Measuring Pulmonary Function Prac PDF

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Prac 4: Measuring Pulmonary Function RATIONALE The aims of this practical are: (i) To briefly revise your knowledge of spirometry (from MEDI111 and MEDI211), particularly involving forced expiratory manoeuvres (ie. forced spirometry), (ii) Perform additional inspiratory and expiratory manoeuvres to obtain flow-related indices of lung function, and (iii) Introduce the concept and derivation of pulmonary static Pressure-Volume relationships. PREPARATORY READING Reading the following material will help your understanding of this session: 1. The respiratory lab module of the MEDI111 Laboratory Manual. 2. West, J .B. 1992. Pulmonary Pathophysiology: - The Essentials. 4th Edition Williams and Wilkins. Pages 3-13. 3. Wanger, J. Pulmonary Function Testing - A practical approach. 2nd Edition. Williams and Wilkins. Pages 27-37, 40-50. 4. West, J.B. 1995. Respiratory Physiology: - The Essentials. 5th Edition. Williams and Wilkins. Pages 151-154. And Chapter 7

INTRODUCTION Why measure pulmonary function? Measuring ventilatory function is important in determining a person's ability to supply oxygen to, and remove carbon dioxide from, the blood:air interface. Clinically, such knowledge is routinely used when diagnosing cardiorespiratory impairment (though usually not able to establish specific diagnoses), or to assess a patient's rehabilitation or their ability to perform functional/work duties. If pulmonary function is severely impaired, the assessment may be used in determining a patient's ability to withstand surgery. The performance of forced expiratory manoeuvres, before and after a strenuous exercise bout, is also useful in the diagnosis of exercise-induced asthma. Of further importance, pulmonary function tests are used to examine cardiorespiratory function during rest and work, in relation to environmental stress (acute and chronic effects of pollutants, and of hypo and hyper-baric environments), exercise training, and ageing. Spirometry refers to the measurement of lung capacities and volumes. Spirometry allows the measurement of many important aspects of pulmonary function. These measurements are clinically useful in the diagnosis of restrictive and obstructive pulmonary disorders. Such measurements include (see table on next page):

Total Lung Volume

TLV

The amount of gas in the lungs after a maximal (forced) inhalation

Vital Capacity

VC

The maximum amount of gas that can be exhaled following a maximal (forced) inhalation. 1

Tidal Volume

TV

The volume of gas inspired or expired during each normal (unforced) ventilation cycle.

Inspiratory Capacity

IC

The maximum amount of gas that can be inhaled after a normal (unforced) exhalation.

Expiratory Capacity

EC

The maximum amount of gas that can be exhaled after a normal (unforced) inhalation.

Inspiratory Reserve Volume

IRV

The maximum amount of gas that can be forcefully inhaled after a normal (unforced) inhalation.

Expiratory Reserve Volume

ERV

The maximum amount of gas that can be forcefully exhaled after a normal exhalation.

Functional Residual Capacity

FRC

The amount of gas left in the lungs after a normal (unforced) exhalation.

Residual Volume

RV

The amount of gas left in the lungs after maximal (forced) exhalation.

See Figure 4.1 for a graphical representation of these volumes.

Fig 4.1: Lung volumes and capacities as shown on a spirogram Ventilation results from a pressure difference between the pulmonary air and the atmosphere. This pressure difference is created by a change in the volume of the thoracic cavity, since, according to Boyle's law, the pressure of a gas is inversely proportional to its volume. An increase in thoracic volume results in a decrease in intrapulmonary pressure. Air is therefore pushed into the lungs by the greater pressure of the atmosphere (inhalation). When the thoracic volume decreases, the intrapulmonary pressure rises above atmospheric pressure, pushing air out of the lungs (exhalation). In normal ventilation, the thoracic volume is regulated by the actions of the diaphragm and the external intercostal muscles. When the diaphragm is at rest, it forms a convex floor to the thoracic cavity. During inhalation, the diaphragm contracts and pulls itself into a more flattened form. This lowers the floor of the thorax and increases the thoracic volume. At the same time, the contraction of the external intercostal muscles increases the volume of the thorax by rotating the 2

