Hemodynamic Correlates of the Various Components of the First Heart Sound

Beat-by-Beat Estimation of Hemodynamic Parameters in Left Ventricle Based on Phonocardiogram and Photoplethysmography Signals Using a Deep Learning Model: Preliminary Study

Beat-by-beat monitoring of hemodynamic parameters in the left ventricle contributes to the early diagnosis and treatment of heart failure, valvular heart disease, and other cardiovascular diseases. Current accurate measurement methods for ventricular hemodynamic parameters are inconvenient for monitoring hemodynamic indexes in daily life. The objective of this study is to propose a method for estimating intraventricular hemodynamic parameters in a beat-to-beat style based on non-invasive PCG (phonocardiogram) and PPG (photoplethysmography) signals. Three beagle dogs were used as subjects. PCG, PPG, electrocardiogram (ECG), and invasive blood pressure signals in the left ventricle were synchronously collected while epinephrine medicine was injected into the veins to produce hemodynamic variations. Various doses of epinephrine were used to produce hemodynamic variations. A total of 40 records (over 12,000 cardiac cycles) were obtained. A deep neural network was built to simultaneously estimate four hemodynamic parameters of one cardiac cycle by inputting the PCGs and PPGs of the cardiac cycle. The outputs of the network were four hemodynamic parameters: left ventricular systolic blood pressure (SBP), left ventricular diastolic blood pressure (DBP), maximum rate of left ventricular pressure rise (MRR), and maximum rate of left ventricular pressure decline (MRD). The model built in this study consisted of a residual convolutional module and a bidirectional recurrent neural network module which learnt the local features and context relations, respectively. The training mode of the network followed a regression model, and the loss function was set as mean square error. When the network was trained and tested on one subject using a five-fold validation scheme, the performances were very good. The average correlation coefficients (CCs) between the estimated values and measured values were generally greater than 0.90 for SBP, DBP, MRR, and MRD. However, when the network was trained with one subject's data and tested with another subject's data, the performance degraded somewhat. The average CCs reduced from over 0.9 to 0.7 for SBP, DBP, and MRD; however, MRR had higher consistency, with the average CC reducing from over 0.9 to about 0.85 only. The generalizability across subjects could be improved if individual differences were considered. The performance indicates the possibility that hemodynamic parameters could be estimated by PCG and PPG signals collected on the body surface. With the rapid development of wearable devices, it has up-and-coming applications for self-monitoring in home healthcare environments.

New non-invasive approach to detect cardiac contractility using the first sound of phonocardiogram

Aim

During surgery, a non‐invasive and easy‐to‐use method is required for evaluating left ventricular status. The systolic time interval, including pre‐ejection period (PEP), of left ventricle has been known to be correlated with cardiac contractility. In this study, we focused on the non‐invasive time interval from the Q wave of an electrocardiogram to the third component in the first heart sound (QS₁‐3rd) and evaluated the correlation between PEP and peak differentiated left ventricular pressure (LV dp/dt).

Methods

Six adult anesthetized pigs were intubated. Mechanical ventilation was started. An electrocardiogram, carotid artery blood pressure, left ventricular pressure, and phonocardiogram on the fourth left intercostal space were monitored using a polygraph system. Cardiac output was measured by the thermodilution method. Data were simultaneously measured at baseline and after the infusion of noradrenaline, nitroprusside, esmolol sulfate, and dobutamine, respectively. Data were analyzed by Spearman’s rank correlation coefficient using four‐quadrant plot analysis.

Results

A total of 270 points were simultaneously measured. The QS₁‐3rd showed a significant correlation with PEP (QS₁‐3rd = 7.62 + 0.92 PEP; ρ = 0.91, P < 0.0001). Concordance rate was 92% between PEP and QS₁‐3rd (excluded zones were set within ± 5 ms). Both PEP and QS₁‐3rd showed a good correlation with LV dp/dt (LV dp/dt = 3861.3–24.4 PEP; ρ = 0.85, P < 0.0001, LV dp/dt = 3763.6–23.5 QS₁‐3rd; ρ = 0.82, P < 0.0001).

