RCSI Bahrain Mechanics of Ventilation-II - Year 1 Past Paper Notes

RCSI Bahrain, Building No. 2441, Road 2835, Busaiteen 228, Kingdom of Bahrain Mechanics of Ventilation- Ⅱ Year Year 1 Course Respiratory Biology Module Code MED104 Lecturer Date Dr Patrick Walsh 30th April 2023 Learning Outcomes 1. Ability to describe spirograms for lung function testing 2...

RCSI Bahrain, Building No. 2441, Road 2835, Busaiteen 228, Kingdom of Bahrain Mechanics of Ventilation- Ⅱ Year Year 1 Course Respiratory Biology Module Code MED104 Lecturer Date Dr Patrick Walsh 30th April 2023 Learning Outcomes 1. Ability to describe spirograms for lung function testing 2. Understand dead space volumes and their effect on pulmonary and alveolar ventilation 3. Understand and describe various methods for lung function and volume measurements Spirometers and Spirometry • Composed of two vessels – One contains water, the other floats upside down in the first. • As subject breathes through attached tube, air flows in and out of inner vessel which moves up and down. LO: Understand and describe various methods for lung function and volume measurements Spirometers and Spirometry PC spirometers make your desktop or laptop PC into a spirometer when you run the software application provided with the device. Spirotrac is the most widely used spirometry software and also is capable of many other types of medical examination. • Can link to computer via USB for generation of spirograms etc. LO: Understand and describe various methods for lung function and volume measurements RV, FRC, TLC • Measuring the functional residual capacity (FRC) by the helium dilution technique LO: Understand and describe various methods for lung function and volume measurements HELIUM DILUTION TECHNIQUE Add Helium here • • • • Helium/oxygen mixture in spirometer C1 = initial Helium concentration [mol/L] V1 = Volume in spirometer C1 x V1 = Total amount of Helium • • • • • • • • • Patient starts breathing at FRC New volume of the system now V2 = V1 + FRC Helium diluted accordingly But total amount of Helium cannot change (closed system) C1 x V1 = C2 x V2 C1 x V1 = C2 x (V1 + FRC) FRC = (C1 x V1)/C2 – V1 Inappropriate in patients with obstructive pulmonary disease (This works because Helium is insoluble) LO: Understand and describe various methods for lung function and volume measurements BODY PLETHYSMOGRAPHY • • • Patients sits in a “body box” (airtight chamber) and breathes through a mouthpiece At FRC, the mouthpiece is closed Patient tries to breathe in • Consequences: - Chest and lungs expand, pressure in lungs drop as volume increases - Expanding chest compresses the air inside the chamber, air volume in chamber decreases, pressure in chamber increases • FRC can be calculated using Boyle’s law https://www.youtube.com/watch?v=OcvIrz1N7-g LO: Understand and describe various methods for lung function and volume measurements Assumption: Pressure in lungs and box identical at time zero Air in box compressed, Pressure in Box increases P1 à P2 2 Closed Systems: Sealed box + Lungs – mouth - valve Lungs expand, Pressure in Lungs drops P1 à P3 The basic physical principle exploited by body plethysmography is the law of Boyle-Mariotte. For a fixed amount of gas in a closed compartment, the relative changes in the compartment’s volume are always equal in magnitude but opposite in sign to the relative changes in pressure. Thus, one can infer relative volume changes from pressure changes. LO: Understand and describe various methods for lung function and volume measurements BODY PLETHYSMOGRAPHY • • • Change in lung volume: FRC à FRC + ∆V Change of air volume in box: V1 à V1 - ∆V ∆V in the box can be determined from the measured change in pressure à P1 x V1 = P2 x (V1 - ∆V) [Boyle’s Law] • • • Knowing ∆V, then allows to calculate FRC P1 x FRC = P3 x (FRC + ∆V) [Boyle’s Law] FRC = P3 x ∆V/(P1 – P3) • • • P1 = Initial pressure in box and airways P2 = Increased pressure in box P3 = Decreased airway/alveolar pressure LO: Understand and describe various methods for lung function and volume measurements LUNG VOLUMES • Helium dilution technique/Plethysmography + Spirometry sufficient to fully characterise lung volumes and capacities • Lung volumes vary with – body size – age – sex – muscular training – posture – race – respiratory diseases • • Vital capacity (VC) is a useful measurement clinically E.g. decreased in poliomyelitis, TB, lung cancer, chronic asthma, chronic bronchitis, COPD, pleural effusion, pneumothorax, pulmonary fibrosis, pulmonary oedema, pregnancy, ascites, lying down, female, ageing LO: Understand and describe various methods for lung function and volume measurements WHAT IS THE DEAD SPACE? • Definition - the volume occupied by gas in the lungs which does