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Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.51 no.6 Campinas Dec. 2001
Acromion-clavicular joint as an alternative reference point for the phlebostatic level*
La articulación acromion-clavicular como punto de referencia alternativo para el nivel flebostático
Getúlio Rodrigues de Oliveira Filho, M.D.I; Rolando Eliezer Jimenez Bernal, M.D.II; Sandro Luiz Pivatto, M.D.II; Alexandre Teobaldo Tomasi, M.D.III; Luiz Fernando Soares, M.D.IV; Pablo Escovedo Helayel, M.D.IV
IIME2 do CET/SBA
IIIME1 do CET/SBA
IVAnestesiologista do Hospital Governador Celso Ramos
BACKGROUNDS AND OBJECTIVES:
The phlebostatic level corresponds to the medium point of the anterior-posterior
thoracic diameter at the level of the 4th intercostal space. It is
the standard reference point for central venous pressure zero level. In general,
the access to the lateral aspect of the thorax is impossible during anesthesia.
This study aimed at building an equation to estimate the phlebostatic level
based on anthropometric variables and the acromion-clavicular joint level.
METHODS: Participated in this prospective study 200 patients who were distributed in two groups. Group 1 was used for building the predictive equation. Group 2 was used for the validation of its predictive capability. Anthropometric data were collected. Phlebostatic level and acromion-clavicular joint height were measured in the supine position. Multiple linear regression was used in Group 1, with phlebostatic level as the dependent variable, and anthropometric variables, as well as acromion-clavicular joint height, as independent variables. Bland-Altman method was used in Group 2 to test the agreement between the observed and estimated phlebostatic level values.
RESULTS: Group 1 resulting equation was: phlebostatic level = 49.57 + (0.19 x age) + (0.31 x weight) + (0.20 x acromion-clavicular joint height). Mean difference in Group 2 between estimated and observed phlebostatic levels was 2.79 ± 7.62 mm.
CONCLUSIONS: The equation presented in this study can accurately predict the phlebostatic level.
Key Words: MONITORING: central venous pressure, phlebostatic level
JUSTIFICATIVA Y OBJETIVOS:
El nivel flebostático corresponde al punto medio del diámetro antero-posterior
del tórax, al nivel del 4º espacio intercostal y se constituye
en el punto de referencia patrón para el nivel cero de la presión
venosa central. Frecuentemente, durante anestesia, el acceso a la fase lateral
del tórax es imposible. Este estudio tuvo por objetivo, derivar una ecuación
que posibilite la estimativa del nivel flebostático a partir de variables
antropométricas y de la altura de la articulación acromion-clavicular.
MÉTODO: Fueron estudiados 200 pacientes. El grupo 1 fue utilizado para derivación de la ecuación y el grupo 2, para su validación. Fueron colectados datos antropométricos. El nivel flebostático y la altura de la articulación acromion-clavicular fueron medidos en decúbito dorsal. Regresión linear múltipla fue utilizada en el grupo 1, teniendo como variable dependiente el nivel flebostático, e independiente, las variables antropométricas y la altura de la articulación acromion-clavicular. En el grupo 2, la concordancia entre los valores del eje flebostático observados y predichos por la ecuación fue testada pelo método de Bland-Altman.
RESULTADOS: La ecuación resultante del grupo 1 fue nivel flebostático = 49,57 + (0,19 x Edad) + (0,31 x Peso) + (0,20 x altura de la articulación acromion-clavicular). La diferencia media entre los valores predichos y observados del nivel flebostático, en el grupo 2 fue de 2,79 ± 7,62 mm.
CONCLUSIONES: La ecuación presentada en este estudio puede prevenir con precisión el punto flebostático.
The accurate central venous pressure measurement is basically dependent on three factors: patient's positioning in the neutral supine position, correct location and catheter tip's patency and the choice of the external reference point for zero level1.
