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Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.54 no.1 Campinas Jan./Feb. 2004
Automatic blood pressure monitors. Evaluation of three models in volunteers*
Monitores automáticos de presión arterial. Evaluación de tres modelos en voluntarias
Luiz Eduardo Imbelloni, TSA, M.D.I; Lúcia Beato, TSA, M.D.II; Ana Paula Tolentino, M.D.III; Dulcimar Donizete de Souza, M.D.IV; José Antônio Cordeiro, M.D.V
IDiretor do Instituto de Anestesia
Regional, Hospital de Base da FAMERP; Anestesiologista da Clínica São
Bernardo, Rio de Janeiro, RJ
IIAnestesiologista da Clínica São Bernardo, Rio de Janeiro, RJ
IIIAnestesiologista do Hospital de Base da FAMERP
IVChefe do Departamento de Especialidades Cirúrgicas da FAMERP
VCoordenador de Ensino na FAMERP
BACKGROUND AND OBJECTIVES: Since 1903,
blood pressure has been noninvasively monitored (NIBP), either with manual sphygmomanometer
or automated noninvasive devices. One NIBP measurement problem is the considerable
variance in blood pressure data, both within and between available techniques.
The oscillometric method for NIBP monitoring evaluates blood pressure during
cuff deflation. Difficulties in blood pressure measurement by oscillometry may
arise from: inadequate cuff size, inadequate cuff application, undetected fails
in cuff, hoses, or connectors, arm movement, shock and vascular compression
proximal to the cuff. This study aimed at evaluating the reliability of three
noninvasive blood pressure monitoring devices during five measurements.
METHODS: Blood pressure of 60 healthy female volunteers aged 20 to 40 years was evaluated from 7 am to 11 am, in the sitting position during a normal workday. Five measures were taken with each device at 2-minute intervals. Three automatic blood pressure monitors were studied. No patient was obese, hypertensive or suffering from cardiac disease and cardiac arrhythmia. Indirect measurements were made according to manufacturers' instructions.
RESULTS: There were no differences in demographics among the three studied groups. Mean intrapersonal variation from one measurement to the other was up to 6.7 mmHg for systolic blood pressure (SBP), 4.9 mmHg for mean blood pressure (MBP) and 3.3 mmHg for diastolic blood pressure (DBP) with 95% confidence interval. The highest difference between measures in the same volunteer was 49 mmHg for SBP, 46 mmHg for MBP and 28 mmHg for DBP.
CONCLUSIONS: This study has shown significant variations in SBP, MBP and DBP and that SBP is the most reliable parameter to check blood pressure changes in volunteers.
Key Words: MEASUREMENT TECHNIQUES:non-invasive blood pressure, oscilometry; MONITORING: blood pressure
JUSTIFICATIVA Y OBJETIVOS: Desde 1903
la monitorización de la presión arterial ha sido realizada por
método no invasor, con esfigmomanómetro o aparatos automáticos
no invasores. Uno de los problemas de la medida de la presión arterial
no invasora es considerar la variación de la presión arterial
con el método utilizado. El método oscilométrico de medida
de la presión arterial evalúa la presión arterial durante
la deflación del manguito. Dificultades de la medida de la presión
arterial por el método oscilométrico pueden acontecer por: tamaño
inadecuado del manguito, incorrecta aplicación del manguito, faltas no
detectadas por el manguito y conectores, movimiento del brazo, estado de choque
y compresión vascular proximal al manguito. Este estudio evaluó
la confianza de los aparatos en las medidas de la presión arterial por
el método no invasor en cinco medidas con tres aparatos diferentes.
MÉTODO: Fue evaluada la presión arterial en 60 voluntarias con edades entre 20 y 40 años en el período de las 7 a las 11 horas de la mañana, en la posición sentada de un día normal de trabajo. La medida de la presión arterial constató de cinco afericiones con intervalo de 2 minutos. Fueron estudiados tres aparatos automáticos de medida de la presión arterial. Ninguna paciente era obesa, hipertensa o sofría de enfermedad cardíaca o disritmias cardíacas. La medida indirecta de la presión arterial era hecha de acuerdo con las instrucciones de los fabricantes.
RESULTADOS: No hay diferencia entre los tres grupos estudiados en relación a los parámetros demográficos. La variación media intrapersonal en la PAS, de una medida para otra, fue de hasta 6,7 mmHg, en la PAM de hasta 4,9 mmHg y en la PAD de hasta 3,3 mmHg, todas con intervalo de confianza 95%. La diferencia máxima entre las medidas en la misma voluntaria fue de 49 mmHg en la PAS, 46 mmHg en la PAM y 28 mmHg en la PAD.