Conclusion

This non‐invasive and easy‐to‐use hemodynamic parameter (QS₁‐3rd) could be helpful for continuous monitoring of left cardiac contraction performance.

A non-invasive approach to investigation of ventricular blood pressure using cardiac sound features

Heart sounds (HSs) are produced by the interaction of the heart valves, great vessels, and heart wall with blood flow. Previous researchers have demonstrated that blood pressure can be predicted by exploring the features of cardiac sounds. These features include the amplitude of the HSs, the ratio of the amplitude, the systolic time interval, and the spectrum of the HSs. A single feature or combinations of several features have been used for prediction of blood pressure with moderate accuracy. Experiments were conducted with three beagles under various levels of blood pressure induced by different doses of epinephrine. The HSs, blood pressure in the left ventricle and electrocardiograph signals were simultaneously recorded. A total of 31 records (18 262 cardiac beats) were collected. In this paper, 91 features in various domains are extracted and their linear correlations with the measured blood pressures are examined. These features are divided into four groups and applied individually at the input of a neural network to predict the left ventricular blood pressure (LVBP). The analysis shows that non-spectral features can track changes of the LVBP with lower standard deviation. Consequently, the non-spectral feature set gives the best prediction accuracy. The average correlation coefficient between the measured and the predicted blood pressure is 0.92 and the mean absolute error is 6.86 mmHg, even when the systolic blood pressure varies in the large range from 90 mmHg to 282 mmHg. Hence, systolic blood pressure can be accurately predicted even when using fewer HS features. This technique can be used as an alternative to real-time blood pressure monitoring and it has promising applications in home health care environments.

Nonlinear Time Domain Relation between Respiratory Phase and Timing of the First Heart Sound

The previous studies on respiratory physiology have indicated that inspiration and expiration have opposite effects on heart hemodynamics. The basic reason why these opposite hemodynamic changes cause regular timing variations in heart sounds is the heart sound generation mechanism that the acoustic vibration is triggered by heart hemodynamics. It is observed that the timing of the first heart sound has nonlinear relation with respiratory phase; that is, the timing delay with respect to the R-wave increases with inspiration and oppositely decreases with expiration. This paper models the nonlinear relation by a Hammerstein-Wiener model where the respiratory phase is the input and the timing is the output. The parameter estimation for the model is presented. The model is tested by the data collected from 12 healthy subjects in terms of mean square error and model fitness. The results show that the model can approximate the nonlinear relation very well. The average square error and the average fitness for all the subjects are about 0.01 and 0.94, respectively. The timing of the first heart sound related to respiratory phase can be accurately predicted by the model. The model has potential applications in fast and easy monitoring of respiration and heart hemodynamics induced by respiration.

Modeling of heart sound morphology and analysis of the morphological variations induced by respiration

In this study, each peak/valley of a heart sound was modeled by a Gaussian curve and characterized by amplitude, timing, and supporting width. This model was applied to analyze the morphological variations induced by respiration in 12 subjects. It was observed that the morphology exhibited regular behaviors with respiration. The amplitude of the prominent peaks and valleys of S2 (the second heart sound) were commonly attenuated during expiration and were accentuated during inspiration whereas no consistent observations were obtained for S1 (the first heart sound). The supporting width of S1 commonly decreased with expiration and increased with inspiration whereas the supporting width of S2 displayed no significant changes during respiration. For all subjects, the delay of S1 increased during inspiration and decreased during expiration. However, the delay of the aortic component increased during expiration and decreased during inspiration. The pulmonary component of S2 was observed in 7 of 12 subjects, and the delay was opposite to that of the aortic component. The opposing delays yielded a splitting between the two components of S2 that increased during inspiration and decreased during expiration. The delay pattern was the most consistent observation in all subjects. These results suggest that a quantitative analysis of morphological variations, particularly the delay pattern, could be used as a non-invasive continuous monitoring method of hemodynamic change during respiratory cycles.

