Cardiac Cycle

Cardiac Cycle

• 
• orderly depolarisation/repolarisation triggers the cardiac cycle, of atrial and ventricular contractions and relaxation
• diastole is when the heart ventricles are relaxed and fill with blood
• systole is when the ventricles contract and pump blood into the aorta and pulmonary artery
• at a HR of 75bpm, diastole is 0.5secs and systole is 0.3secs

Passive Filling

• pressure in the atria and ventricles is close to zero
• atrioventricular valves open, so venous return flows into the ventricles
• when aortic pressure reaches 80mmHg, the aortic valve is closed
• on the right side of the hear, similar things happen but at much lower pressures
• ventricles become 80% full by passive filling

Atrial Contraction

• P wave in the ECG signals depolarisation
• the atria contratcs between the P wave and the QRS
• atrial contraction completes the EDV
• end diastolic pressure is only a few mmHg

Isovolumetric Ventricular Contraction

• ventricular contraction starts after the QRS, which signals ventricular depolarisation
• ventricular pressure rises, until it exceeds atrial pressure
• here the AV valves shut and produce the first heart sound
• tension rises around a closed volume, as the aortic valve is still shut
• ventricular pressure rises steeply

Ventricular Ejection

• when ventricular pressure exceeds the aorta/pulmonary artery pressure, the aortic/pulmonary valves open
• SV is ejected by each ventricle, leaving the ESV behind
• aortic pressure rises
• T wave signals ventricular repolarisation
• ventricles relax and the ventricular pressure starts the fall
• when it falls below aortic/pulmonary pressure, the aortic/pulmonary valves shut
• this produced the second heart sound
• the valve vibration produced the dicrotic notch in the aortic pressure curve (slight rise when it is decreasing)

Isovolumetric Ventricular Relaxation

• closure of the aortic/pulmonary valves signals the start of this event
• ventricle is a closed box, as the AV valve is still shut, so tension falls around a closed volume
• when ventricular pressure falls below atrial pressure, the AV valves open and a new heart cycle starts

Heart Sounds

• 
• first heart sound is caused by closure of mitral and tricuspid valves (atrioventricular valves)
• this signals the beginning of systole (S1)
• second heart sound is caused by the closure of aortic and pulmonary valves
• it signals the end of systole and the beginning of diastole (S2)
• additional heart sounds may be heard (S3 and S4)

Cardiac Auscultation

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• aortic area is in the second intercostal space on the right
• pulmonary area is in the second intercostal space on the left
• tricuspid is in the fourth intercostal space on the left
• mitral area (apex beat) is in the fifth intercostal space, midclavicular line

Atrial Pressure

• the aorta stretches due to its elastic tissues
• when the heart relaxes, the stretch will recoil back
• this recoil allows the blood to keep flowing forwards, so the pressure does not fall to zero
• the aortic valve also shuts, which dissconnects the ventricle from the aorta, so no backflow occurs