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Pressures, Flows and Volumes during the Cardiac Cycle.

Christian StrickerAssociate Professor for Systems Physiology

ANUMS/JCSMR - ANU

Christian.Stricker@anu.edu.au http://stricker.jcsmr.anu.edu.au/Cardiac_cycle.pptx

THE AUSTRALIAN NATIONAL UNIVERSITY

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AimsAt the end of this lecture students should be able to

• outline the events are used to time the CC;

• describe what delimits systole and diastole; i.e. heart

sounds (S1 and S2);

• draw the important features of pressure, volume and flow

changes during cardiac cycle in ventricles, atria and aorta;

• outline when and how atria and ventricles are filled;

• explain how stroke volume is determined by both, systolic

and diastolic pressures; and

• recognise the relative timing between left and right atria

and ventricles.

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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

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Events within Cardiac Cycle• Electrical events (“Pump control”)

– Ionic current flow and action potentials: ECG• EC-coupling

• Mechanical events (“Pump action”)– Muscle contraction: Auscultation

• Pressure generation– Movement of valves (directionality of flow)

» Sounds, clicks and murmurs

– Flow of blood out of ventricle

– Volume changes

– Pressure and volume waves (pulse; see later)

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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

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Electrical Events

• From action potentials to ECG.

• ECG provides precise way to determine timing/phase– P wave: Atrial depolarisation.

– QRS complex: Ventricular depolarisation - start of systole.

– T wave: Ventricular repolarisation - end of systole.

Mod

ified

from

Ber

ne e

t al.,

200

4

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Mechanical Properties of Heart

• Two serially connected

pumps with a high (left)

followed by 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).

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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…

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Heart Sounds

S1: Closure of the mitral/tricuspid valves.

S2: Closure (”slamming shut”) 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.

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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

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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 increased

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.

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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↑.

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Left Atrium

• SV (homeostatic requirement): 80 mL

• If PLA < PLV: Mitral valve closes.

• Atrium fills towards end of systole.

• If PLA > PLV: Mitral opening (early diastole).

• Under resting conditions, atrial contraction

represents only a “last little push” to fill

ventricle (15%).

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Valvular Plane Displacement

• Systole: Movement of valvular

plane towards cardiac apex

(~16 mm) causes mechanical

“suction” 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

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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

Synopsis

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• 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..

Right Ventricle

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Timing of Right and Left Ventricle

• Cycle starts right atrium (pacing in SAN) and

ends in right ventricle (delayed contraction).

• S1 and S2 are “composed” of contributions

from both respective valves in both ventricles.

Bor

on/B

oulp

aep

2003

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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).

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Systole and Diastole Timing

• 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.

Mod

ified

from

Kol

ler,

1979

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Take-Home Message• Systole: S1 → S2

• Diastole: S2 → S1

• The 7 phases of the cardiac cycle are: isovolumetric

contraction, fast and slow ejection, isovolumetric rela-

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).

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MCQJoe 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?Peak atrial pressure

Peak ventricular pressure

Peak aortic pressure

a) ↑ ↑ ↑

b) ↓ ↓ ↓

c) ↓ ↓ ↑

d) ↓ ↑ ↑

e) ↑ ↑ ↓

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That’s it folks…

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MCQJoe 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?Peak atrial pressure

Peak ventricular pressure

Peak aortic pressure

a) ↑ ↑ ↑

b) ↓ ↓ ↓

c) ↓ ↓ ↑

d) ↓ ↑ ↑

e) ↑ ↑ ↓