Renewed Interest in Preejectional Isovolumic Phase: New Applications of Tissue Doppler Indexes: Implications to Ventricular Dyssynchrony

There is renewed interest in isovolumic contraction (IC) in tissue Doppler echocardiography of the myocardial walls, which is revisited in this editorial with new regional velocity data. The aims are to recall traditional background information and to emphasize the need to master the rapidly evolving tissue Doppler procedures for the accurate display of brief IC. IC, a preejectional component of great physiologic interest, is very demanding in terms of ultrasound technology. The onset and end of its motion velocities should be unambiguously defined versus the QRS complex and ejection wall motion. This is a prerequisite for exploiting the new information as guidance toward new therapeutic strategies from a practical viewpoint. However, IC preload dependence should be kept in mind, because of its limited potential for contractility studies. Finally, when only duration measurements are made in the assessment of ventricular dyssynchrony, regional preejectional duration is the pertinent tool to single out the onset of ejection local wall motion.

Quantification of first heart sound frequency dynamics across the human chest wall

Power spectral analysis has attracted attention because of its potential for non-invasive cardiac diagnosis. However, time-frequency analysis of first heart sound frequency dynamics from canine epicardium has demonstrated that cardiac vibrations are fundamentally multi-component and non-stationary, questioning the validity of power spectral techniques. In this study, we employed time-frequency transforms to characterise first heart sound frequency dynamics from 27 sites across the human thorax. In contrast to the dynamics observed epicardially, the first heart sound frequency law was dominated by quasi-stationary and impulse-like components implying that the instantaneous power and the power spectrum contain most of the diagnostic information in the first heart sound.

Analysis of mitral and aortic valve vibrations and their role in the production of the first and second heart sounds

Using a theoretical approach and stress and vibration analysis, formulae have been derived for the frequencies that would be generated by considering the mitral and aortic valve leaflets as vibrating membranes. In the analysis the physical characteristics of the valves, such as loading, valve geometry and valve tissue mechanical properties, were taken into consideration. The analysis showed that the fundamental frequencies for both the vibrating valves increase with the pressure difference across the valve membranes. This suggests that for a better understanding of the role played by the vibrating heart valves in the production of heart sounds, data are needed on the relation between these frequencies and the pressure difference across the valves. Unfortunately such data do not seem to be available.

Study of respiratory influence on the intensity of heart sound in normal subjects

The amplitudes of the first and second heart sounds were recorded during quiet natural breathing in 31 normal subjects. A total of 3,656 and 3,016 heart beats were available at the apex and pulmonic areas respectively. The intensities of the first and second heart sounds were found to be increased during expiration. This respiratory tendency in the heart sounds was less prominent during the transitional phase between expiration and inspiration. Therefore we suggest that respiratory changes in heart sounds should be evaluated with a heart beat located at or close to the center of each inspiration and expiration. Respiratory alteration in the intensity of heart sounds is one of the commonest auscultatory pitfalls. Auscultatory evaluation of the intensity of heart sounds should thus be performed carefully, with the respiratory changes kept in mind.

Significance of momentary pressure changes during isovolumic relaxation

Sudden momentary fluctuations of left ventricular, aortic, right ventricular, and pulmonary arterial pressure were noted during isovolumic relaxation of the respective ventricles. The presence of such transients raised questions related to their meaning and significance. The purpose of this report is to emphasize the nonartifactural nature of these pressure transients and to describe their origin and significance in the cardiac cycle. Pressure transients were observed in 31 of 32 patients with normal aortic valves, and in 17 patients with normal pulmonary valves in whom right-sided measurements were made. Such transients, however, were absent on the left ventricular and aortic pressure recordings of three patients with calcific aortic stenosis. These sudden changes in pressure are indicative of momentary compressions and rarefactions of the blood that occur within the ventricles and their respective arterial chambers. Whenever present, pressure transients were noted to occur coincident with the major aortic or pulmonary components of the second sound. Since intraaterial sound pressure is derived from the pressure signal by litering the low frequencies and amplifying the high frequencies, one can deduce that intraarterial sound pressure is in fact a representation of these pressure changes. The recognition of these pressure transients on an otherwise smooth ventricular, aortic, or pulmonary arterial pressure places in proper perspective their role in the production of the second heart sound.

