The first cardiac sound represents what part of the cardiac cycle? The cardiac cycle: Changes in contractility lead to pressure differences in the heart chambers that lead to the movement of blood. The first cardiac sound, called S1, produces a « lubricating noise » caused by the mitral and tricuspid valves closing when ventricular systole begins. There is a very slight split between the closure of the mitral and tricuspid valves, but it is not long enough to produce several sounds. There are five important points about the ECG: the P wave, the QRS complex and the T wave. The small P wave represents the depolarization of the auricles. The atria begin to contract about 25 ms after the onset of the P wave. The large QRS complex represents the depolarization of the ventricles, which requires a much stronger electrical signal due to the larger size of the ventricular heart muscle. The ventricles begin to contract when the QRS reaches the peak of the R-wave. Finally, the T-wave represents the repolarization of the ventricles. Repolarization of the atria occurs during the QRS complex, which masks them on an ECG.
This impulse spreads from its initiation into the SA node through the atria via specialized internodal pathways to the atrial myocardial contractile cells and the atrioventricular node. The internodal pathways consist of three bands (anterior, middle and posterior) that lead directly from the SA node to the next node of the conduction system, the atrioventricular node. The pulse takes about 50 ms (milliseconds) to move between these two nodes. The relative importance of this signaling pathway was discussed, as the impulse would reach the atrioventricular node by simply following the cell-by-cell pathway through the myocardial contractile cells in the atria. In addition, there is a specialized pathway called the Bachmann bundle, or interatrial band, which directs the impulse directly from the right atrium to the left atrium. Regardless of the signaling pathway, when the impulse reaches the atrioventricular septum, the connective tissue of the cardiac skeleton prevents the spread of the impulse in the myocardial cells of the ventricles, except at the atrioventricular node. Figure 3 illustrates the initiation of the pulse into the SA node, which then distributes the momentum through the atria to the atrioventricular node. The ECG works by detecting and amplifying tiny electrical changes on the skin that occur during the depolarization of the heart muscle. The output for the ECG is a graph showing several different waves, each corresponding to a different electrical and mechanical event in the core. Changes in these waves are used to identify problems with different stages of cardiac activity. Figure 5.
a) Note the long plateau phase due to the influx of calcium ions. The prolonged refractory period allows the cell to contract completely before another electrical event can occur. b) The action potential of the heart muscle is compared to that of skeletal muscle. Delayed After depolarization is due to abnormal manipulation of calcium. Here, the increase in intracellular calcium, as after a myocardial infarction, increases the activity of the Na/Ca exchanger. The net effect of this channel is an inward depolarizing current, which can trigger an extrasystole when the threshold potential is reached. Myocardial-conductive cells: specialized cells that transmit electrical impulses through the heart and trigger contraction through myocardial contractile cells Opening and closing heart valves: Closing heart valves produces « lub, dub » sounds that can be heard through a stethoscope. The cardiac cycle consists of a phase of relaxation and pronounced contraction. What term is usually used for ventricular contraction when no blood is expelled? Myocardial contractile cells: The majority of heart muscle cells in the atria and ventricles that conduct impulses and contract to drive Pacemaker blood cells are highly specialized myocardial cells with an intrinsic ability to rhythmically depolarize and initiate action potential. [6] Pacemaker cells are mainly located in the SA and atrioventricular (AV) nodes, with some cells also in the His and Purkinje fiber bundle. Pacemaker cells have a property known as automaticity and independently initiate action potentials.
[7] This action potential is conducted as an electrical impulse in the cardiac conduction system and also between one cardiomyocyte to another through lacunar junctions. This conduction helps the heart contract synchronously The cardiac cycle describes the phases of contraction and relaxation of the heart that drive blood flow throughout the body. AV blocks are often described by degree. A first-degree block or subblock indicates a delay in the line between the SA and AV nodes. This can be seen on the ECG as an unusually long RP interval. A second-degree or incomplete block occurs when some pulses from the SA node reach the AV node and continue, while others do not. In this case, the ECG would show some P waves that no QRS complex would follow, while others would appear normal. In the third block or complete block, there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex). Even in the case of a total SA block, the AV node takes on the role of pacemaker and continues to initiate contractions with 40 to 60 contractions per minute, which is enough to maintain consciousness. Cardiac output (CO) is a measure of cardiac performance.
Although there are many clinical techniques for measuring CO, it is best described as a physiological and mathematical relationship between different variables. When one of the variables changes, the CO as a whole changes accordingly. It can also be used to predict other regulated variables such as blood pressure and blood volume. The mathematical description of CO is [latex]text{CO}=text{Heart Rate (HR)}timestext{Stroke Volume (SV)}[/latex]. Changes in HR, sv or their components change CO. The cardiac conduction system: The system of nerves that work together to adjust the heart rate and stimulate the depolarization of muscle cells in the heart. Heart blockage: interruption of the normal conduction pathway During auscultation, it is common for the clinician to ask the patient to breathe deeply. This procedure not only makes it possible to listen to the airflow, but can also increase the sound of the heart. Inhalation increases blood flow to the right side of the heart and can increase the amplitude of the heart murmur on the right side.
Exhalation partially restricts blood flow to the left side of the heart and can increase the sound of the left heart. (Figure) shows the correct location of the stethoscope bell for ease of auscultation. Artificial pacemaker: medical device that transmits electrical signals to the heart to ensure that it contracts and pumps blood into the body The normal rate of fire in the AV node is lower than that of the SA node because it slows down the rate of neural impulses. Without autonomic nerve stimulation, it adjusts the rate of ventricular contraction to 40-60 bpm. Some types of autonomic nerve stimulation change the rate of fire in the AV node. Sympathetic nerve stimulation always increases heart rate, while parasympathetic nerve stimulation decreases heart rate by acting on the AV node. The action potentials differ considerably between cardiac conductive cells and cardiac contractive cells. While Na+ and K+ play a vital role, Ca2+ is also crucial for both cell types. Unlike skeletal muscles and neurons, the conductive cells of the heart do not have a stable resting potential. The conductive cells contain a series of sodium ion channels that allow a normal and slow influx of sodium ions, causing the membrane potential to slowly increase from an initial value of -60 mV to about -40 mV. The resulting movement of sodium ions produces spontaneous depolarization (or prepotential depolarization).
At this point, the calcium ion channels open and ca2+ enters the cell and depolarizes it faster until it reaches a value of about +5 mV. At this point, the calcium ion channels close and the K+ channels open, allowing K+ to flow and leading to repolarization. When the membrane potential reaches about −60 mV, the K+ channels close and the Na+ channels open, and the prepotential phase begins again. This phenomenon explains the autorythm properties of the heart muscle (Figure 4). During the early phase of ventricular diastole, when the ventricular muscle relaxes, the pressure on the blood remaining in the ventricle begins to decrease. When the pressure in the ventricles falls below the pressure in the pulmonary trunk and aorta, the blood returns to the heart and produces the dicrotic notch (small sagging) seen in blood pressure monitoring. The crescent valves close to prevent reflux into the heart. Since the atrioventricular valves remain closed at this time, the blood volume in the ventricle does not change, so the early phase of the ventricular ventricular diastole is called the isovolucular ventricular relaxation phase, also called the isovolumetric ventricular relaxation phase (see (figure)). The QT interval is the time elapsed between the beginning of the QRS wave and the end of the T wave. .