial pressure of a gas in a mixture is equal to the pressure of that gas if it were alone (in the container) Henry’s Lawª If temperature is held constant, the quantity of a gas that will dissove in a solution is proportional to the partial pressure of that gas. This is why more O2is dissolved into the tissues from the arterial blood and more CO2 into the venous blood from the tissues.ntotalPPPP +++=L21222NCOOtotalPPPP ++≅PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Partial Pressure ChangesPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Measurable variables in the Respiratory System Very few parameters can actually be directly measured, typicallythose that have direct connection to external airª Flow of gas through the mouth and nose (flowmeter)• The integral of this flow provides the volume of gasª Pressure at the mouth, nose and body surfaceª Partial pressures of various gases in mixtures passing through air way opening (AWO)ª Partial pressures of gases in blood samples (in vitro)ª Temperature / core body temperature Most other parameters need to be inferred / computed from other measurementsPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Other parameters Airway resistanceª Airway resistance relates to the ease at which air flows through the tubular respiratory structures. Higher resistances occur in smaller tubes (bronchioles and alveoli) Lung complianceª The ability of the alveoli and lung tissue to expand on inspiration()()AWOALVAWOAWQPPQPR∂−∂=∂∆∂=()∆∂∆∆∂∆=∆∂∂=compliancewallchestPVcompliancelungPVPVCWallLungsLungsLungs)()()()(surfaceBodyPleuralWPleuralAWOLPPPPPP−=∆−=∆PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Other Parameters Lung Elasticityª The ability of the lung’s elastic tissues to recoil during expiration Intrathoracic Pressureª Positive and negative pressure occurring within the thoracic cavity, critical to proper inspiration (negative internal pressure during inspiration and positive internal pressure during expiration) Intraalveolar Pressureª Pressure within the alveoli. Important in maintaining proper respiration and gas exchange to and from bloodPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Flowmeters The easiest parameter that can be measured is the airflow, particularly at the airway opening Various flowmeters have been developed for this purposeª Rotating-vane flowmetersª Ultrasonic flowmetersª Thermal convection flowmeters (hot-wire anemometer)ª Differential pressure flowmeters : most commonly used in clinical applications : they provide bidirectional flow information, sufficient accuracy, sensitivity, linearity and frequency responsePBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Pneumotachometers(Pneumotachs) Based on the principle that convective flow occurs as a result of a difference in pressure between two points There is a linear relationship between pressure difference and volume-flow rate through any system, which is then used to estimate the flow by measuring the pressure difference using a pressure transducer They consist of ª (one or more) fine mesh screen(s) placed perpendicular to the flowª or a tightly packed bundle of capillary tubes / channelsPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404PneumotachsPressure across the barrier must be < 1 cm H2O, otherwise normal breathing may be affected. A typical mesh is a 50 mm diameter with 158 wires / cm (400 / inch).PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Pneumotach∫=⇒∆∆==tQdtKVtVKQAirflow 'causes pressure drop ∆PQcPQP⋅=∆∆αPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404PneumotachsPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404PneumotachsPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Lung Volumes & Capacities Many functional indications of the respiratory system can be obtained by measuring various lung volumes and capacitiesPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Other major Measurements Maximum Voluntary Ventilation:ª Deep, rapid breathing Forced Expiratory Volume -1:ª Volume of air expired in maximal forceful effort in 1 second Maximum Expiratory Flow Rateª Rate of air flow during maximum forceful expiration Minute Volumeª Inspiratory volume measured in a period of 1 minutePBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Spirometry A practical way to measure / test the pulmonary function is to integrate the output of a flowmeter placed at a subject’s mouth. However, the most common procedure used for estimating the lung volumes (since 19thcentury) is to continuously collect the gas passing through the airway opening and to compute the volume it occupies in the lungs using a system called spirometer.  The spirometer automatically does the integration due to its structural nature.Â