The effects of exercise on the amplitude of the first heart sound in normal subjects

The cardiovascular responses to isometric and dynamic exercise have been studied in six normal subjects, before and after intravenous propranolol, using the amplitude of the first heart sound (S1), measured from an ultra-low frequency phonocardiogram, as an index of left ventricular mechanical performance. Dynamic exercise caused significant increases in heart rate and S1 amplitude which were largely inhibited by beta-adrenergic blockade. Isometric exercise also produced increases in heart rate and blood pressure, but a decrease in S1 amplitude. Propranolol had no significant effect on the cardiovascular response to isometric exercise. These results confirm previous suggestions that, in contrast to dynamic exercise, the normal cardiovascular responses to isometric exercise are relatively independent of the beta-adrenergic nervous system. Possible reasons why the improvement in left ventricular performance, which has previously been shown to occur during isometric handgrip, was not reflected in the phonocardiogram, are also discussed.

Characterization of left ventricular external wall motion in man by video dimension analyzer (Vidian)

Several investigators have described close relationships between left ventricular wall motion and physiologic cardiac events. Using an improved wall motion tracking devide (Vidian) in studies of 30 patients, we have compared the dynamics of left ventricular wall motion, recorded noninvasively, with high fidelity left ventricular and aortic pressures, intracardiac phonocardiograms, apexcardiograms, and cyclic left ventricular volume curves obtained during cardiac catheterization. Wall motion tracking signals comprised: pre-ejection outward deflection commencing with the first component of the first heart sound and coincident with the pre-ejection phase of the left ventricular pressure and apexcardiogram; a sharp descent during ejection, commencing with the "E" point of the apexcardiogram and with the onset of the upstroke of the aortic pressure; end ejection nadir, synchronous with the dicrotic notch of the aortic pressure; a nadir representing cessation of inward displacement, presumably reflecting slight inertial motion of the wall; a brief period of isovolumic relaxation which terminated synchronously with the "O" point of the apexcardiogram; rapid, then slow filling waves, coincident with those of the apexcardiogram, and demarcated by a transitional angulation synchronous with the third heart sound; and "a" wave, occurring simultaneously with that of the apexcardiogram. Ventricular wall motion tracking signals also corresponded to curves representing cyclic changes in left ventricular minor radius, and chamber volume derived from cineventriculograms. In 10 patients with abnormal contraction patterns detected by biplane cineventriculography, anomalous deflections were also recorded during ejection by the Vidian. Left ventricular wall motion tracking with the Vidian: 1) provides a sensitive index for timing of intracardiac events, 2) reflects cyclic changes in ventricular volumes and minor dimensions, 3) provides a convenient noninvasive technique for detection of regional asynergy involving the lateral left ventricular wall, and 4) by correlation with simultaneous ventricular pressure measurements, may provide useful information regarding left ventricular pressure/segment dimension relations.

First heart sound: a phono-echocardiographic correlation with mitral, tricuspid, and aortic valvular events

We evaluated the relationship of S1 recorded by phonocardiography at the mitral area with motion of the mitral, tricuspid, and aortic valves, recorded by simultaneous echocardiography in 20 cardiac patients with a normal PR interval. The first major component of S1 coincided with mitral valve closure in 20 of 20 patients (100%) and also with tricuspid valve closure in 14 of 20 patients (70%). The second major component of S1 coincided with aortic valve opening in 20 of 20 patients and also with tricuspid valve closure in six of 20 patients (30%). We conclude that the first major component of S1 coincides with mitral valve closure in all patients but may also coincide with tricuspid valve closure in many patients, and the second major component of S1 coincides with aortic valve opening in all patients but may also coincide with tricuspid valve closure in some patients.

Tricuspid component of first heart sound

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Early systolic sounds in aortic valve stenosis

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Reversed mitral diastolic gradient in aortic incompetence

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Q-1 interval of the phonocardiogram in patients with atrial septal defect

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