RESEARCH ON HUMAN RESPIRATORY PARAMETERS

Transcription

1 RESEARCH ON HUMAN RESPIRATORY PARAMETERS Purpose of experiment Determine the main respiratory parameters at rest and in a state of physical activity. Tasks of experiment Determine the breathing period and frequency at the rest and in a state of physical activity. Determine an exhaled air volume during one breathing cycle at the rest and in a state of physical activity. Determine the maximum exhaling air volume. Theoretical topics Gas, physical properties and the laws of gasses. Mean free path of molecules and its dependence on pressure and temperature. Hagen Poiseuille equation. Respiratory physics, basic respiratory parameters. Diaphragm and thoracic breathing. Equipment and materials Pressure measurement module with Cobra3 BASIC-Unit measuring system (pressure measuring limits: 0 2 bar, device error 0.5 mbar), rubber tubes, ruler, vernier caliper, PC with USB port and with software. Theoretical part Structure of the Lungs. Air is inhaled through the nose or mouth and then through the pharynx, larynx, and the trachea (windpipe). The trachea divides into the right and left bronchus, each of which continues to bifurcate into smaller and smaller bronchi and bronchioles over 23 levels of bifurcation until they form alveoli. There are about alveoli, each mm in diameter, with walls that are 0.4 μm thick. They are in contact with blood in the pulmonary capillaries. Oxygen diffuses from the alveoli to the blood where molecules of oxygen are caught by the red blood cells, while carbon dioxide diffuses from the blood into the air in the alveoli. The total surface area of the alveoli is 80 m 2 (ranging from m 2 ). The total external surface area of the lungs is only 0.1 m 2, so subdividing into alveoli results in a tremendous increase in the surface area in contact with the blood, by a factor of almost 1,000. This is also the factor by which the oxygen intake increases. Without this, we would never even come close to meeting our metabolic needs for oxygen. Our chests expand when we breathe because incoming air filling the alveoli makes each one bigger, just as with ordinary bubbles. Physics of Breathing. The lung is an elastic structure that collapses like a balloon and expels all its air through the trachea whenever there is no force to keep it inflated. Each lung is surrounded by a sac membrane within the thoracic cavity. The inside wall of this sac, the visceral pleura FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 1

2 (membrane), attaches to the outer lung wall. The outside wall of this sac, the parietal pleura (membrane), attaches to the thoracic wall. The lung floats in the thoracic cavity, surrounded by a thin layer of pleural fluid that lubricates movement of the lungs within the cavity. It is the springiness of the lung that pulls the two pleural membranes apart, and this causes a slight decrease of pressure of the pleural sac relative to atmospheric pressure of 4 mmhg to 6 mmhg. This pressure difference is what keeps the lungs expanded, and keeps them from collapsing. The mechanical driving force in controlling lung volume is the transpulmonary pressure, which is the difference in pressure in the alveoli in the lungs and that around the lung in the pleural sac, which is called the intrapleural (or pleural) pressure. (The alveolar and pleural pressures are gauge pressures, referenced to atmospheric pressure.) The lungs are expanded and contracted by the motion of structures surrounding them by way of inspiratory and expiratory muscles. This occurs in two ways, of which only the first is used during quiet breathing: (1) The diaphragm moves downward to lengthen the chest cavity (by pulling the bottom of the lungs downward) during inspiration. During quiet breathing, the lungs contract by the natural elastic recoil of the lungs and chest wall, with the diaphragm relaxed, while in heavy breathing this contraction is accelerated by the contraction of the abdominal muscles that push the abdominal contents and then the diaphragm upward to shorten the chest cavity. (2) The ribs are elevated by the neck muscles to increase the anteroposterior (front-to-back) diameter of the chest cavity and are depressed (lowered) by the abdominal recti to decrease it. This causes chest cavity expansion and contraction, respectively, because the ribs slant outward and have larger transverse cross-sectional areas in the lower sections; this can increase the anterior posterior chest thickness by about 20% during inspiration. How does this help bring air into the lungs? Before inspiration, there is atmospheric pressure in the lungs (Fig a). The attractive force of the visceral pleura for the parietal pleura and the outward force of the outer lung wall due to the lower-than-atmospheric pressure in the pleural sac ( 4mmHg) cause each lung to expand. In equilibrium their sum is balanced by the tendency of the lungs to contract due to their springiness. The inspiratory muscles (diaphragm and external intercostals) increase the dimensions of the rib cage (the thoracic cavity). This causes the visceral and parietal pleurae to separate (Fig b). The lung volume then increases (Fig c) because (1) the attraction of the visceral and parietal pleurae increases as they are separated further and (2) this separation causes palveoli ppleura to decrease even more, from 4 to 6 mmhg. Because both of these forces in the direction of lung expansion increase, they now overcome the springiness of the lungs that favors lung contraction and the lung expands. The pressure in the lungs and alveoli decreases from 0 to 1 mmhg and then air flows from the mouth and nose into the lungs (Fig d). Exhaling is the opposite process - volume of the lung decreases (Fig e) and the air flows out from the lungs through the mouth and the nose (Fig f). During normal breathing exhaling is automatic, requiring no contraction by muscles. Muscle contraction is necessary during heavy exercise to inhale more fresh air and to actively exhale stale air. Work of Breathing. We have already pointed out that during normal quiet breathing, all respiratory muscle contraction occurs during inspiration; expiration is almost entirely a passive process caused by elastic recoil of the lungs and chest cage. Thus, under resting conditions, the respiratory muscles normally perform work to cause inspiration but not to cause expiration. The work of inspiration can be divided into three fractions: (1) that required to expand the lungs against the lung and chest elastic forces, called compliance work or elastic work; (2) that required to overcome the viscosity of the lung and chest wall structures, called tissue resistance work; and (3) that required to overcome airway resistance to movement of air into the lungs, called airway resistance work. FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 2

