Noninvasive Central Venous Pressure Measurement By Compression Ultrasound

Background and purpose Central venous pressure is one of the many parameters for monitoring hemodynamics and the pressure index to estimate patients cardiac preload which is the circulating blood volume in front of myocardial contraction. It is also an important reference index for the clinical infusion velocity and fluid volume. So it plays an important role for monitoring patients condition. The current standard technique for measurement of CVP is the central venous catheterization. However, this method is not only invasive, but also with potential complications and the higher technique. By 1930, Sir Thomas Lewis firstly described a noninvasive technique for estimating the CVP by measuring the height of the column of blood in the jugular veins. But the physical examination has not been widely used all along due to the limitation of clinical examination skills and patients condition. In recent years, ultrasound has been proven to be a powerful tool to noninvasively visualize neck veins and determine elevated jugular venous pressure, while the improvement was limited. Indeed, the main reason for the limitation of noninvasive CVP measurement is the failure to locate the external reference point of RA in different positions of the patient in anindividualized and accurate way. To solve this problem, we originated a simple and accurate method based on solid geometry to locate the central point in RA by echocardiograph. This brand new method could be helpful to improve the accuracy of noninvasive determination of CVP. Furthermore, it provides a new idea for the ultrasound locatization. Based on the principle of brachial cuff sphygmomanometer, we measured CVP noninvasively by compressing the superficial vein of the upper limb, using our self-made device. It might serve as a noninvasive method for more rapid, convenient for the determination of CVP.Materials and methods 1 patients Thirty patients who had been examined using multislice 3D-CT for chest imaging as part of their clinical management, including sixteen male cases, fourteen female cases, age 25 ~ 68 years old, the average(48.37 ± 5.85) years old, were recruited. All patients have been informed and all agreed to conduct the stud y. Thirty-five patients including twenty male cases, fifteen female cases, age 20~67 years old, average age(44 ± 14.1), blood pressure(125 ± 12/78 + 9) mm Hg, were recruited. The criteria were as follows: the patients were not suffering from phlebitis, a history of neck operation, deep venous thrombosis; the thorax were no deformity, no barrel chest; all subjects were normal by ultrasound. All patients have been informed and all agreed to conduct the stud y. 2 Data collection 2.1 Surface location of right atrial central point by echocardiography CT 3D-CT was performed on the patients with the use of a General Electric Light Speed VCT scanner(General Electric, Milwaukee, WI, USA) for chest examination. The dates were further reconstructed into 3D images with the General Electric Advantage Windows 3D workstation(General Electric, version 4.5). With the 3D volume rendering images and the corresponding 2D images, the center point of RA was located.(Figure 1). Echocardiography To avoid the influence of different positions, patients were placed in the samesupine position as during previous CT examination. A commercially available echocardiography systems equipped with a 3.5 MHz transducer(Vivid E9; General Electric Healthcare, Milwaukee, WI) was used to visualize the heart. Firstly, an adjusted apical “four-chamber” view in which the right atrium(RA) was placed in the middle of the sector image was acquired(Figure 3). The distance between the top of image and the central point of RA was measured and recorded. Then the probe was fixed in the image-acquired direction and position by the doctor. The assistant put a stick next to the probe and made it perpendicular to the coronal plane. After that, a ruler whose direction was kept parallel with the probe long axis direction was used to locate the surface projection of the middle point of RA. The ruler was made intersected with both the stick and the anterior chest wall, and distance between the two intersections was kept the same distance as previously recorded. The intersection of the ruler and the anterior chest wall was the surface projection of the center point of RA we wanted(Figure 2). The whole time(T) of locating process was recorded. Comparison After the surface projection of the middle point of RA located by echocardiography was marked, the surface projection point acquired through CT was marked as previous recorded too. Then the absolute distance(Da) and vertical distance(Dv) between the two points in the direction of coronal plane were measured. As the point marked according to CT location is the standard point, if the echo-location point is superior to it, Dv is defined as “+”, otherwise “-”. 10 patients were selected randomly to assess the intra-observer and inter-observer reproducibility of echo-location method. For the intra-observer reproducibility assessment, location point marked by one of the two investigators was identified as the standard point, if the other point is superior to it, Dv is defined as “+”, otherwise “-”. 2.2Noninvasive Central Venous Pressure Measurement by Compression Ultrasound Surface location of right atrial central point Patients were placed in supine position. We located and marked the surface projection of the central point of RA by echocardiograph. After the surface projection of the center ofRA on the anterior chest wall was located, the transverse plane that contained the RA center point was also obtained. The intersections of the bilateral midaxillary lines and that plane were marked. Noninvasive Central Venous Pressure Measurement The patients were placed in a comfortable supine position with a slightly elevated chest and were breathing regularly. After applying ultrasound transmission gel, the transducer with the pressure meter was placed on the skin with minimal pressure. The vein had to be easily compressible. After zero adjustment, slowly increasing pressure was applied by the transducer until complete compression of the vein and recording the data. Measuring the vertical distance between right atrial central point and pressure point. As right atrial central point was the standard point, if the pressure point was superior to it, the vertical distance is defined as “+”, otherwise “-”. Patients were selected to assess the inter-observer reproducibility of compression sonography. In this study, all patients had CVP invasively monitored through the central venous catheterization. The data were recorded. 2.3 statistical analysis Data analysis was performed using SPSS 17.0. Data were presented as mean ± SD and 95% confidence interval. Correlation and difference between invasive and noninvasive pressures was analyzed using paired t-test. Bland-Altman plots were used to show the agreement between two methods and inter-observer. P<0.05 was considered statistically significance.Result 1. Mean Da, Dv and T of the whole subjects were 0.76 cm(95% confidence interval(CI): 0.62 to 0.81 cm), 0.16cm(95% CI:-0.02 to 0.34 cm), and 43.80 sec(95% CI: 40.01 to 47.40 sec). Intra-observer locations showed slight lower variability than inter-observer for both Da(0.51 ± 0.30 cm vs0.32 ± 0.26 cm) and Dv(0.07 ± 0.35 cm vs-0.05 ± 0.27 cm). 2. The mean invasive CVP was(7.75 ± 1.69) mm Hg, mean noninvasive CVP of the two observers were(7.42 ± 1.60) mm Hg,(7.54± 1.54) mm Hg. The difference was not statistically significant for the paired t-test(P > 0.05). Linear regression analysisshowed a significant positive correlation between invasive and noninvasive pressures(r=0.83, P< 0.01). The Bland-Altman plot shows the mean difference of the two methods was-0.33 mm Hg, 95% confidence interval(-2.20 to 1.55 mm Hg). The inter-observer difference was 0.11 mm Hg, 95% confidence interval(-1.33 to 1.55 mm Hg).Conclusion 1. The geometric echo-location method we proposed could be used to locate the surface projection of RA individually with good accuracy and feasibility. It could be helpful to improve the accuracy of noninvasive determination of CVP and give a new perspective for localization with ultrasound. 2. Based on the principle of brachial cuff sphygmomanometer, CVP could be measured noninvasively by compressing the superficial vein of the upper limb. It might serve as a noninvasive method for more rapid, convenient for bedside monitoring CVP.