The phlebostatic level was determined in 1945 by measuring venous pressure of the basilic vein and of hand dorsal veins of individuals positioned with the bed head raised to different levels. It corresponds to the intersection of a frontal plane crossing the medium point of the chest anterior-posterior diameter, at the level of the xiphoid process, and of a cross section plane crossing, in most patients, the 4th intercostal space at the level of its junction with the sternum lateral border. The intersection of both planes generates a transverse axis which crosses the chest from side to side at the medium point of the chest anterior-posterior diameter, at the level of the junction of the 4th intercostal space with the sternum border. This transverse axis crosses the junction of superior and inferior vena cava with the right atrium. Phlebostatic level means any horizontal plane crossing the phlebostatic axis. The phlebostatic level crossing the medium point of the thoracic anterior-posterior diameter at the level of the 4th intercostal space is considered a reliable reference for the medium point of right and left atrium. It has been validated by studies during heart catheterization and bidimensional echocardiography and has been used for measuring right and left atrial pressures, as well as pulmonary artery pressure2,3. The phlebostatic level can only be used as a reference point in the supine position because, when used as a reference for central venous pressure zero level in the semi-lateral position of 30º, it is related to unacceptable statistical and clinical discrepancies of central venous pressure measurements4-6.
Usually, one cannot reach phlebostatic level reference points during anesthesia due to patient's arm abduction at 90º or to the surgical staff around patient's chest.
Acromion-clavicular joint is an easily palpable anatomic reference even in strong or obese patients, being usually accessible to the anesthesiologist. At ectoscopy, it seems to correspond to the medium axillary line.
This study aimed at building an equation to estimate the phlebostatic level based on anthropometric variables and acromion-clavicular joint height.
After Hospital Governador Celso Ramos Medical Ethics Committee approval, participated in this prospective study 200 adult patients of both genders, aged 15 to 86 years. Measurements were performed with a millimetric ruler and water level. Patients were placed in the supine position with no pillow on a plain surface. The vertical distance - height - between such surface and the following points was recorded:
1) Chest anterior surface at the level of the 4th intercostal space anteriorly, that is, chest anterior-posterior diameter;
2) Acromion-clavicular joint with the arm abducted at 90º.
Phlebostatic level (PL) was determined as the medium point of chest anterior-posterior diameter.
Patients were distributed in two groups. Group 1 (n = 120) was used to build an equation able to estimate the phlebostatic level based on the acromion-clavicular joint height and anthropometric data (age, gender, weight, height). Group 2 (n = 80) was used to validate this equation.
In Group 1, acromion-clavicular joint height was compared to the phlebostatic level by Student's t test for independent variables. A linear equation was built by anterograde progressive multiple regression, which had the phlebostatic level as the dependent variable, being age, gender, weight, height and acromion-clavicular joint height the independent variables.
In Group 2, the acromion-clavicular height and the phlebostatic level estimated by the equation were compared to the observed phlebostatic level by Student's t test for independent variables. The agreement between the estimates and the measures of phlebostatic level was tested by Bland-Altman's test. Significance level was established in 5%.
Demographic data are shown in table I. Age and acromion-clavicular joint height were significantly different between groups.
Gender and height were rejected during the linear equation building process. Resulting multiple linear regression equation was PL = 49.57 + (0.19 x age) + (0.31 x weight) + (0.20 x acromion-clavicular joint height). Correlation coefficient (R) was 0.61, determination coefficient (R2) was 0.37 and standard deviation was 9.39 mm. P value equaled zero. In Group 2, mean and standard deviations of measured and estimated phlebostatic levels were 88.84 ± 10.83 mm and 90.98 ± 6.14 mm (p = 0.12).
The difference between phlebostatic point estimate and its real measure varied from 16.24 to 23.2 mm, with mean and standard deviation of 2.79 and 7.62 mm, respectively. The limits of the 95% confidence interval of the mean difference were 1.09 and 4.49 mm.
Bland-Altman's diagram analysis has shown that most points corresponding to the differences between estimated and observed measures and their respective means were located between two standard deviation limits of the mean difference between measures (Figure 1).
Venous pressure assessed by clinical means tends to underestimate values measured by a central venous catheter. The reasons for that are the lack of standard external reference points for zero level, the posture effects on central venous pressure and the change in vasomotor tone of patients with heart failure. So, it is not recommended to attempt a CVP numeric estimate through clinical evaluation7.
The importance of a known zero level for gauging pressure gages and pressure transducers has been confirmed by several studies1,8.