ribs upward and outward. At the end of inhalation, the diaphragm and external intercostal muscles relax, causing the thorax to resume its original volume and the air inside of the lungs to be exhaled. The amount of air inhaled or exhaled in this manner is the tidal volume. Inhalation can become difficult if the air passages are obstructed or if the lungs lose their normal elasticity. In these cases, the affected person relies increasingly on muscles that are not used in normal (tidal) ventilation--scalenus, sternocleidomastoid and pectoralis major. These muscles are also used in healthy people during forced inhalation to obtain the inspiratory reserve volume. During forced exhalation, the internal intercostal muscles contract, depressing the rib cage, and the abdominal muscles contract, pushing the viscera against the diaphragm. The push of the viscera increases the convexity of the diaphragm and decreases the thoracic volume to a smaller level than that achieved during a normal exhalation. The amount of air exhaled by contraction of both groups of muscles is the expiratory reserve volume. Even after a maximum forced exhalation, there is still some air left in the lungs. This residual volume of air makes it easier to inflate the lungs during the next inhalation and oxygenates the blood between ventilation cycles. Measuring Pulmonary function: Spirometry, or more specifically, Forced Spirometry, allows the measurement of several clinically useful indices of pulmonary function. From your lectures and reading, you will have realised that the magnitude of both static and flow-resistive forces are important determinants of one's ability to ventilate their alveoli. In this lab, you will examine both static pulmonary pressures underlying lung volume changes and airflow-lung volume profiles. A spirometer, utilizing flow-sensing will be used to analyze the air. Flow-sensing spirometers measure flow rather than displacement (i.e. bell spirometer). Since they don't require objects to be displaced, they generally avoid errors due to sluggish response kinetics. So, instead of the waterseal (bell) spirometer from previous physiology labs, for this lab you will be using a pneumotachograph, in conjunction with a PowerLab data acquisition system. You have probably also been exposed to other pneumotachographs (in the Quinton Q-Plex I, Quinton Instrument Company, U.S.A.) and a hot wire anemometer (in the Sensor Medics 2900, Sensor Medics Corporation, U.S.A.). 1. Pneumotachograph: measures the pressure differential across a fixed resistance, and works on the basis that flow is equal to the drop in pressure which exists across that given resistance (ie. F= ∆P/R). The pressure gradient increases in direct proportion to the flow. The fixed resistance is sufficiently small that the subject or patient does not sense its presence. The resistance may be a grid of capillary tubes placed parallel to the flow (Fleisch-type), or an arrangement of fine mesh screens. Since resistance must remain constant, condensation in the resistor must be prevented. This is accomplished by increasing the distance between the mouth and pneumotach, or by maintaining the pneumotach screen close to deep body temperature (-37°C). 2. Vortex: Airflow is made turbulent by placing physical obstructions in a tube. Turbulent motion then increases with increasing flow (Re = 2rvd/η) and is able to be detected by the extent to which it disrupts an ultrasonic beam, which is passed across the tube lumen from an emitting to a receiving crystal. The major weakness of this spirometer is its lack of sensitivity to low flows. 3. Turbine: The angular velocity of a turbine within a tube increases with flow. The velocity is detected from the extent of interruption to a light beam that is passed through the turbine, to a photosensitive cell. As mentioned above, this method has the disadvantage that there is a moving 3

