THE AUSTRALIAN NATIONAL UNIVERSITY

1. THE AUSTRALIAN NATIONAL UNIVERSITY Pressure Changes and Airflow during BreathingChristian StrickerAssociate Professor for Systems PhysiologyANUMS/JCSMR - ANUChristian.Stricker@anu.edu.auhttp://stricker.jcsmr.anu.edu.au/Airflow.pptx

3. Respiratory Part in Block 2 • Week 8 • Airflow • Week 9 • Mechanical properties of lung / thorax. • Spirometry practical • Pulmonary ventilation • Pulmonary perfusion and matching of perfusion with ventilation • Week 10 • Respiratory control and regulation • Respiratory regulation practical • Week 12 • Wrap-up of Block 2

4. Aims At the end of this lecture students should be able to • discuss different types of spirometers; • outline how flows, volumes & pressures are measured; • illustrate the phases of the respiratory cycle and what determines them; • define forced volumes and peak flows; • recognise some important features in a flow-volume loop; and • explain the P-V work during respiration.

5. Contents • Measuring techniques • How to measure RV • Repetition of volumes/pressures • Respiratory cycle • Forced volumes and peak flows • Flow-volume curves • P-V (V-P) work and its optimisation

6. Static Lung Volumes • Measured with a spirometer. • Static volumes (no flow): only maximal values relevant. • Volumes = cannot be broken down any further. • Capacities ≥ 2 volumes. • TLC reached with maximal inspiration. • RV reached with maximal expiration. • FRC reached when in- and expiratory muscles are “relaxed” (training). • Need training to achieve maxima (coaching). Modified from Boron & Boulpaep, 2003

7. Bell Spirometers • Bell dome filled with air and immersed in water (separation). • Oldest spirometer type, still used. • Different makes and variants. • Pros: • Direct volume measurement. • Very precise when well adjusted. • Allows for measurements of O2 uptake (metabolic studies). • Cons: • Poor dynamics due to inertia; no flow measurements possible. • Expensive (several K AU$). Wintrich, 1854

8. Pneumotachographs • Modern spirometers (Lilly; used in practical): measure flow. • Based on Ohm’s law: • ΔP = RI (I = flow = V / Δt). • Volumeobtained by integration. • Pros: • Excellent dynamic response. • Cheap (few 100’s AU$). • Cons: • “Fiddly” - as small pressure changes can cause “drifts” (T). • Need repeated calibrations. • Need a computer. http://www.spirxpert.com/technical3.htm

9. How to Measure RV (TLC) • Body plethysmograph. • Insp → air flow → ΔV → ΔP. • From ΔPBox, PAand FRC are determined. • If Ppl measured (oesoph.), all respiratory pressures known. • Allows RAWestimation: Modified from Boron & Boulpaep, 2003

10. Another Method for RV • Helium dilution technique • Requires [He] to be measured. • TLV can be estimated if initial [He] and that after equilibration are known. • Simple and effective method • Problem: Small amount of He is dissolved in plasma → over-estimates of true volume (correction required).

11. Important Pressures • (always take “inside view”). • Pb = barometric pressure. • Ppl= intrapleural pressure. • PA (alveolar pressure) = 0 at beginning/end of in-/expiration (FRecoil = FThorax). • Volume corresponds to FRC (when all muscles are relaxed). • PL = translung pressure. • Pw = transthoracic pressure. • Prs = resp. system pressure. Modified from Boron & Boulpaep, 2003

12. Respiratory Cycle within TV Modified from Boron & Boulpaep, 2003 • FRC: Ppl = -PL (no muscle force). (PA=Ppl+PL; PA= 0) • Inspiration: muscles contract (-2.5 cm H2O) →Ppl↓ and-PL↓lags (due to RAW & CL): PA < 0 → air flow into alveoli. • End of I / start of E: Ppl = -PL, but at a larger magnitude. • Expiration: muscles relax → recoil of system →Ppl↑ with lagging-PL↑: PA > 0 → air flow out of alveoli. • 2 parts ofPpl: (Ppl=PA-PL) • PA : air flow. • PL : lung volume (integrated flow).

