Introduction & Importance
The blood coming from the tissues are transported by the venous system which is composed grossly by the venules, the small and the great veins. The pulmonary artery contains mixed venous blood, which is actually the sum of the superior vena cava and inferior one venous blood. Venous congestion is seen in some diseases and is consequence to abnormal high venous pressure.Venous Return
The venous return (VR) can be defined as the volume of blood reaching the right heart. If one defines the term central venous pool - roughly the blood contained in the great thoracic veins and in the right atrium - then venous return will be the volume of blood entering this compartment, coming from the periphery. According to the Ohm’s law, there must be a pressure gradient between these two compartments. Keeping others variables constant, the venous return is inversely proportional to the central venous pressure. These two parameters can be plotted in a diagram, yielding the venous return family curves. In hemodinamically stable conditions the VR must be virtually equal to the CO - changes from one heartbeat to another do exist - otherwise blood would be damped back. CVP is always inherently driven to the equilibrium value that makes CO and VR equal. At CVP of 2 mmHg, CO is about 5 l/min.Major factors influencing venous return
1) Respiratory cycle - Central venous pressure (CVP) decreases with inspiration thereby increasing venous return. This is explained by the negative intrathoracic pressure originated at inspiration, which is transmitted to the great veins of the thorax; moreover, the downward diaphragm movement during this phase helps the pulling of blood toward the heart by increasing the intrabdominal pressure. At expiration, the mechanisms reverse. 2) Venous tone - is governed by autonomous system. 3) Right heart function - The blood reaching the right ventricle is pumped to the pulmonary circulation and therefore will not be damped backward in the venous system. 4) Gravity - discussed below 5) Muscle pump - discussed belowGravity & Muscle pump
Venous pressure increases by approximately 0.77 mmHg for each centimeter (cm) below right atrium In a standing person, the venous pressure around the ankle is about 90 mmHg. Gravity actually causes blood pooling in the legs and if one stands quietly, fainting will occur within a few minutes, despite the compensatory mechanisms, because of low brain perfusion pressure. In fact, whenever a patient is in shock the Trendelemburg position increases venous return due to gravity effects. Muscle contractions helps venous return by compressing the surrounding veins - the so-called muscle pump. In fact, leg muscles contractions can lower local venous pressure to less than 30 mmHg. In the upright position, the venous pressure above the right atrium is decreased. Neck veins collapse and pressure is close to zero. Dural sinuses have rigid wall and cannot collapse. The pressure in these ones is subatmospheric and may reach -10 mmHg in the superior sagittal sinus. If these structures are perforated during a neurosurgical procedure, air embolism can occur. The levels of air injected in the blood stream of laboratory animals that was fatal ranged between 5 to 100 mL and depends in part, upon the rate of infusion. Air, unlike blood, is compressible and can virtually stop the circulation or lodge in the cerebral circulation causing permanent neurological deficits or even death.The Venous Pulse Waves.
Venous flow becomes pulsatile close to the heart. The atrium dynamics is transmitted to the great veins, generating six waves which can be described. (The description is classically about the right atrium) : 1) "a" wave - The atria contraction generates this upward contour on the tracing 2) x descent - coming soon after the "a" wave, corresponds to the atrium diastole period. 3) c wave - In the past, it was believed that the carotid impulse, coming from the carotid artery, which is next to internal jugular vein, produced this upward movement on the tracing, thus the "c" for carotid. Today, the bulge of the tricuspid valve into the atrium at early systole is actually responsible for this wave. 4) x’ descent - Corresponds to the downward displacement of the base of the right ventricle at systole which reduces atria pressure and consequently produces this descent; 5) v wave - Venous return coming into the atrium at systole. 6) y wave - The atrioventricular valves open and blood quickly fills the ventricles, generating this other descent. Some authors also describe a h wave, that would be a subdivision of the y wave. It would correspond to the diastasis period. It is important to keep in mind that only two waves are diastolic events: the "a" wave and the y one. The reminder are systolic events.How can I measure CVP ?
Following the classic and definitive work on venous pressure by Landis and Hortenstine, Hughes and Magovern used right atrial pressure monitoring to guide blood volume replacement in patients who had undergone thoracotomy. Wilson and coworkers developed the CVP catheters. Because of its simplicity and availability, CVP monitoring is routinely used to guide fluid therapy in emergency conditions associated with blood volume deficits. CVP give us a somewhat good estimate about the effective circulatory blood volume and the venous return. Wide variation in the CVP may occur if (1) the central line slips into the right ventricle, (2) in severe right-sided heart failure and dilation of the atrioventricular ring and (3) in cases of tricuspid insufficiency. Besides cathetherization, CVP can be estimated at bedside: The internal jugular pulsations are transmitted to the neck of the patient and are observed by the examiner so the top level of pulsations can be determined. (The major movement of jugular pulse corresponds to x+x’ descent and sometimes is the only one observed. The remaining waves are too small to detect by inspection). An imaginary horizontal level is traced from this point. The sternal angle (Louis) is defined as the zero level. Trace a vertical line form this latter until this line meets the horizontal one previous traced. Measure in centimeters the vertical distance between the sternal angle and the meeting point. Normal upper limits are 2 cm when patient is supine and 4.5 cm when the head is elevated 45 degrees. Invasive methods have shown that normal value of CVP ranges between -2 and 6 mmHg, according to the respiratory phase. CVP is decreased in negative pressure breathing and in shock. It is increased in congestive heart failure, positive pressure breathing, cardiac tamponade and right ventricular failure.Right Ventricular Failure
Venous congestion is the sine qua non of right heart failure. The most common cause of right ventricular failure is complication of a previous chronic left ventricular failure, when the syndrome is named congestive heart failure. Another causes are right ventricle infarct, massive pulmonary embolism and left-right shunts such as atrial sept defects. As stated before, when the right cardiac output falls, there is a backward dumping - thus the name "backward failure" - The right ventricle cannot pump effectively the blood coming from the veins (venous return) and so the congestion. One can easily see the venous congestion by inspecting the jugular pulsations on the neck (see above).Deep vein thrombosis
This is a vascular disease affecting more the leg veins in which the thrombus prevent (sometimes actually stops) venous return distal to the obstruction point. The patient is anticoagulated and kept on bed rest with the affected leg raised to aid venous return by the gravity effect.Constrictive Pericarditis
In this disease, the pericardium becomes thickened and fibrous, therefore diastolic function can be severely impaired because when the ventricle relax, it will found a rigid compartment that will prevent adequate filling - diastolic restriction. The final diastolic pressure (Pd2) rises. The more compliant chambers, the atria and the right ventricle, bear the brunt of the burden, but Pd2 in all cardiac chambers tend to be the same, about 15-25 mmHg in severe cases. The y descent becomes greater than usual (diastolic collapse). Kusmaull described in this disease an inspiratory increase in venous pressure - Kusmaull’s sign, which is non-specific. The neck veins distends more with inspiration showing that right heart function seems to be impaired. In fact, the clinical picture of this disease mimicks right heart failure.1) Invasive and Nonivasive Physiologic Monitoring: William C.Shoemaker, MD . M.H.Parsa, MD in Critical Care Medicine 1996
2) Bedside Cardiology - Jules Constant, 1993
3) Review of Medical Phisiology - William F. Ganong, 1993
4) Cardiovascular Phisiology - David C.Mohrman & Lois Jane Heller, 1991
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