Fundamentals of clinical cardiology

The phases of the cardiac cycle

Aldo A. Luisada, M.D. Donald M. MacCanon, Ph.D.

Chicago, Ill.

I nvestigators and clinical cardiologists usually base their considerations and

calculations on the well-known scheme of the cardiac cycle that was developed by Wiggers. 1,2 Accuracy in these calculations is necessary in physiologic studies on ani- mals or man based on catheterization of the heart and large vessels.

It should be kept in mind that several physiologic studies in the past were per- formed with manometers that possessed an inadequate frequency response. Even in the early fifties physiologic studies per- formed in man usually employed strain gauges which either had a low-frequency response or had a purposely damped out- put in order to obtain a “smooth” pressure pattern. Low speed of film recording and lack of consideration of the catheter and electronic lags were often additional causes of inaccuracy in measuring time intervals. Similarly, sound phenomena were usually transcribed without knowledge of the pos- sible distortion or delay of the various cy- cles by the recording equipment. Even the conventional angiocardiograms, recorded at either 30 or 60 frames per second, gave

a poor resolution for timing short events. Accuracy is further decreased if one em-

ploys indirect or noninvasive techniques. These suffer from the variability in pattern of the apexcardiogram, the variable pulse delay of the carotid tracing (according to the distance of the point of recording from the aortic root, variations in the speed of pulse transmission determined by the com- pliance of the arterial walls, and modifica- tions caused by the interposed tissues), and finally the lack of agreement on how to record heart sounds (area, type of micro- phone, filters, criteria for measurement of the intervals, and so on). For these reasons it seems useful to re-evaluate the scheme of the normal cardiac cycle. This re-evalua- tion should indicate the need for some changes in the scheme because of: (1) more accurate methods of pressure recordings based on catheter-tip sensors, (2) studies in the last 10 years on the timing of valve opening and closure effected by high-speed pressure recordings and high-speed angio- cardiograms, and (3) considerations on the timing of heart sounds in regard to the phases of the cycle. These studies will be

From the Division of Cardiovascular Research (Physiology) and the Division of Cardiology (Medicine) of The Chicago Medical School, University of Health Sciences, Chicago, III.

This study was made during tenure of an Undergraduate Training Grant from the National Heart and Lung Institute. United States Public Health Service.

Received for publication Mar. 25, 1971. Reprint requests to: Aldo A. Luisada. M.D., The Chicago Medical School, 2020 West Ogden Ave., Chicago, III. 60612.

Vol. 83, No. 5, pp. 70.5-711 May, 1972 American Heart Journal 70.5

706 Luisada and MacCanon Am. Heart J.

May, 1972

AV VALVES CLOSURE

SEMIL.VALVES OPENING

SEMIL.VALVES CLOSURE

INTERVAL ( LT )

MECHANO-ACOUSTIC INTERVALS

CONTRACTION FILLING

Fig. 1. Scheme of the cardiac cycle and of the relationship between valve openings and closures; ventricular, atrial, and arterial events; waves of the ECG; and heart sounds recorded by the phonocardiogram. ECG, elec- trocardiogram; PCG, phonocardiogram; LV, left ventricle; RV, right ventricle; PA, pulmonary artery; LA, left atrium; RA, right atrium; Ao, aorta; I, II, first and second heart sound; III, IV, third and fourth heart sound; M, T, mitral and tricuspid valves; PA, Ao, pulmonary and aortic valves; lit, right heart; Lt, left heart.

briefly reported below. The over-all con- elusions are presented in graphic form in

Atrioventricular valve closure,

Fig. l.* onset of ventricular contraction, and first heart sound

*Both the profiles of the cuwes and the reported intervals are based on modern catheter-tip manometers (animals) or catheter tracings corrected for catheter and filter delay

Di Bartolo and associates3 in 1961 showed that both atrioventricular (A-V) valves

(humans). Older systems with inadequate frequency re- sponse. electronic or hydraulic damping, or both, can pro- duce skewed curves of more rounded appearance.