13. Reasons Why -PLLags Ppl • There are two major factors: • Airway resistance (RAW) • Total compliance (CT) • Characteristic time (τ) is product: τ = RAW·CT. • For influence of each factor see next lecture. • See also influence on ventilation (later).

14. Forced/Large Respiratory Cycle • Pmus = 27 cm H2O causes inspiration to ~90% TLC. • Similar story as before. • Muscles build-up considerable recoil: energy not only in elastic lung/thorax tissue, but also in muscles. • At >60% TLC, considerable pressures generated by muscles, which can render thorax recoilinward at end of inspiration. Modified from Hlastala & Berger 2001

15. Forced Volumes & Peak Flow • Dynamic volumes important in evaluating RAW. • Training required. • Best out of 3 trials. E for 10 s (count down!). • Peak flows: PEF and PIF • PEF more sensitive to RAW. • PIF is normally > PEF (airway distension - next). • FEV1 and FVC (more dynamic) • FEV1 good test of RAW. • FVC > VC (acceleration). Modified from Boron & Boulpaep, 2003

16. Flow-Volume Diagram • Air flow is plotted against volume change from maximal filling (TLC): 0 @ TLC; maximum = RV; span = FVC. • Maximal efforts required to have indicative curves (training). • Expiration above and inspiration below zero flow. • PEF reached ≥ 20% volume. • Expiratory flow rates at < 2 L are effort dependent: muscle. • Expiratory flow rates at > 2 L are effort independent: limited by lung/thorax elasticity & RAW. Berne et al., 2008

17. TV and Different Efforts • TV in “middle” of graph. • Inspiratory part of loop unimportant. • Detailed interpretation given later. • Make sure that person puts in best effort (judge it…). • Effort-independent region: flow determined by RAW and recoil / CT. Berne et al., 2008

18. Respiratory Work - V-P Loop • Inspiration: Winsp + Wela (“loading of recoil”). • Expiration: Wexp - Wela (recoil at TV is sufficient). • For volumes > TV, Wmus(exp) becomes more important.

19. Optimisation of Respiration • Resistive work↑ with respiratory rate as flow↑(transitional flow). • Elastic work↓ with respiratory rate (time constant of recoil). • Elastic work (Wela) at mimimum~ 30 bpm (“resonance” of elastic system). • Minimum of total work at ~ eupnoea (12 – 20 bpm). Berne et al., 2004

20. Take-Home Messages • Several methods to measure volumes and pressure; all have their place in clinical practice. • Pneumotachography and He-dilution allow measurement of RV and TLC. • Intrapleuralpressure (Ppl) has 2 components: • PAdetermining flow; and • PL determining volume. • PLlags behind -Pplduring respiratory cycle. • Air flow into lung is determined byRAW and CT. • V-P loop indicates external work during respiration: • at TV: expiration is passive; inspiration via muscle force. • Flow-volume loop reveals most dynamic aspects.

21. MCQ A 17 year-old woman presents to emergency with an acute asthma exacerbation of moderate severity. Which of the following sets of lung function tests best describes her current condition? • Decreased FVC, FEV1, FEV1/FVC, PEF and increased RV. • Decreased FVC, FEV1, PEF and normal FEV1/FVC, RV. • Decreased FVC, FEV1, FEV1/FVC, and increased PEF, RV. • Decreased FVC, FEV1, PEF and increased FEV1/FVC, RV. • Decreased FVC, FEV1, FEV1/FVC and normal PEF, RV.

22. That’s it folks…

23. MCQ A 17 year-old woman presents to emergency with an acute asthma exacerbation of moderate severity. Which of the following sets of lung function tests best describes her current condition? • Decreased FVC, FEV1, FEV1/FVC, PEF and increased RV. • Decreased FVC, FEV1, PEF and normal FEV1/FVC, RV. • Decreased FVC, FEV1, FEV1/FVC, and increased PEF, RV. • Decreased FVC, FEV1, PEF and increased FEV1/FVC, RV. • Decreased FVC, FEV1, FEV1/FVC and normal PEF, RV.