3 Alveoli pressure Pleural pressure Moment between breathing cycles Expansion of the chest caivity a b Air moves to the lungs Diaphragm Enlargement of the lung c Inhalation d Air moves from the lungs Contraction of the chest cavity Exhalation e f Fig Breathing cycle. See the text for details. Difference of inhaling and exhaling air volume dependence on the diaphragm and thoracic pressure is displayed in Fig The amount of work done by the lungs during the one breathing cycle is equal to: 1 p A ΔV 2 T ; (5.10.1) 2C Fig Difference of inhaling and here C is lung compliance. exhaling air volume dependence on the diaphragm and thoracic pressure FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 3 V Inspiration Exspiration Vr

4 With a children s breathing rate ν=40 min -1, lung compliance C = 0,1 l/cmh2o 10-6 m 3 /Pa and tidal volume V 500 work done by the lung per 1 minute is: T cm 2 3 0, A 5 J/min. (5.10.2) Artificial Respiration. When the normal breathing stops functioning, artificial respiration apparatus such as resuscitators or tank respirators can be used. Resuscitator. The resuscitator consists of a tank supply of oxygen or air; a mechanism for applying intermittent positive pressure and, with some machines, negative pressure as well; and a mask that fits over the face of the patient or a connector for joining the equipment to an endotracheal tube. This apparatus forces air through the mask or endotracheal tube into the lungs of the patient during the positive-pressure cycle of the resuscitator and then usually allows the air to flow passively out of the lungs during the remainder of the cycle. Tank Respirator (the Iron-Lung ). A patient s body is positioned inside the respiratory tank with the head protruding through a flexible but airtight collar. At the end of the tank opposite the patient s head a motordriven leather diaphragm moves back and forth with sufficient excursion to raise and lower the pressure inside the tank. As the leather diaphragm moves inward, positive pressure develops around the body and causes expiration; as the diaphragm moves outward, negative pressure causes inspiration. Fig Tank respirator (schematic). The rigid box is connected to a large volume, low pressure pump which swings the pressure about 0 to -10 cm water. When air is forced into the lungs under positive pressure by a resuscitator, or when the pressure around the patient s body is reduced by the tank respirator, the pressure inside the lungs becomes greater than pressure everywhere else in the body. Flow of blood into the chest and heart from the peripheral veins becomes impeded. As a result, use of excessive pressures with either the resuscitator or the tank respirator can reduce the cardiac output sometimes to lethal levels. For instance, continuous exposure for more than a few minutes to greater than 30 mm Hg positive pressure in the lungs can cause death because of inadequate venous return to the heart. Methodology The experiment on human respiratory parameters is performed with the Cobra3 BASIC-Unit measurement system connected to a pressure measurement module (Fig ). The pressure measurement module is connected to one end of a rubber tube, and the other end of the rubber tube is connected to a T form tube connector\s ending C (Fig ). While the subject breathes through the T form tube connector (A, Fig ), due to the moving air (speed v) a pressure change appears in the measuring tube, and is recorded. The pressure variation can be explained by the Bernoulli equation: when the air starts to move in the tube connector, the dynamic pressure in the tube changes. This change is compensated by the change in the static pressure, which is recorded. Fig Pressure measurement module. FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 4

