CV Physiology | Hemodynamics (Pressure, Flow, and Resistance)
Impedance to flow is the relationship between varying pressure and the flow it there are several sites where there is obstruction resistance or impedance. Blood flow refers to the movement of blood through a vessel, tissue, or organ, and is The graph shows the components of blood pressure throughout the blood. Recall this relationship from Case AP = blood flow x resistance. AP could increase because pressure in the pulmonary artery increases or because.
Many surgical procedures are peroperatively tested regarding objective reache, such as during valvuloplasty or valve replacement, in which the performance of peroperative echocardiogram can verify the surgical "status quo", and thus assume results. In CABG surgery, this has been tried with the assessment of coronary artery caliber [11,12], but it has not been fully achieved.
Despite this, various medical publications have studied late results of patients undergoing coronary artery bypass grafting, which are sometimes generically considered for future surgeries, as we could expect the same results for all cases, without considering the differences between them.
Thus, we have studied herein the coronary vascular resistance CVRassessing the possible behavior of blood flow in coronary grafts. METHODS This study was based on data obtained retrospectively from the perfusion record of cardiopulmonary bypass surgery, according acceptable and widely used technical standards for coronary perfusion using anterograde cardioplegic normothermic solution, when 10 patients consecutively underwent CABG, from June 17 to July 15 in our institution.
The study was developed based on the peroperative assessment of the perfusion pressure of coronary segments in patients undergoing CABG with saphenous vein, when the distal anastomosis of the graft had already been performed, by means of direct measurement and, in-line through pressure transducers when the selective infusion of cardioplegia was performed Figure 1.
Then, by means of mathematical calculations, the CVR and the blood flow resulting from such calculations were determinated for appropriate conditions of blood pressure. For demonstrative technical reasons, the arteries revascularized with the left internal thoracic artery were not included in the study. For verification reasons, all cases had their coronary artery caliber measured by dilators that were introduced inside the vessels lumens, as shown in Table 1. However, it was not our target search.
The CVR was obtained in two conditions: Statistical data were obtained simply by the arithmetic mean of the flows and coronary resistances. Some factors that are involved with the coronary flow, such as the flow competition and vasomotricity, as well as factors such as age, gender or medical conditions, among others were not considered in this study because the aim was to determine the CVR related exclusively to the coronary bed, accepting as persistent the physiological or pathological changes.
Determinants of Resistance to Flow (Poiseuille's Equation)
It was noted great variability of the CVR and CGO Table 1 as well as the inverse proportionality between them Figure 2indicating a close relationship with the distal bed of the artery studied. In our study, we found a mean of Due to physiological or pathological conditions, the vascular resistance estimated in arteries or isolated segments may be very different from the mean expected. There are many factors that directly or indirectly affect the CVR and may be cause of occlusion of arteries and grafts .
Slow coronary flow or lack of blood flow have been observed in cases of myocardial infarction or after angioplasty, suggesting mechanisms of recent onset, as the factors of post-reperfusional injury by free radicals, alpha-adrenergic vasoconstriction, angiotensin, neutrophil activation, embolic or thrombotic factors and lack of distal bed .
The different results found in CABG, even if when considering randomized studies, have maintained a permanent doubt among surgeons in respect to the reason of a well-performed procedure does not result in the expected purpose sometimes.
However, it is important to question whether these excellent results are due only to the quality of the graft or the favourable conditions of the native artery, or if such results may be due to the flow and resistance.
- Hemodynamics (Pressure, Flow, and Resistance)
Therefore, we included the question whether we could understand better the patency of venous grafts by estimating the risk peroperatively.
InCarrel  noticed the changes suffered by veins when undergone process of arterialization and Faulkner et al. Questions about sequential grafts and their possible benefits have been presented by several authors, as Rabelo et al.
In this study we perform a sequential venous graft in one case and observed distribution of resistance by both revascularized segments, thus reducing the final resistance, which allowed not only a reasonable output, but, perhaps, and the most important: Pressure Changes in Liquids Without a Change in Volume In the walls of a fluid-filled container contract, the pressure exerted on the fluid in the container increases.
You can demonstrate this principle by filling a balloon with water and squeezing the water balloon in your hand. Water is minimally compressible, and so the pressure you apply to the balloon is transmitted throughout the fluid. As you squeeze, higher pressure in the fluid causes part of the balloon to bulge.