must already be closed at the time of the first rapid vibration of the first sound. This

Volume 83 Number 5 The phases of the cardiac cycle 707

Fig, 2. Experiment demonstrating the times of A-V valve closures and the time of onset of the first heart sound in an anesthetized dog. RV-RA crossing point and LV-LA crossing point are nearly simultaneous and are marked by pencil signals. Gauges are catheter tip and subminiature pressure type with insignificant time delay. After correction for electronic and filter delay of the phonocardiogram, the interval between crossing and onset of high-frequency vibrations of the first heart sound is 29 msec. The upper tracing is a velocity phono- cardiogram in the high-frequency band. RV and RA tracings = ATTENUAT. X2; LV and LA tracings = ATTENUAT. X5; time lines = 100 msec.; film speed = 400 mm. per second. (From Luisada et al.: New studies on the first heart sound, Am. J. Cardiol. Vol. 28, August, 1971. Reproduced by permission.)

was demonstrated by the fact that the crossing points of the atria1 and ventricular pressures of both sides of the heart pre- ceded the onset of the first sound by a sizable interval (30 to 35 msec.). It is obvious that, when pressure rises in a ven- tricle while it is stable or dropping in the respective atrium, the valve separating them has already completed its closure and acts as a barrier preventing pressure rise in the atrium. These findings were confirmed by van Bogaert and colleagues4 and by Laurens5 on the basis of experimental pres- sure and sound tracings, as well as by studies of catheterization in our clinical laboratory.6 In addition, other studies, performed with various techniques, led to the same conclusion; these range from films taken with rapid cineangiocardiography (MacCanon and colleaguesn) to tracings with ultrasound equipment using the Doppler effect (Yoshida and co-workers*). Recent studies in our laboratory performed with catheter-tip sensors9 have further con- firmed the above observations (Fig. 2).

In man, tricuspid valve closure always follows mitral valve closure; in the anes- thetized dog, it may slightly precede or

slightly follow but is frequently simulta- neous. Di Bartolo and colleagues3 found a 29 msec. average interval between the onset of the Q wave of the electrocardiogram (ECG) and mitral valve closure, and a 34 msec. average interval between the Q wave and tricuspid valve closure. More recently, using catheter-tip sensors,g we have found intervals of 25 and 35 msec., respectively. Both studies were performed in dogs anes- thetized with pentobarbital. In man6 we found average intervals of 26.6 msec. for the Q-mitral closure interval and 44.1 msec. for the Q-tricuspid closure interval. The former had been called the “electro- dynamic interval” by Blumberger.‘O

The intervals between the onset of the Q wave of the ECG and the three main components of the first heart sound were found to be 58, 88, and 116 msec., respec- tively, with the use of a band pass filter of 50 to 400 Hz having a filter slope of 48 db per octave.” The first interval, called the “electroacoustic interval” by Blumberger,‘O corresponds to the often-measured Q-I interval of clinical researchers. The average interval between mitral and tricuspid valve closures in man was found to be very short,

708 Luisada and MacCanon

Fig. 3. Simultaneous tracings of the phonocardiogram, the ECG, and the pressure tracings of the pulmonary artery and right ventricle in an anesthetized dog. Gauges are catheter-tip. Electronic and filter delay from the sound tracing is about 10 msec. Pencil lines mark the closure of the tricuspid valve and the opening of the pulmonic valve (pressure tracings) as well as the onset of the first and second component of the first heart sound (PCG).

i.e., 17.5 msec.6 The interval between the onset of pressure rise in the left ventricle and the first high-frequency component of the first heart sound should be called the “systolic mechanoacoustic interval.” It was found to average 17 msec. in clinical studies.

Recent studies9 have confirmed that the first component (a) of the first heart sound occurs at about the first third of the left ventricular pressure rise; the second com- ponent (b) occurs at the opening of the aortic valve; and the third component (c) occurs at the point in which the still rising aortic pressure curve shows a change in slope.

Right and left ventricular pressure rise

The pressure rise of the right ventricle follows a slower course than that of the left.