5 When real liquids or gases flow in a horizontal tube, the potential energy of the liquid or gas particles is used to defeat the forces of internal friction and, therefore, along the axis of the tube the static pressure decreases. J. Poisseuille in 1840 experimentally found that the average speed vvid of the laminar fluid flowing in a horizontal circular tube is directly proportional to the difference of the input pressure p1 and the output pressure p2, a square of the tube radius R and inversely proportional to the fluid viscosity η and the tube length l. This is Poiseuille s law.: 2 R p1 p2 v vid. (5.10.3) 8 l l v A B C Fig The connection setup of the T form tube connector: A a tube through which we are breathing, B an open tube, C the ending of connector which is plugged in the pressure measurement module, v velocity of the air. When vvid is known, the liquid volumetric flow rate through the S cross-section tube can be found: Q v S vid. While the cross section of the tube is the Hagen Poiseuille equation: 4 R Q p. 8l and using notation p p 1 p2 2 S R (5.10.4) we can write down (5.10.5) With the integration of the pressure dynamics over the one cycle period, we can calculate the air volume V which flowed through the tube: V 4 R pdt 8l ; (5.10.6) Due to the measurement system mechanics, varying diameter tubes (Fig ), the air in the tube moves in a different way during inspiration and expiration: during inspiration, the air flow is turbulent, while during expiration it is laminar. Due to the turbulent flow, air resistance in the tube significantly increases and therefore increases the measured pressure. For this reason, the Hagen Poiseuille equation (5.10.6), which describes the amount of laminar airflow, is suitable only for measuring the expiration dynamics. FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 5

6 The respiratory parameters registered in the clinical practice. Breathing is not easily described because it is such a complicated process. What actually matters is the efficiency with which oxygen is transferred from the air to the blood and carbon dioxide is removed, but this is very difficult to measure unless continuous samples of air and blood can be taken. In clinical practice, breathing is described by very many measurements which need to be carefully defined if normal ranges are to be established. A number of measurements can be Fig Main respiratory parameters. made directly from a recording of the lung volume changes which occur when the patient is asked to carry out set procedures. Some of the respiratory parameters are shown in Fig Vital capacity (VC). This is the maximum volume of air that can be expired after a maximum inspiration. Normal values for young men are about 5 l. The values are less in women (about 3.5 l) and decrease with age. (Note, some laboratories use inspired VC. This is the volume that can be inspired after a maximum expiration.) Residual volume (RV). This is the volume of air remaining in the lungs after a maximum expiration. Normal values for young men are about 1.2 l. This volume increases with age and is slightly lower in women. Forced expiratory volume (FEV1). This is the volume of air expired in the first second following full inspiration. For young men the FEV1 is about 4 l. The range of normal values is different for men and women and changes with height, weight and age. Peak expiratory flow rate (peak-flow rate) (PEFR). This is the maximum flow rate during forced expiration following full inspiration. This is a very commonly used measurement which, in normal adult males, gives a value of about 7 l s 1. However, it must again be taken in mind when interpreting results that the normal values are different for men and women and also change with age. Tidal volume (Vt). This is the volume of air inspired or expired at each breath. This can be at any level of breathing activity. For a normal adult at rest Vt is about 300 ml. Note the very large difference between this and the VC. Only a small fraction of the lung capacity is used when resting. The five measurements defined so far are all measurements made from a single breath. There are many more measurements which can be made, but these five are the most commonly used. Ventilation of the lungs depends upon how rapidly the person breathes and the depth of each breath. The total ventilation can be defined in many ways but only two measurements will be defined here. Maximum voluntary ventilation (MVV). This is the volume of air inspired per minute during maximum voluntary hyperventilation. The patient is asked to breathe at 50 min 1 as deeply as possible for 15 s. The total volume of inspired air is measured and multiplied by four to give the MVV. Normal values for men are about min 1, but values change with age and body size. There is no standard definition of MVV, with the result that some laboratories use a different breathing rate and make their measurements over a longer or shorter time. Normal values must be established for each particular method of testing. Alveolar ventilation (VA). This is the volume of air entering the alveoli in 1 min. VA is important because all the gas exchange takes place in the alveoli. However, VA cannot be measured directly but it can be calculated from other measurements and is normally about 80% of the inspired air. All the measurements given so far can be obtained from a record of lung volume against time. The six definitions which follow are much more difficult to implement. They are given here to help you understand the language used in the respiratory function laboratory. FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 6