If the pressure becomes high enough, the stress on the balloon will cause it to pop. The water volume inside the balloon did not change, but the pressure in the fluid increased. In the human heart, contraction of the blood-filled ventricles is similar to squeezing a water balloon: This high-pressure blood then flows out of the ventricle and into the blood vessels, displacing lower-pressure blood already in the vessels.
The pressure created in the ventricles is called the driving pressure because it is the force that drives blood through the blood vessels. When the walls of a fluid-filled container expand, the pressure exerted on the fluid decreases. Thus, when the heart relaxes and expands, pressure in the fluid-filled chambers falls. Pressure changes can also take place in the blood vessels. If blood vessels dilate, blood pressure inside them falls. If blood vessels constrict, blood pressure increases.
Volume changes of the blood vessels and heart are major factors that influence blood pressure in the cardiovascular system. Blood Flows from an Area of Higher Pressure to One of Lower Pressure As stated earlier, blood flow through the cardiovascular system requires a pressure gradient. This pressure gradient is analogous to the difference in pressure between two ends of a tube through which fluid flows Fig.
This relationship says that the higher the pressure gradient, the greater the fluid flow. A pressure gradient is not the same thing as the absolute pressure in the system. Fore example, the tube in Figure b has an absolute pressure of mm Hg at each end. However, because there is no pressure gradient between the two ends of the tube, there is no flow through the tube. On the other hand, two identical tubes can have very different absolute pressures but the same flow.
The top tube in Figure c has a hydrostatic pressure of mm Hg at one end and 75 mm Hg at the other end, which means that the pressure gradient between the ends of the tube equals 25 mm Hg.
The identical bottom tube has a hydrostatic pressure of 40 mm Hg at one end and 15 mm Hg at the other end. This tube has lower absolute pressure all along its length but the same pressure gradient as the top tube - 25 mm Hg. Because the pressure difference in the two tubes is identical, the fluid flow through the tubes is the same.
CV Physiology: Hemodynamics (Pressure, Flow, and Resistance)
Resistance Opposes Flow In an ideal system, a substance in motion would remain in motion. However, no system is ideal because all movement creates friction. Just as a ball rolled across the ground loses energy to friction, blood flowing through blood vessels encounters friction from the walls of the vessels and from cells within the blood rubbing against one another as they flow.
The tendency of the cardiovascular system to oppose blood flow is called the system's resistance to flow. Resistance R is a term most of us understand from everyday life.
Flow and perfusion
We speak of people being resistant to change or taking the path of least resistance. This concept translates well to the cardiovascular system because blood flow also takes the path of least resistance.
An increase in the resistance of a blood vessel results in a decrease in the flow through that vessel. What parameters determine resistance? For fluid flowing through a tube, resistance is influenced b by three components: The following equation, derived by the French physician Jean Leonard Marie Poiseuille and known as Poiseuille's law shows the relationship of these factors: To remember these relationships, think of drinking through a straw.
You do not need to suck as hard on a short straw as on a long one the resistance offered by the straw increases with length. Drinking water through a straw is easer than drinking a thick milkshake resistance increases with viscosity.
And drinking the milkshake through a big fat straw is much easier than through a skinny cocktail straw resistance increases as radius decreases. How significant are tube length, fluid viscosity, and tube radius to blood flow in a normal individual? The length of the systemic circulation is determined by the anatomy of the system and is essentially constant.Putting it all together: Pressure, flow, and resistance - NCLEX-RN - Khan Academy
Blood viscosity is determined by the ratio of red blood cells to plasma and by how much protein is in the plasma. Normally, viscosity is constant, and small changes in either length or viscosity have little effect on resistance. This leaves change in the radius of the blood vessels as the main variable that affects resistance in the systemic circulation. Let's return to the example of the straw and the milkshake to illustrate how changes in radius affect resistance.
If we assume that the length of the straw and the viscosity of the milkshake do not change, this system is similar to the cardiovascular system - the radius of the tube has the greatest effect on resistance.
Because flow is inversely proportional to resistance, flow increases fold when the radius doubles. As you can see from this example, a small change in the radius of a tube has a large effect on the flow of a fluid through that tube.
Thus a small change in the radius of a blood vessel will have a large effect on the resistance to blood flow offered by that vessel.