component, which introduces inertia. Nevertheless, the method is not susceptible to alterations in the gas composition (including water vapour) or turbulence. 4. Hot-wire anemometer: Either one or two wires are maintained at high temperature in an airstream. Because convective heat transfer acts to cool the wire(s), additional energy is required to maintain the desired temperature(s), The magnitude of energy is related to the airflow. The quantitative relation between flow and convection is dependent on airflow being laminar, which therefore imposes some constraints on the physical setup. Determinants of pulmonary function: The rate with which air ventilates the respiratory tract (Inspired minute ventilation: VI) is the product of the average volume inspired per breath (Tidal Volume: VT[l]) and the number of breaths per unit time (ƒb: [l.min-1]). While both variables are controlled by the respiratory centres, the VT is not simply a reflection of the strength and duration of the central drive for inspiration (and expiration, which is usually passive at rest), since it depends most directly on two interrelated variables: the pressure gradient for airflow, and the airway resistance (Remember: Flow = ∆ P/R). Thus, it is necessary to measure parameters of at least two of these three variables when examining the determinants of pulmonary function, and these are necessarily the flow and airway pressures. This lab is divided into three sections. The first section reviews lung volumes and capacities, since lung volumes are principal determinants of flow (= ∆V/∆t) and pressure (=∆V/Compliance). The second and third sections examine flow and pressure respectively as functions of lung volume. Basis of Section 3: The ease with which thoracic volume changes is its compliance (C), ∆V/∆P. Compliance is specific to the volume at which it is measured and differs for inspiration and expiration, even when obtained under static volume conditions (Cst). Cst of the total respiratory system (CStRS) reflects the inspiratory effort that is required to overcome lung and chest wall elastic recoil pressures in order to change lung volume. Thus, a low compliance (ie. high Elastance) immediately reveals that inspiration will be energetically expensive, even before we consider the flow-resistive forces which must also be overcome to ventilate the alveoli. This low compliance would usually confer a high elastic recoil pressure, which aids in ensuring that expiration is completed passively. Conversely, an abnormally high compliance (as in Emphysema), indicates that inspiration is achieved easily, but that elastic recoil pressure will be low. Thus, active expiratory effort may be necessary if VE is to be sufficient to prevent an increase in PACO2 (hypoventilation). It is therefore apparent that a substantial increase or decrease in CStRS (from the usual 0.2 l.cmH2O-1) will involve an increase in the energy cost of breathing. During this lab you will attempt to obtain Total Respiratory System Pressure (Prs) at each of several static expiratory volumes and will therefore be in a position to estimate the CStRS. While this experiment will not allow you to determine the individual compliances of the chest wall and lung, separately, it is possible to do so.

DATA COLLECTION AND INTERPRETATION: While you will not be collecting any data yourself, I have included the protocol safety and procedures for your information. (Observe the video clip on Moodle) Safety: Pulmonary Function & Spirometry: •

The subject must be at least 1 metre away from the computer at all times.

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Always be alert for adverse physiological effects in the subject. DISCONTINUE at the first sign of a problem – instruct the subject to breathe normally without the spirometer and either close the software or flick down the safety switch on the front panel of the PowerLab.

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Use ONLY equipment supplied; do not attempt to plug any other equipment into the PowerLab. Plug equipment in ONLY according to directions – do not attempt to plug them in elsewhere on the PowerLab.

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Subjects must be seated in chair with back support while using the spirometer

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A clean mouthpiece and straw should be used for each volunteer. Do NOT share mouthpieces or straws!!!!

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If you are suffering from a respiratory infection or from asthma, do not volunteer for this experiment.

Each person who connects him or herself to the spirometer must always breathe through a disposable filter which has a mouthpiece attached, and must not use another group member's. Label your filter with the felt pen, so that you can reuse them as necessary. Be aware that the filter will cause some impairment of the absolute airflow that you are able to generate. If you used a bell spirometer it would involve some re-breathing and it would be necessary to use a soda-lime CO2 scrubber.

Spirometric Technique is important: When performing lung function tests, each forced expiratory manoeuvre should be conducted three times, and should be characterised by: 1. 2. 3. 4.

A good start (ie. No hesitation, and a back-extrapolation of...