7 Minute volume. This is the volume of blood passing through the lungs in 1 min. This is obviously an important measurement as the exchange of oxygen in the lungs will depend upon both the flow of air and the flow of blood through them. In a normal resting adult the blood flow through the lungs is about 5 l min 1. Ventilation: perfusion ratio. This is the ratio of the alveolar ventilation to the blood flow through the lungs. In resting adults, the alveolar ventilation is about 4 l min 1 and the lung blood flow 5 l min 1 so that the ventilation: perfusion ratio is about 0.8. The ratio increases during exercise. Arterial oxygen pressure (PAO2). This is the partial pressure of oxygen dissolved in the arterial blood. The partial pressure of a component of a gas mixture is the pressure it would exert if it alone occupied the whole volume of the mixture. It is equivalent to the concentration of the oxygen molecules dissolved in the blood. To measure the partial pressure of gases in a liquid such as blood, the liquid is allowed to equilibrate with the surrounding gas and the pressure measurement made in this gas. Normal values for adults are mmhg ( kpa). Alveolar oxygen pressure (PaO2). This is the partial pressure of oxygen in the air present in the alveoli. Normal values are mmhg ( kpa). These last two definitions are particularly important because it is the difference between alveolar and arterial pressures which causes oxygen to diffuse from the alveolar air into the blood. Compliance of the lung (C). This is the expansibility of the lungs expressed as the volume change per unit pressure change. It is a measure of the effort needed to expand the lungs. Normal values are about 200 ml cmh2o 1 (2 l kpa 1 ). Airways resistance. This is a measure of the resistance to airflow in the airways expressed as the air pressure divided by the flow. Normal adult values are about 2 cmh2o l 1 s (0.2 kpa l 1 s). Procedures 1. Prepare the experimental setup: connect the pressure measurement module (Fig ) to the Cobra3 BASIC-UNIT measurement system ports 1 and 3 (Fig ); connect the pressure measurement module connector to the rubber tube whose opposite end is connected to the T-form tube connector. One of the open ends of the T-form tube connector is connected to the short tube through which the subject will breathe. The Cobra3 BASIC-Unit measurement system is connected to the PC using a USB port (9, Fig ). 2. Measure the length l of the tube (Fig ) with the ruler. 3. Measure the inner diameter d of the breathing tube with the vernier caliper, calculate the inner radius R of the tube. 4. Run the measure Cobra3 software. 5. Run the new experiment in the software: on the menu bar choose File New measurement. In the newly opened window set the parameters of the experiment. The parameters that are displayed in Fig should be set as follows: in the window Get value choose every 100 ms, in the window Start of measurement choose on key press, in the window End of measurement choose on key press. 6. Calibrate the measurement system: in the New measurement window (Fig ) press the Calibrate button and in the newly opened window in the Calibrate text box enter 0 (pbackgr=0). Press the Calibrate button. 7. In the New measurement window press the Continue button. 8. In the measure Cobra3 software press the Start measurement button and calmly breathe through the short tube that is connected to the T-form tube connector. More than 10 full breathing cycles should be measured (inhale and exhale). FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 7

8 Fig measure Cobra3 software New measurement window. 9. After finishing all breathing cycles, press the End measurement button. The software will create the pressure dynamics graph. 10. The registered signal can be saved if necessary. In the menu bar choose Measurement Export data and in the newly opened window (Fig ) choose the saving parameters. Data can be copied to the clipboard ( Copy to clipboard ) or saved in a file ( Save to file ). Data can be saved in two columns as a text file: the first column records the measurement number and the second the pressure value ( Export as numbers ). The, graph can be saved as a bitmap file ( Export as bitmap ) or metafile ( Export as metafile ). 11. Analyze the data: in the measure Cobra3 software menu bar choose Analysis Curve analysis, in the newly opened window press the Calculate button and the software will analyze the data and create the table with the minimum and maximum values of the pressure at certain time values (Fig ). Also the Visualize results box should be checked; in this case in the pressure dynamics graph maximum pressure values will be marked as green arrows and minimum values as red arrows. 12. Calculate the periods T of each breathing cycle the time durations between maximum values or between minimum values Calculate the average breathing cycle period Tvid and breathing frequency ( ). 14. With the button enlarge the zone with several breathing cycles. With the button choose the zone of one breathing cycle where the pressure value is greater than pbackgr (Fig ). Choose Analysis Show integral, the software will calculate integral p dt i of the marked curve, write down the absolute integral value. Fig measure Cobra3 software Export data window. T vid FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 8

