Cardiac cycle

About: Cardiac cycle is a research topic. Over the lifetime, 3290 publications have been published within this topic receiving 96159 citations.

Synchronization of vagus nerve stimulation with the cardiac cycle of a patient

Abstract: Disclosed herein are methods, systems, and apparatus for treating a medical condition of a patient, involving detecting a physiological cycle or cycles of the patient and applying an electrical signal to a portion of the patient's vagus nerve through an electrode at a selected point in the physiological cycle(s). The physiological cycle can be the cardiac and/or respiratory cycle. The selected point can be a point in the cardiac cycle correlated with increased afferent conduction on the vagus nerve, such as a point from about 10 msec to about 800 msec after an R-wave of the patient's ECG, optionally during inspiration by the patient. The selected point can be a point in the cardiac cycle when said applying increases heart rate variability, such as a point from about 10 msec to about 800 msec after an R-wave of the patient's ECG, optionally during expiration by the patient.

Assessment and consequences of the constant-volume attribute of the four-chambered heart

Abstract: The constant-volume hypothesis regarding the four-chambered heart states that total pericardial volume remains invariant throughout the cardiac cycle. Previous canine studies have indicated that the pericardial volume remains constant within 5%; however, this hypothesis has not been validated in humans using state-of-the-art technology. The constant-volume hypothesis has several predictable functional consequences, including a relationship between atrial ejection fraction and chamber equilibrium volumes. Using cardiac magnetic resonance (MR) imaging (MRI), we measured the extent to which the constant-volume attribute of the heart is valid, and we tested the accuracy of the predicted relationship between atrial ejection fraction and chamber equilibrium volumes. Eleven normal volunteers and one volunteer with congenital absence of the pericardium were imaged using a 1.5-T MR scanner. A short-axis cine-loop stack covering the entire heart was acquired. The cardiac cycle was divided into 20 intervals. For each slice and interval, pericardial volumes were measured. The slices were stacked and summed, and total pericardial volume as a function of time was determined for each subject. In the normal subjects, chamber volumes at ventricular end diastole, end systole, and diastasis were measured. Pericardial volume remained invariant within 5 ± 1% in normal subjects; maximum variation occurred near end systole. In the subject with congenital absence of the pericardium, total heart volume, defined by the epicardial surface, varied by 12%. The predictions of the relationship between atrial ejection fraction and chamber equilibrium volumes were well fit by MRI data. In normal subjects, the four-chambered heart is a constant-volume pump within 5 ± 1%, and constant-volume-based modeling accurately predicts previously unreported physiological relationships.

Most ovine mitral annular three-dimensional size reduction occurs before ventricular systole and is abolished with ventricular pacing.

Abstract: Background Conventional surgical thinking indicates that mitral annular (MA) size reduction plays a key role in mitral valve closure, and most MA size and shape changes are thought to occur during left ventricular (LV) systole. The influences of left atrial (LA) and LV systole on MA size and shape, however, remain debated. Methods and Results Eight radiopaque markers were placed equidistantly around the MA and imaged using high-speed simultaneous biplane videofluoroscopy in seven closed-chest, sedated sheep before and during asynchronous LV pacing. Marker images were used to compute the three-dimensional coordinates of each marker every 16.7 ms throughout the cardiac cycle, allowing calculation of three-dimensional MA area, septal-lateral (SL) dimension, and commissure-commissure (CC) dimension under control and LV pacing conditions. Maximum MA area occurred in early diastole, and minimum MA area near end-diastole; maximum area reduction was 12±1%, (P<.001). Interestingly, 89±3% of area reduction occurred before LV systole. During this presystolic period, SL decreased by 8±1% and CC by 2±1%; the SL/CC ratio fell from 0.73±0.02 to (1.69±0.01 (P≤.005), indicating a less circular shape at end-diastole. With LV pacing, total MA area reduction was similar (13±2 versus 12±1%, P=NS versus control); however, all MA area reduction occurred during LV systole with minimum MA area occurring at end-systole. Presystolic shortening in both SL and CC dimensions was lost, and presystolic ellipticalization disappeared. Conclusions Changes in MA size and shape coincident with LA systole included area reduction and shape change prior to the onset of LV contraction. These presystolic changes vanished when LA systole was absent (LV pacing). Thus, LA systole plays a pivotal role in MA size reduction and shape alteration. The unexpected timing of these MA dynamics should be taken into account during mitral valve reparative procedures.

Arterial pressure ramps provoke linear increases of heart period in humans

Abstract: A variety of methods has been used experimentally to increase baroreceptor activity and provoke transient, vagally mediated cardiac slowing in humans. We studied baroreceptor-cardiac reflex physiology in eight resting volunteers by measuring R-R interval changes during and after spontaneous brief elevations of arterial pressure, preceded by bursts of muscle sympathetic nerve activity. Arterial pressure was measured with a catheter in a brachial artery, and muscle sympathetic activity was measured with a microelectrode positioned transcutaneously in a peroneal nerve. R-R intervals were related to preceding systolic pressures with linear regression analysis. When systolic pressures were correlated with R-R intervals of the next cardiac cycle, slopes (+/- SE) averaged 12.4 +/- 2.2 ms/mmHg and correlation coefficients averaged 0.81 +/- 0.09. Baroreflex slopes were inversely related to base-line systolic pressures. These results are similar to those obtained when baroreceptors are stimulated experimentally; they suggest that elevations of arterial pressure, which are preceded by bursts of muscle sympathetic activity, trigger with one heart-beat latency highly linear increases of efferent vagal-cardiac nerve activity.

Detection of a relation between respiration and CSF pulsation with an echoplanar technique

Abstract: The flow of cerebrospinal fluid (CSF) through the aqueduct was studied with an echoplanar imaging technique. Images (1024) of a slice perpendicular to the aqueduct were acquired with a repetition time of 107 msec and a flip angle of 90 degrees. This imaging technique is very sensitive for flow into the selected slice, although a quantitative assessment of flow velocities is not possible. Simultaneously with the image data acquisition, data from a pulse oximeter and a respiration belt were recorded. For each data point, a delay time to the preceding cardiac pulse was determined from the recorded pulse wave. The signal intensities could then be assigned to the cardiac cycle. Each cardiac interval was assigned to one of eight respiratory phases, and an average signal curve during the cardiac interval was calculated for each respiration phase. The evaluation showed to signal maxima within the cardiac interval, which could be identified as a downward flow at 10% and an upward flow at 80% of the cardiac pulse interval by measurements with additional saturation pulses. In examinations of 22 healthy volunteers, an influence of respiration on the flow through the aqueduct was found. In spite of interindividual variability, comparable effects could be observed in all volunteers. In the late expiration phase the caudally directed flow was at its maximum, whereas the cranially directed flow was maximal in the post-inspiration phase.