not participate in gas exchange. • Different types, including: 1. anatomical dead space 2. physiological dead space 3. alveolar dead space LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation ANATOMICAL DEAD SPACE • Normal respiration: 500 ml tidal volume • Only 350 ml enters the alveoli • Mouth, nose, pharynx, trachea etc = Anatomical dead space (no pulmonary capillaries) • 150 ml of air in anatomical dead space cannot contribute to gas exchange • VT = VA + VD • VT = Tidal vol.; VA = Alveolar vol.; VD = Dead space vol. • Deep breaths more efficient LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation ANATOMICAL DEAD SPACE LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation ALVEOLAR DEAD SPACE • Another type of dead space exists: Alveolar dead space • Air in alveoli that are surrounded by pulmonary capillaries without blood flow • Usually negligible in healthy people • Can increase in disease (e.g. pulmonary embolism) LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation PHYSIOLOGICAL DEAD SPACE • Physiological dead space is the total dead space Physiological dead space = anatomical dead space + alveolar dead space • The anatomical dead space is measured by Fowler’s method • the physiological dead space is measured by Bohr’s method • Dead space volume = VD Fowler’s method – Nitrogen washout Anatomical Dead Space • • • • • • Pure oxygen breathed in after exhalation to FRC Continuously exhaled N2 is measured Exhaled gas has 3 phases Phase 1 – no N2 Phase 2 – rapid rise in N2 – mixture of dead-space and alveolar gas Phase 3 – saturation/plateau of N2 LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation Fowler’s method LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation Fowler’s method N2 Ideal scenario Typical scenario N2 LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation BOHR’S METHOD PHYSIOLOGICAL DEAD SPACE • Physiological dead space is calculated by measuring partial pressures (or fractional concentration [%]) of CO2 in alveoli and expired air • Alveoli: Partial pressure of CO2 (PaCO2) measured in arterial blood • Expired air: Partial pressure of CO2 (PECO2) measured from collected expired air • Assumption: Atmospheric CO2 can be neglected à All of the expired CO2 comes from the alveolar space (VA) and none from the VD (which contains atmospheric air) • Therefore: VT x PECO2 = VA x PaCO2 LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation BOHR’S METHO VT x PECO2 = VA x PaCO2 But how do we get VD from this? Since VT = VA + VD, we can replace VA with VT - VD in the above equation: VT x PECO2 = (VT - VD) x PaCO2 • This can be rearranged to calculate physiological dead space VD = VT x [(PaCO2 - PECO2)/PaCO2] LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation PULMONARY AND ALVEOLAR VENTILATION • How do dead space and breathing patterns affect ventilation? • Pulmonary or minute ventilation: Total volume of air breathed per minute • Pulmonary Ventilation = VT x f = VT x RR VT = Tidal volume [L] f = RR = respiratory frequency, respiratory rate [breaths.min-1] • Normal values VT = 0.5 L f = 12 min-1 à Normal minute ventilation = 6 L min-1 LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation PULMONARY AND ALVEOLAR VENTILATION • How much of the 6 L/min contributes to alveolar ventilation and therefore gas exchange? • • Alveolar ventilation = (VT – VD) x f Normal Alveolar Ventilation therefore (500 ml – 150 ml) x 12 min-1 = 4200 ml/min LO: Understand dead space volumes and their effect on pulmonary and alveolar ventilation PRACTICE QUESTION • Compare & contrast the compositions of the following air samples in terms of their fractional concentrations (%) and partial pressures in both mmHg and kPa: • (a) Alveolar air • (b) Expired air • (c) Room air Assume a barometric pressure of 760 mmHg. MCQ EXAMPLE 1 During inspiration, how does alveolar pressure compare to atmospheric pressure? A. Alveolar pressure is greater than atmospheric. B. Alveolar pressure is less than atmospheric. C. Alveolar pressure is the same as atmospheric. D. Alveolar pressure is one of the few pressures where the reference pressure is not atmospheric. E. Alveolar pressure is exactly half atmospheric pressure during inspiration MCQ EXAMPLE 2 How do you calculate how much inspired air actually ventilates the alveoli during one minute? A. Subtract the volume of dead space from the tidal volume. B. Subtract both the dead space volume that was already in the lungs plus the dead space of the inspired air that won't reach the alveoli from the tidal volume. C. Subtract the volume of dead space from the tidal volume and multiply it by the number of breaths per minute. D. It is equal to the tidal volume times the frequency of breathing. E. It is equal to the tidal volume.