THE AUSTRALIAN NATIONAL UNIVERSITY

1. THE AUSTRALIAN NATIONAL UNIVERSITY Pressures, Flows and Volumes during the Cardiac Cycle.Christian StrickerAssociate Professor for Systems PhysiologyANUMS/JCSMR - ANUChristian.Stricker@anu.edu.auhttp://stricker.jcsmr.anu.edu.au/Cardiac_cycle.pptx

3. Aims The students should • understand what events are used to time the CC; • realize that heart sounds (S1 and S2) delimit systole and diastole; • be able to draw the important features of pressure, volume and flow changes during cardiac cycle in ventricles, atria and arteries; • know when and how ventricles are filled; • appreciate how stroke volume is determined by both, systolic and diastolic pressures; and • appreciate the relative timing between left and right atria and ventricles.

4. Contents • Linking elements within the CC • ECG for objective timing • Cardiac sounds (phonocardiography) • Heart as two serially connected pumps • Left ventricle, aorta and left atrium • Blood pressure change and stroke volume • Right ventricle and central venous pressure • Relative timing between two ventricles

5. Events within Cardiac Cycle • Electrical events (“Pump control”) • Ionic current flow and action potentials: ECG • EC-coupling • Mechanical events (“Pump action”) • Muscle contraction • Pressure generation • Movement of valves (directionality of flow) • Sounds, clicks and murmurs • Flow of blood out of ventricle • Volume changes

6. Clinical Relevance • Why are these concepts important? • Interpretation of clinical signs/findings to get insight into cardiac function: • Auscultation and phonocardiography • ECG • Pulse pressure curves • Echocardiography (ultrasound) • Cardiac catheter • … • Used daily many times… Complexity

7. Electrical Events Modified from Berne et al., 2004 • From action potentials to ECG. • ECG provides precise way to determine phase • P wave: Atrial depolarisation. • QRS complex: Ventricular depolarisation - start of systole. • T wave: Ventricular repolarisation - end of systole.

8. Mechanical Properties of Heart • Two serially connected pumps with a high (left) following a low (right) pressure vascular bed. • Directionality enforced by a pair of valves at in- and out-flow of each chamber. • Failing of one part dramatically imposes load on preceding element(s).

9. Systole - Diastole • Clinical term. • Systole = period between S1→S2 = contraction and ejection period. • Diastole = period between S2→S1 = relaxation and filling period. • Duration of systole is quite constant, however, diastole varies with heart rate. • To identify systole and diastole, no fancy tool required… just ears and stethoscope: you hear it…

10. Heart Sounds S1: Closure of the mitral/tricuspid valves. S2: Closure of the aortic/pulmonary valves. Typically split during respiration; during inspiration, venous return to RA↑ but LA↓ (pooling in pulmonary bed; see later). S3: Faint rumble; opening of mitral/tricuspid valve with flow murmur into ventricles (typically heard in young people). S4: Faint rumble; flow murmur caused by atrial contraction.

11. Left Ventricle • Contraction after electrical pacing • Pressure range: 0 - 120 torr • Systole duration: ~ 0.3 s @ 75 bpm • Diastole duration: variable (~0.5 s) • Peak diastolic volume: 120 mL • End-systolic volume: 40 mL • Stroke volume (SV): 80 mL

12. Role of Valves • The role of valves is to • separate pressures when closed; i.e. Pbefore≤ Pafter; • and direct flow when open; i.e. Pbefore> Pafter. • Valve abnormalities cause murmurs. • Valvular stenosis: narrowing of open valve diameter; requires increa-sed ventricular pressure to maintain flow through narrowed valve. • Valvular regurgitation: failure of valve to seal properly: no pressure separation → flow in both directions; may involve large volumes.

13. Aortic Trunk • SV (homeostatic requirement): 80 mL • If PLV > PAO : Aortic valve opens. • AV does not close at Ppeak→ due to highest flow and elasticity of AO. • Notches indicate valve movements. • Pressure difference (ΔP): 40 torr • Duration of blood ejection: ~200 ms • Ejected volume = SV = 80 mL • ΔP determines SV: longer ejection → SV↑.

14. Left Atrium • SV (homeostatic requirement): 80 mL • If PLA < PLV: Mitral valve closes. • Atrium fills towards end of systole. • If PLA > PLV: Mitral valve opens (early diastole). • Under resting conditions, atrial contraction represents only a “last little push” to fill ventricle (15%).

15. Valvular Plane Displacement • Systole: Movement of valvular plane towards cardiac apex (~16 mm) causes mechanical “suc-tion” on central veins: • Increased atrial filling during late systole. • Mechanism: “horror vacui” imposed by pericardial space. • Diastole: Movement back (early filling phase) rises atrial pressure slightly, aiding ventricular filling. Modified from Schmidt & Thews, 1977

16. Synopsis 7 phases of the cycle • Systole • Contraction phase (isovolumetric) • Ejection phase • Fast • Slow • Diastole • Relaxation phase (isovolumetric)“elastic recoil” • Filling phase • Passive ventricular filling • Rapid ventricular filling • Slow ventricular filling • Atrial contraction

17. Right Ventricle • SV (homeostatic requirement): 80 mL • Largely analogous to phases in left ventricle, except for slightly changed valve timings. • Pressure difference (ΔP): 20 torr • Ohm’s law: I = ΔP / R; to maintain flow, Rpulm.must be no more than half Rsyst..

18. Timing of Right and Left Ventricle Boron/Boulpaep 2003 • Cycle starts right atrium (pacing in SAN) and ends in right ventricle (delayed contraction). • S1and S2 are “composed” of contributions from both respective valves in both ventricles.

19. Right Atrium & Central Veins • SV (homeostatic requirement): 80 mL • Can be seen on jugular vein. • Measured via a pulse transducer. • Pulse wave in central veins results from volume and pressure changes in RA. • Timing and amplitudes depend on location (delay and attenuation).

20. Systole and Diastole Timing Modified from Koller, 1979 • Pulse rate can be altered over a 3 - 4 fold range. • At heart rates (HR) >100 bpm, tdiastole< tsystole: Filling ↓. • Systole at 150 bpm is shorter than that at 50 bpm. • At high HR, atrial contractions become important.

21. Take-Home Message • Systole: S1→S2 • Diastole: S2→S1 • The 7 phases of the cardiac cycle are: isovolumetric contraction, fast and slow ejection, isovolumetricrela-xation, fast and slow ventricular filling, atrial contraction. • Whilst diastole is variable, systole is ± constant. • SV is dependent on difference between systolic and diastolic pressure: difference ↑ → SV ↑. • Most of atrial filling occurs during systole. • “Insignificance” of atrial contraction at rest. • “Pump failure” causes load on preceding element(s).

22. MCQ Joe Ackermanis, a 26 year-old male, was diagnosed with an aortic valve stenosis (narrowing of valve opening). Compared to a normal heart, which of the following descriptions best describes the pressures in the heart of this person?

23. That’s it folks…

24. MCQ Joe Ackermanis, a 26 year-old male, was diagnosed with an aortic valve stenosis (narrowing of valve opening). Compared to a normal heart, which of the following descriptions best describes the pressures in the heart of this person?