CONCLUSIONES: Este trabajo mostró que hay variación significativa entre as medidas de las PAS, PAM y PAD y que la PAD es el parámetro más fidedigno para verificar alteraciones de la presión arterial en voluntarias.
The difference in blood pressure among vessels was firstly referred to by the Renascent painter Giovanni Di Paolo in representing St. John Baptist's decapitation with some vessels pouring blood (arteries) and others dripping blood (veins) 1. Riva-Rocci, in 1896, has described in two successive publications a new mercury column sphygmomanometer to evaluate blood pressure variations 2,3.
The standardization of technical procedures is important for epidemiologic studies involving different observers because systematic errors and especially observer-introduced errors, especially in blood pressure which has intrinsic variations, may compromise results. Routine blood pressure monitoring is still not standardized, almost always not following basic recommendations to avoid reading errors 4,5. Several factors related to equipment, observer, environment, patient and the technique itself, may interfere with blood pressure measurement accuracy. Criteria to be followed during NIBP measurement are clearly stated in our country 6. The strict adherence to the correct blood pressure measurement procedure should be adopted not only in the clinical practice but especially in clinical investigations and experiments including blood pressure measurement in their methods. In evaluating blood pressure measurement reported in the literature, it has been observed that, from a list of 19 items to be followed, only 20% have followed 12 of them 5. In a different study analyzing 223 studies published by 18 Brazilian journals related to clinical practice in the period 1989-1994, it was observed that half the studies have not specified the type of sphygmomanometer used 7. In studies where the type of device was mentioned, 29% were mercury column, 16% aneroids and 4% electronic 7. In addition, calibration checks were not mentioned in 82%, only 18% have expressed concern in checking system's accuracy and 25% of the studies have mentioned measurements position 7. The Brazilian Journal of Anesthesiology has published 274 studies from 1999 to 2002, of which 118 have studied blood pressure and none has mentioned the device or its accuracy. From those 274 studies, 15 have studied hemodynamic changes during spinal anesthesia. In a study on the efficacy of ephedrine to prevent arterial hypotension during Cesarean sections, the devices were also not mentioned 8,9. The identification of patients at higher risk for arterial hypotension during spinal anesthesia has already been studied where predicting factors were considered age above 45 years, female gender and sensory block level above T7 10.
Our study has evaluated three noninvasive blood pressure measurement devices commonly used in operating rooms and anesthesia machines, in an investigation project with volunteers aged 20 to 40 years without history of arterial hypotension or use of medication, with the objective of evaluating the reliability of intermittent blood pressure measurements.
Participated in this study 60 female volunteers, aged 20 to 40 years (assistant nurses, nurses, instrumentation technicians and physicians) working at surgical centers of Clinica São Bernardo and Hospital Barra D'Or in Rio de Janeiro, and Hospital de Base from São José do Rio Preto in the period 7 a.m. to 11 a.m. of a normal workday. All volunteers had normal blood pressures and were not under any anti-hypertensive medication, were not obese and had no history of arrhythmias. Volunteers with arterial hypertension during gestation were excluded from the study. Blood pressure was measured five times at 2-minute intervals, in surgical center rooms and according to Brazilian Consensus recommendations for arterial hypertension treatment 6. All measures were obtained in the sitting position and with right arm maintained at the level of the heart. It has been asked whether the 2-minute interval adopted would bring any discomfort.
Three devices were used in the study: Anamed-Vital Line AM 78100B, Hewlett-Packard (HP) model Viridia 24 C and Dixtal DX 2010. Indirect blood pressure was obtained according to manufacturers' instructions.
Repeated measures analysis method was used to evaluate effects of time, device (profession) and interaction between time and device (time and profession). Analysis of variance was used to compare age, weight, 1st to 5th measures variation and height according to device (profession). Paired t test was used to compare general variations among measures and confidence intervals for mean variations were determined by normal distribution, since all were approved by the normality test.
There have been no differences in mean age, weight and height, or according to device of profession.
Device and Time Comparison
Systolic blood pressure (SBP): there is evidence of time effect (p = 0.017) with intrapersonal mean decreasing with time; there is no evidence of device effect or of interaction between time and device (p = 0.36 and p = 0.84 respectively) (Figure 1).
Diastolic blood pressure (DBP): there is evidence of device effect (HP with mean values lower than other two brands) (p = 0.000); there is no evidence of time effect or of interaction between time and device (p = 0.12 and p = 0.68 respectively) (Figure 2).
Mean blood pressure (MBP): there is evidence of device effect (p = 0.000) with HP showing lower mean values as compared to other brands. There is evidence of time effect (p = 0.041) with intrapersonal mean decreasing along time. There is no evidence of interaction between time and device (p = 0.82) (Figure 3).
