Intra-Atrial Pressure Changes During Cardiac Cycle
Pressure in the atria is called the intra-atrial pressure. Intra-atrial pressure is responsible for opening of the atrioventricular valves and ventricular filing. It the main factor for the development of venous pulse.
Methods Of Study
Right atrial pressure is recorded directly by cardiac catheterization. Left atrial pressure is determined indirectly by measuring pulmonary capillary wedge pressure, which reflects the left atrial pressure accurately.
Pulmonary Capillary Wedge Pressure
Pulmonary capillary wedge pressure is the pressure exerted in the pulmonary capillary bed after obstructing the proximal part of pulmonary artery. Pulmonary capillary wedge pressure is measured by using a balloon-tipped multilumen cardiac catheter (Swan-Ganz catheter). TIP of the catheter is not open but a pressure transducer is attached to it.
By means of venous puncture, the catheter is guided through right atrium into right ventricle, Prom the right ventricle, it is advanced towards the proximal portion ot pulmonary artery and the balloon is inflated with air by using a syringe. This occludes the pulmonary artery. Then, the catheter alone is advanced further into distal portion of pulmonary artery, leaving the inflated balloon at the proximal portion, It allows the catheter to float in a wedge position. Now the pressure existing in the pulmonary capillary bed ahead of catheter is called pulmonary capillary wedge pressure (the word wedge refers to being obstructed).
When the proximal part of pulmonary artery is obstructed, pressure in the distal part falls rapidly and after about 10 second, it becomes equal to left atrial pressure. It is because of the absence of any valve between pulmonary capillary bed and left atrium So, the left atrial pressure can be determined by measuring pulmonary capillary wedge pressure.
Intra-Atrial Pressure Curve
Intra-atrial pressure curve is similar to the tracing of jugular venous pulse, which is known as phlebogram. It has three positive waves, a, c and v and three negative waves, x, x1 and y.
‘a’ wave is the first positive wave and occurs during atrial systole. The pressure rises sharply upto 5 mmHg in right atrium and 7 mm Hg in left atrium. After reaching the peak, the pressure starts decreasing.
‘x’ wave is the first negative wave and appears during the onset of atrial diastole. Because of relaxation of atria, the pressure falls. Atrioventricular valves close at the end of this wave.
‘c’ wave is the second positive wave and this appears during isometric contraction. Rise in pressure is due to the closure of atrioventricular valves and the increased intraventricular pressure.
When atrioventricular valves close, there is a little back flow of blood towards atria. When the intraventricular pressure increases, there is bulging of AV valves into the atria. Because of these two factors, the atrial pressure rises.
‘x1’ wave is the second negative wave and appears during ejection period. During ejection period, the contraction of ventricular musculature pulls the atrioventricular ring towards the ventricles. This causes fall in atrial pressure.
‘v’ wave is the third positive wave, which is obtained during atrial diastole. It shows a gradual increase in atrial pressure due to filling of blood in atria (venous return).
‘y’ wave is the third negative wave and appears after the opening of AV valves when the blood rushes from atria into ventricles. So the pressure in the atria falls.