However, as its peak is reached at a much lower level, this peak grossly coincides in time with that of the left ventricle (Fig. 2).

lsovolumic contraction or tension period

The isovolumic contraction period is measured from the closure of the A-V valve to the opening of the semilunar valve of each side. In clinical studies,6 we have found average values of 57.7 msec. for the Q-open- ing of the pulmonic valve and 64.7 msec. for the Q-opening of the aortic valve. By subtracting the Q-mitral closure interval from the Q-aortic opening, and the Q-tri- cuspid closure interval from the Q-pulmonic opening, we found that the average iso- volumic contraction period of the right ventricle was 13.6 msec., and that of the left ventricle 38.1 msec. This last value is shorter than Wiggers’ estimate2 of 50 msec.

Volume 83 Number 5 The phases of the cardiac cycle 709

Semilunar valve closure and the second sound

According to Wiggers,‘v2 the aortic valve leaflets approximate somewhat during sys- tolic ejection because of eddy currents. The valve closes at the onset of diastole because of a pressure difference between the two sides of the leaflets created by the sharp decline of intraventricular pressure. How- ever, even studies of angiocardiographyl2 have shown that the momentum of blood flowing from each ventricle into its artery maintains the leaflets open for a brief inter- val after the initial divergence of the pres- sure curves. In addition, studies by Mac- Canon and colleagues’ with an ingenious electric device introduced into the aortic valve have demonstrated that this valve closes soon after the end of ventricular con- traction. The interval between the end of contraction and valve closure is extremely short in the normal heart (in the range of 8 to 10 msec. in the dog; up to 1.5 msec. in man) and is caused by the momentum of the outflowing blood.* Thus, they con- firmed studies by Spencer and Greiss14 on ventricular ejection and aortic flow. It seems appropriate to call this interval “inertial interval.” It is grossly similar to that called “protodiastole” by Wiggers, except that the latter was considered as ending with the incisura. The aortic com- ponent of the second sound occurs 8 to 15 msec. after valve closure, at the time of the incisura of the pressure pulse in the very first part of the ascending aorta. In cases with extremely low peripheral resistance, however, the sound may be slightly delayed over the incisura.

The interval between the end of left ventricular systole and the aortic compo- nent of the second sound could be called the “diastolic mechanoacoustic interval,” simi- lar to that occurring at the onset of systole.

In their studies of blood ejection and pressure gradient across the pulmonary valve, Okino and Spencer-l5 obtained find- ings similar to those observed across the aortic valve.

While the inertial periods in the two

*It should be noted that this interval may increase to a remark- able degree if there is aortic valve stenosis because ejection continues until left ventricular pressure falls below aortic pressure.

PCG

Second Sound (PA.Comp)l

PA. Pr eJ=b

R.V. Press.

0

IIsd

9

PCG

I

Ao. Press.

>

PA.

0 Press.

Fig. 4, A and B. Scheme of cardiovascular events. A, Right ventricular and pulmonary artery pres- sures; end of systole; pulmonary valve closure; and pulmonary component of the second sound. B, Re- lationship between the two components of the second heart sound and the two incisuras of the main arter-

arteries are similar, so that closures of the aortic and pulmonary valves occur approxi- mately at the same time, the intervals sepa- rating valve closure from the sound compo- nent is longer for the right than for the left side of the heart. This is because of the greater compliance of the pulmonary ves- sels and will cause a more delayed rebound over the closed valve, causing both a late incisura and a late pulmonary component of the second sound (Figs. 3, 4).

According to a.recent study in this lab- oratory,16 inspiration and ‘expiration cause minimal changes in the duration of ven- tricular systoles while they- cause marked changes in the interval between aortic and pulmonic incisuras, as well as changes in

710 Luisada and MacCanon Am. Heart J. May, 1972

the interval between aortic and pulmonary components of the second heart sound. These changes are caused primarily by the increase in capacity of the pulmonary vessels, so that the pulmonary artery takes longer to reach its elastic limits and thus delays the rebound of its pressure pulse.

