A New Method Of Non-invasive Measurement For Central Venous Pressure And Its Mechanic Principles

Research Purpose and SignificanceCVP(central venous pressure, CVP) refers to the blood pressure where the inferior and the superior vena cava enter the right atrium, the normal range of which is 4 ~ 12 cm H2O. It is influenced by many factors such as circulation blood volume, the functional status of the heart(mainly the right ventricle). It is commonly used in clinically to monitor effective blood volume and guide fluid infusion therapy. It is especially necessary to monitor CVP for critically ill patients with shock, acute heart failure, acute renal failure and so on. In recent years, the world witnesses a high incidence of chronic diseases such as diabetes, hypertension due to the aging of the population, so it is significant to effectively monitor the hemodynamic changes and the cardiovascular function of the population.Up to now, the method for measuring central venous pressure is mainly dependent on invasive central venous catheterization, which is recognized as gold standard. Invasive catheterization method is to place the catheter directly in the right atrium, so the accuracy is high and the result is reliable, hence still in use. However, the procedures are complex and lengthy and the patients have to endure pain and the risks of bleeding and infection. So establishing a rapid, non-invasive method will tremendously improve the monitoring of central venous pressure.Physicians usually comprehensively analyses multiple indexes of the medical history and the physical examination including blood pressure, pulse, heart rate, liquid balance, skin color, and temperature and humidity, and then preliminarily evaluate the circulating blood volume and the cardiac preload. However, the accuracy is only 50-60%. In addition, the method is mainly dependent on the clinical experience of the physicians, which is influenced by individual sense of the illness. This subjectivity will bring out mistakes, delay the effective treatment and lead to disastrous consequences.Another method is to estimate the central venous pressure via visually observing the height of the collapse point of the jugular vein. The theory is based on a theory that in the human closed-loop blood system, the heart is the power pump but the body’s own veins are piezometric tube. The operation method is: The operator stands on the right side of the patient in the semi-recumbent position and place the right hand on the patient’s sternum with a flashlight; the beam get through the pulse point of the right internal jugular vein and generates projection in its rear; visually observe the the highest pulse point of the right jugular vein and mark it, measure the vertical distance between the point and the sternal angle, and the observed value plus 5cm is the estimated value of CVP(cm H2O). Since the method is to determine the collapse point of the internal jugular vein via visual observation, there exists persional error and the method is notrigorous; meanwhile, in clinical practice, due to complex changes in the patient’s condition of jugular vein collapse, in many cases it is difficult to observe the collapse point. Our pre-experiment confirmed that even in healthy people, there are a considerable number of people, regardless of their changing position jugular vein, whose internal jugular vein can not always be revealed.In addition, Arun D.Nagdev explores a way to semi-quantify central venous pressure through the measurement of inferior caval index by ultrasound. The method of Nagdev requires that the patient coordinates by breathing with Mueller action. He groups the results with the cutoff point of 8mm Hg, and assess the relationship between the inferior caval index being less than 50% and the central venous pressure below 8mm Hg, so as to decide whether the CVP is abnormally low. Although the methods mentioned above can describe CVP in some extent, they all have some inevitable drawbacks, such as the need for special equipment and strict breathing of patients and time-consuming operation, all of which make it difficult to be widely applied in clinic.Based on the anatomic structure of cardiac vascular and principles of hydromechanics, we established a new method of non-invasive measurement for central venous pressure, namely, using the jugular vein itself as a piezometer tube and measuring the collapse point of jugular point through ultrasound. The projection of the center of right atrium on the surface was located via geometric method on the basis of cardiac vascular anatomy. Relativity of the results between this non-invasive measurement and central vena catheterization was analyzed as well as the diagnostic value of this new non-invasive method. Materials and Methods 1. Patients94 patients who were about to undergo central venous catheters between December 2013 to May 2014 in the Department of Digestive Surgery, Tangdu hospital were included. Of all the patients, 58 were male, 36 were female. The age ranged from 20 years old to 80 years old, with average age being 53.05 years old. All the 94 patients have went through conventional two-dimensional Doppler ultrasound and Color Doppler ultrasound to have their heart and jugular vein examined. No remarkable defects of cardiac structure were found. The patients were all healthy enough to endure central vena catheterization, and the pressure acquired from catheterization was regarded as gold standard. The patients have been informed and all agreed to conduct the study. 2 Measurement of Central Venous Pressure 2.1 Measurement of the center of right atrium:All participants adopted a supine positionand breathed naturally. The position of the fourth intercostal space was determined via palpation alongside the right edge of the sternum. The midpoint of the fourth intercostal space was marked and a tape measure was used to circle the cross section which went through the midpoint. The intersection between the tape measure and midaxillary line is the projection of the center of right atrium on the body surface. 2.2 Determination of the collapse point of jugular vein:The jugular veins in both sides were scanned in horizontal axis and vertical axis respectively. The bed should be adjusted so that the jugular vein can be best displayed. Rotate the transducer to locate the collapse point the jugular vein, and mark it on the body surface. The patient should keep the same position for the operator to measure the vertical distance between the center of right atrium and the collapse point of jugular vein. This step should be conducted twice to obtain an average number. 2.3 Measurement of Central Venous Pressure through catheterization:This part of the study was conducted by anesthetists from the operating room. The lesion of the puncture was previously disinfected and narcotized. Then, the catheter was penetrated into the right atrium and was connected to the cardiogram monitor, so the value of CVP can be read from the monitor. Record the value of CVP successively in 5 cardiac cycles and calculate the average number. Values of CVP were recorded as mm Hg, and then converted into cm H2O。 3 Statistical analysisStatistical analysis was performed using SPSS 20.0. Correlation analysis and paired t-test were used to compare the invasive and noninvasive methods. A two-sided P-value of <0.01 was considered significant, with invasive determination of CVP as the gold standard, the ROC curve of the noninvasive ultrasound method was sketched to explore the optimal cut-off points. 4 Results 1. A total of 94 noninvasive measurements by ultrasound were performed(58 preoperative measurements and 36 postoperative measurements). The paired t-test revealed no significant difference between values of the two method(10.71±2.79 cm H2O vs 10.48±2.67 cm H2O,P=0.152).The correlation analysis reveal high positive correlation between CVPs determined by ultrasound imaging and central venous catheter(r = 0.842, P<0.01). Values of noninvasive CVP increased together with the invasive CVP. 2.The invasive central venous pressure≥12cm H2 O was defined as clinically significant elevation(gold standard). By the ROC curve test of the 94 noninvasive measurements of CVP, fluid column height of 10.75 cm by ultrasound method was determined as the cut-off point, with the sensitivity and specificity of diagnosing elevation of CVP being 96.9% and 87.1% respectively. The corresponding area under the curve was 0.969. 5 Conclusion 1.Ultrasound imaging could show the collapse part of jugular vein in real time and locate the collapse point, leading a high correlation with the gold standard invasive measurement. 2.The innovation of this study is the accurate location of right atrium center, which is very close to the anatomic right atrium center, reducing the measurement errors caused by inaccurate location. Thus, it is very helpful for the precise determination of CVP in clinic. 3.The cut-off point of CVP acquired by noninvasive ultrasound measurements could be used to evaluate the increase of CVP semiquantitatively, providing a new method for the noninvasive measurement of cardiac pressure.