Profession and Time Comparison
Systolic blood pressure (SBP): there is evidence of profession effect (p = 0.000), with lower mean values for instrumentation technician as compared to other professions. There is evidence of time effect (p = 0.014) with intrapersonal mean decreasing along time. There is no evidence of interaction between time and profession (p = 0.47) (Figure 4).
Diastolic blood pressure (DBP): there is evidence of profession effect (p = 0.000), with lower mean values for instrumentation technique as compared to assistant nurse and physician. There is no evidence of time effect (p = 0.14) or of interaction between time and profession (p = 0.98) (Figure 5).
Mean blood pressure (MBP): there is evidence of profession effect (p = 0.000) with lower mean values for instrumentation technician as compared to assistant nurse and physician, and there is evidence of time effect (p = 0.041) with decreasing intrapersonal mean. There is no evidence of interaction between time and profession (p = 0.71) (Figure 6).
Intrapersonal Variation Comparison Along Time
Mean (downwards) intrapersonal variation from one measurement to the other was: SBP up to 6.7 mmHg (4.3 to 9 mmHg) with 95% confidence interval, MBP up to 4.9 mmHg (3.1 to 6.7 mmHg) with 95% confidence interval and MBP up to 3.3 mmHg (1.7 to 4.8 mmHg) with 95% confidence interval being significant all mean decreases between 1st and 5th measurements. These variations are device-independent (p > 0.25 for all pressures), but may be profession-dependent. Mean decrease between 1st and 5th SBP measurements was higher (17 mmHg) in instrumentation technician as compared to other professions which were not different among themselves (p = 0.000); MBP was lower in physicians (increase of 2.6 mmHg) as compared to other professions which were not different among themselves (p = 0.009). Mean decrease from 1st to 5th DBP measurements was not significantly influenced by profession (p = 0.46).
Maximum difference in a same volunteer was 49 mmHg for SBP, 46 mmHg for MBP and 28 mmHg for DPB. All maximum variations were observed from the 6th to the 8th minutes. No volunteer presented the same value during the five measurements, showing an intrinsic personal variation.
The 2-minute interval checking has shown wide measurement variability and 25% of volunteers have complained about the tourniquet.
This study has shown that there are variations in SBP, MBP and DBP measurements of healthy volunteers without history of arterial hypertension. Maximum mean intrapersonal variation from one measurement to the other has been up to 6.7 mmHg for SBP, up to 4.9 mmHg for MBP and up to 3.3 mmHg for DBP, with less measurement variation for SBP. Maximum intrapersonal measurements difference has been 49 mmHg for SBP, 46 mmHg for MBP and 28 mmHg for DBP, all between the 4th and the 5th measurement. This way, SBP and MBP decreases are not the best parameter to evaluate blood pressure in volunteers, as well as there are wide variations between 1st and last measurements leading to intrapersonal errors.
Indirect blood pressure measurement technique using arterial occlusion and stethoscope, according to Riva-Rocci 2,3 and Korotkoff 11 remains to date. Although this indirect blood pressure reading is easy to teach and fast to perform, an automatic measurement is desirable, especially in the operating room and intensive care unit, when repeated measurements are needed. A major advantage of automatic devices is the possibility of ruling out the influence of the observer. Blood pressure measurement is needed to handle anesthetized patients. Noninvasive methods are routinely used due to their low risk and simplicity. The interest in noninvasive mean blood pressure techniques is increasing because this pressure is often evaluated by invasive techniques. The oscillometric method started to be developed in 1973 and was placed in the operating room in 1976 12. Since its introduction, several studies have described the accuracy of devices evaluating mean blood pressure by the indirect method in neonates, children and adults 13-17. Although there are exceptions, several studies have shown that the accuracy of such devices is below 5 mmHg of mean error with standard deviation of less than 8 mmHg as compared to catheter insertion in the central artery. It has been observed an intrapersonal variability from one measurement to the other of 4.3 to 9 mmHg for SBP, 3.1 to 6.7 mmHg for MBP and 1.7 to 4.8 mmHg for DBP, regardless of the device, showing that SBP and MBP are not a good parameter to evaluate minor decreases in volunteers during a normal workday. In a same volunteer, SBP decrease from one measurement to the other has reached 49 mmHg while MBP reached 46 mmHg and DBP 28 mmHg, showing major intrapersonal variability during blood pressure monitoring.