The relaxation period

The isovolumic relaxation period should be measured from the actual closure of the valve and not from the sound component, because this component follows semilunar valve closure after an interval that is deter- mined by vascular compliance. The iso- volumic relaxation period is longer for the left heart than for the right heart. In a study of Arevalo and Sakamoto” it was found to average 81 msec. for the left heart in normal human subjects (with a few exceptions showing longer intervals of 105 to 120 msec.). In dogs, varying values of 37 msec. (rate 180 to 150 per minute), 74.3 msec. (rate 106 to 86 per minute), and 60.6 msec. (rate 56 to 54 per minute) were observed.

Early diastole and the third sound

The hemodynamic events associated with the period of rapid filling in early diastole have been studied by Arevalo and co- worker9 in our laboratory. Following open- ing of the atrioventricular valves, the blood of the atria flows rapidly into the ventricles. The pressure difference during the first part of this phase is partly determined by the release of elastic forces.lg The first derivative of left ventricular pressure re- turns to the base line at the end of this phase and coincides with the third heart sound.‘8 This sound, therefore, seems to occur after the end of the phase of active relaxation and at the end of rapid filling when ventricular volume and compliance equilibrate, and intramural tension begins to rise again.18

Rapid filling was considered by Wiggers1m2 to last 110 msec. It was found to last from 100 to 120 msec. in our experimental ani- mals, varying somewhat with changes in heart rate. It is shorter for the right ven- tricle than for the left.

Considerations regarding noninvasive techniques

Researchers using tracings of apex cardio- grams, carotid pulse tracings, and sound records should keep the above considera- tions in mind in order to avoid wrong deductions.

If one considers left ventricular systole as ending with the aortic component, and right ventricular systole as ending with the pulmonic component of the second heart sound, one is liable to err grossly in certain clinical cases. We have demonstrated that the delayed aortic component of aortic ste- nosis is not caused by prolonged left ven- tricular systole and is related mostly to a prolongation of ejection due to a lower aortic pressure level .20 In addition, a greater delay takes place between the occurrence of the aortic incisura and the aortic compo- nent of the second sound. Again, the de- layed pulmonary component of atria1 sep- tal defect is not caused by prolongation of the right ventricular systole but is due instead to a delay of the pulmonary pulse rebound because of pulmonary vascular dilatation.21 In patients without shunts and without semilunar valve stenosis, it is likely that judgment of the duration of systole based on the second sound may be approxi- mately correct. This should, however, be checked by adequate studies.

Judgment about the onset of left ven- tricular systole cannot be accurate when based on sound tracings. The apexcardio- gram will usually give a reference onset point: however, even here the variability of this clinical tracing may exclude a com- plete reliance.

Measurements of the isovolumic relaxa- tion period cannot be based on the interval between second sound and opening snap. Both sound phenomena follow the dynamic event by certain intervals, which are sub- ject to alteration under clinical conditions.

Rapid filling does not reach maximum at the time of the third sound; rather, it ends at the time of the third sound.

The pulse of the carotid tracing has some delay over the pulse of the ascending aorta. This delay varies, not only according to the pulse velocity, but also according to the site at which it is recorded: a high carotid

Volume 83 Number 5 Annotations 717

mg. per gram of food. Zinc should therefore be given with great caution, particularly over periods in excess of three months, until more long-term studies of zinc toxicity are available.

M. W. Greaves, M.D., Ph.D., M.R.C.P. Senior Lecturer and Consultant Dermatologist

University Department of Dermatology Newcastle-upon- Tyne

England

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Initial and terminal notching of the QRS complex

in the conventional electrocardiogram

Notching of the QRS complex in the electrocardio- gram (ECG) interested several investigators as much

subjects.‘,6 On the other hand, a notching in the

as f&y years ago.‘-* A notched QRS complex was QRS complex, when it appeared near the apex of

frequently found in the ECG’s of apparently healthy a deflection or in multiple leads, and especially when the notching was gross, has been considered to be