There are, at least, nine reference points for central venous pressure zero level. However, they are all based on measurements done at the chest lateral aspect. The variability of central venous pressure zero level among those different points is approximately 8 cmH2O9.10.
The impossibility of accessing chest lateral aspect to identify phlebostatic level reference points has motivated the search for an easily accessible reference point for the anesthesiologist usually located around the patient's head. In our study, acromion-clavicular joint as well as patients age and weight allowed to build an equation to estimate the phlebostatic level through multiple linear regression.
The multiple linear regression describes the relationship or association between multiple explanatory variables and a dependent variable. Depending on the robustness of this association, the resulting equation may be used to estimate the dependent variable based on explanatory variables, in the same or in different samples. Association magnitude is determined by R coefficient which, in our study, was 0.61 and highly significant. Determination coefficient R2 represents the variance fraction of the dependent variable generated by explanatory variables and describes how clearly the model's straight line describes the association between variables. Determination coefficient was 0.37, thus showing that more than 60% of estimates variance were not due to independent variables. Estimates standard deviation, which is a parameter reflecting the magnitude of actual estimates variability as a function of the line describing the multiple regression model, was 9.39 mm and may be considered clinically insignificant11.
The linear equation allowed for phlebostatic level estimates based on the tested alternative reference point, and such estimates were not significantly different from the measured level. However, the lack of statistical significance in Student's t test comparisons does not establish the clinical importance of the agreement variability between estimated measurements and the phlebostatic level12.
Bland-Altman's method is a simple technique to evaluate agreement between two imperfect clinical measures. The imprecision of the phlebostatic level height measured on the chest lateral surface has already been documented and is caused by individual variabilities among observers and patient-dependent factors, such as chest deformities and breast volume. The imprecision of the height estimated by the linear equation of this study is also related to investigators differences in accurately identifying acromion-clavicular joint, to the possibility of patients' anatomic variations and to inaccuracies inherent of the millimetric ruler, such as inclination and precision. Bland-Altman's method starts by calculating the differences between both measurements to be compared. After that, mean and standard deviation of those differences are also calculated. Mean difference is a measure of the magnitude of the disagreement between both measures, and standard deviation corresponds to a variation of such disagreement. Following, mean of both measures is calculated, assuming that the arithmetic mean of both values represents the best approximation of the real value of the parameter, in this case, the phlebostatic level. Then, on a chart where the coordinate represents differences between both measures and the abscissa their respective means, points corresponding to the difference and the mean of each pair of measures are plotted. A horizontal line is plotted at the level of the mean difference and two lines parallel to that are plotted at the level of values corresponding to the mean difference more or less two standard deviations, which represent clinically acceptable disagreement limits. If mean differences between measures are close to zero, the conclusion is that there are no systematic discrepancies between both compared measures. If most points corresponding to the difference between measures and their respective means are within upper and lower limits of disagreement, variation between measures is considered clinically acceptable13.
Bland-Altman's diagram for this study showed that the mean difference between estimates and phlebostatic level measures was close to zero (2.79 mm), indicating no systematic discrepancy between pairs of measures. The 95% confidence interval of this mean difference remained between 1.09 and 4.49 mm and allowed for the conclusion that, in other samples, mean difference between both measures will remain within those limits in 95% of tested samples, thus con- firming the clinical insignificance of such difference. Con- sidering that the vast majority of points corresponding to mean measures and their respective differences remained within disagreement limits, it is possible to conclude that the alternative reference point calculated by the multiple linear equation built by this study is able to estimate the phlebostatic level with clinically accepted accuracy.
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Correspondence to Submitted for publication January 31, 2001 * Received
from CET/SBA Integrado de Anestesiologia da SES/SC, Hospital Governador Celso
Ramos, Florianópolis, SC
Dr. Getúlio Rodrigues de Oliveira Filho
Address: Rua Luiz Delfino, 111/902
ZIP: 88015-360 City: Florianópolis, Brazil
Accepted for publication June 1, 2001
Submitted for publication January 31, 2001
* Received from CET/SBA Integrado de Anestesiologia da SES/SC, Hospital Governador Celso Ramos, Florianópolis, SC