Several blood pressure treatment decisions during anesthesia are made based on systolic and diastolic blood pressure. In severely ill patients, mean blood pressure values are obtained through radial artery puncture. It has been suggested that noninvasive monitors may perfectly replace invasive blood pressure methods 18. A 14 mmHg MBP variation with 95% confidence interval has been shown in 19 patients 13. Our study has found 4.9 mmHg variation for MBP with 95% confidence interval. Limit 80 mmHg value of the oscillometric monitor has shown a trend to overestimate blood pressure above this limit and underestimate the evaluation below this limit 19. It has also been shown that the oscillometric monitor overestimates blood pressure in the presence of deliberate arterial hypotension 19. Correlation coefficients between invasive and noninvasive methods have shown a good, however wide variability among individuals 20. Possible variations among individuals may be explained by the width of the cuff used with the oscillometric method because width equivalent to one third or half the arm's circumference does not produce an error above ± 5 mmHg 21. This factor alone does not explain variations among individuals. Cuffs used in our study were within manufacturers' standards for adults, so have not contributed for the wide measurement variations.
Arterial hypotension and bradycardia are the most feared spinal anesthesia side-effects 22,23, especially in obstetrics, because prompt and effective treatment is considered critical to prevent fetal suffering. Arterial hypotension criteria used in most spinal anesthesia studies published by the Brazilian Journal of Anesthesiology from 1999 to 2003 is 20% 8-10,24-26 to 30% 27-35 decrease of control SBP or SBP below 90 mmHg 10. Traditionally, arterial hypotension in obstetrics is defined as SBP decrease equal to or below 20% as compared to control, or even, SBP below 100 mmHg, which should be monitored at 3-minute intervals 36. Currently, blood pressure is monitored in more frequent intervals (every minute till birth) and vasopressant therapy is early started to correct even 10% decrease of control pressure 36. Noninvasive blood pressure devices take approximately 45 seconds for inflation, deflation and reading. The oscillometric method (as compared to manual auscultation method) has a minor effect in measurement accuracy when venous ingurgitation is not allowed to decrease. At least a little venous flow should be allowed at the end of each measurement to avoid discomfort and possible edema or petechia. In the normal automatic mode, when measurement time is set for one minute, the shortest interval between measurements is approximately 25 seconds 25. In the immediate mode, time between measurements is only a few seconds, but since these immediate measurements take approximately 15 seconds only, in general there is no discomfort or major venous ingurgitation at the distal limb. However, to prevent any possibility of complications with venous ingurgitation, the immediate mode should not be used more frequently than necessary 37. In our study, 25% of volunteers have complained of tourniquet during the 2-minute interval measurements.
The criteria of waiting for 20% decrease in systolic blood pressure to administer therapeutic ephedrine doses in spinal anesthesia for Cesarean sections, often recommended in the literature and adopted in the clinic, is equally inadequate 8. So, some authors have proposed 38 and others agree 8 that the best way to treat arterial hypotension is the therapeutic use of bolus vasopressants for any blood pressure decrease. Bolus doses of 15 mg or higher than 2 mg.min-1 have determined high incidence of arterial hypertension and prophylactic ephedrine has developed arterial hypertension in 15% to 17.5% 9 or even unacceptable arterial hypertension 8. Bolus doses equal to or higher than 15 mg, or prolonged infusions equal to or higher than 4 mg.min-1 should not be used since they may induce high incidence of maternal arterial hypertension or impair fetal wellbeing 8. Blood pressure evaluation in volunteers at 2-minute intervals has shown that we should be more careful in treating any systolic blood pressure decrease because it may result in arterial hypertension, which is as undesirable as arterial hypotension. No volunteer had the same value in the five measurements, showing an intrinsic personal variation.
Mean blood pressure is less affected by vascular tone changes than diastolic pressure, because it is obtained when oscillations reach their highest amplitude during cuff deflation 25. This feature allows mean blood pressure to be safely used, even in cases of arterial hypotension with vasoconstriction and pulse pressure decrease 37. Conversely, our study has shown that DBP presented fewer variations in volunteers not submitted to surgical stress.
In evaluating three devices in volunteers aged 20 to 40 years, we have observed intrapersonal variations with significantly different means of up to 6.7 mmHg for SBP, 4.9 mmHg for MBP and 3.3 mmHg for DBP. So, treating any blood pressure decrease without taking into account mean device error and intrapersonal variation may result in approximately 17% arterial hypertension 9 or even unacceptable arterial hypertension 8 during spinal anesthesia for Cesarean section. The evaluation at 2-minute intervals was uncomfortable for 25% of patients. This study has shown that DBP is the best parameter to evaluate blood pressure changes in volunteers. Blood pressure treatment during spinal anesthesia based on any SBP decrease should be revaluated.
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Dr. Luiz Eduardo Imbelloni
Address: Av. Epitácio Pessoa, 2356/203 Lagoa
ZIP: 22471-000 City: Rio de Janeiro, Brazil
Submitted for publication March 21, 2003
Accepted for publication May 30, 2003
* Received from Instituto de Anestesia Regional, Faculdade de Medicina de São José do Rio Preto, SP (FAMERP)