9 Fig Selected zone of one exhalation in the measure Cobra3 soft. 15. Repeat procedure 14 for several breathing cycles. 16. Calculate the average pressure difference integrals value p dt i 17. Calculate the volume of air that is exhaled during one calm breathing cycle ( formula). 18. Carry out a new experiment (procedures 5-9) in a physical activity state. Before the experiment the subject has to do some physical exercises (knee-bends, press-ups or hops). 19. Analyse the physical activity state data: calculate breathing period and frequency, exhaled air volume during one breathing (procedures 11-17). 20. Carry out a new experiment (procedures 5-9) in a calm state 2-3 times maximally inhaling and exhaling. 21. Calculate the maximum exhaling air volume (procedures 14-17). 22. Compare the results with the standard values. References 1. B H Brown, R H Smallwood, D C Barber, P V Lawford and D R Hose, Medical Physics and Biomedical Engineering, New York N.Y.; London: Taylor and Francis (1999). 2. Arthur C. Guyton, John E. Hall, Textbook of medical physiology, Philadelphia Pa.: Saunders/Elsevier (2011). 3. Irving P. Herman, Physics of the human body, Berlin; Heidelberg: Springer (2007).. FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 9

10 Cobra3 BASIC-UNIT measure system Appendix 1 Fig Cobra3 BASIC-UNIT measure system front panel. 1. Module port. Collecting connector (25 pin SUB-D socket) for measuring modules. 2. Extension connector for Units. 48 pin plug in the side wall for docking a further Unit. 3. Analog input 1. Earth related analog input (4 mm safety sockets) with measuring ranges of ±10 V and ±30 V. 4. Sensor port S1. On connection of sensors, measuring modules or special instruments to this socket, not only the analog input 1, but also the analog output, three supply voltages and three digital control leads are led out. 5. Sensor port S2. On connection of sensors, measuring modules or special instruments to this socket, both the analog input 2 (but only earth related), as well as three supply voltages and three digital control leads are led out. 6. Analog input 2. Earth free, potential separated difference input (4 mm safety sockets) with 6- step measuring range: ±30 V / ±10 V / ±3 V / ±1 V / ±0.3 V / ±0.1 V 7. TIMER/COUNTER 1. Three 4 mm sockets with the functions START, STOP and a common earth socket. Controlled by TTL impulses or by contact opening or contact closing. This input can be used as a timer, as a counter and as a TTL input. 8. TIMER/COUNTER 2. As input 7 and additionally with the function counter with gate time. 9. USB connection. Type B Socket, situated in the side wall, for the connection of a USB interface via a data cable. 10. Extension connector for Units. 48 pin SUB-D socket, situated in the side wall, for docking a further Unit. 11. Connecting elements and positioning foot. The movable holding bar which is situated in the side wall gives a tight hold to an additional Unit docked on the side. The foot can be swung-out to enable the Unit to be held at an inclined position. The yellow colour of the foot indicates the movability of the holding bar. 12. Voltage supply. A low voltage socket in the side wall for connection of the Cobra3 power supply 12 V- /> 6 W. 13. Control lamp. Green light emitting diode which shows that the instrument is turned on. Further to this, the diode can be used as a signal during experiments, e.g. as a flashing light (see 17). 14. Fixed voltage output. Pair of 4 mm safety sockets for drawing off a direct voltage of 5 V / max. 0.2 A, e.g. for a light barrier. FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 10

11 15. Threaded connector for stand clamp. With a stand clamp fitted to the back, the Basic-Unit can be held on a stand so that it can easily be seen by everybody. 16. Dovetail joint. For fastening several Units, one onto the other (has no electrical connection). 17. Indicator lamp. This yellow light emitting diode serves for user guidance, and is also usable as an indicator lamp during experiments (see 13). 18. Connecting elements and positioning foot. The immovable holding bar which is situated in the side wall gives a tight hold to an additional Unit docked on the side. The foot can be swung-out to enable the Unit to be held at an inclined position. The gray colour of the foot indicates the immovability of the holding bar. FBML RESEARCH OF THE HUMAN RESPIRATORY PARAMETERS 11