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Indian Journal for the Practising Doctor

Inter-Observer Variability in Blood Pressure Recording

Author(s): K N Chowta, Haseena and Mukta N Chowta

Vol. 3, No. 6 (2007-01 - 2007-02)

K N Chowta, Haseena and Mukta N Chowta

ISBN: 0973-516X

Dr N K Chowta (Associate Professor) and Ms Haseena (Postgraduate Scholar), are from the Department of Medicine, Kasturba Medical College, Mangalore.
Dr M N Chowta (Assistant Professor) is from the Department of Pharmacology, Kasturba Medical College, Mangalore.

Address for correspondence: Dr. Nithyananda Chowta K., Associate Professor, Department of Medicine, Kasturba Medical College Hospital, Attavar, Mangalore – 575 001.
E mail: [email protected]

Abstract

Objective: The present study was undertaken to evaluate the variability in blood pressure recording by different observer on the same patient.
Methodology: Two hundred patients of either sex, aged over 18 years, normotensives or hypertensives, attending the Outpatient Department of the KMC hospital, Attavar, were selected for the study. The blood pressure was recorded in the right arm in sitting position as per the WHO criteria. The recordings were taken by 5 observers at 5 minutes interval using the same instrument. The readings obtained by different observers were compared for the variability in reading and its statistical significance.
Results: Out of the 200 randomly selected patients 170 were male and 30 were females. Systolic blood pressure showed statistically significant variability among observers (p<0.001). Diastolic blood pressure also showed variability, but it was statistically not significant (p=0.5405). In the present study 30 were known hypertensives and 170 were normotensives. There was no significant increase in variability among hypertensives.
Conclusion: There is significant variation in the patient’s blood pressure values obtained by different observers. Our study emphasizes the need for repeated blood pressure measurements before labeling an individual as hypertensive.

Key words: Blood pressure, hypertension, variability.

Introduction

Blood pressure is a cardiovascular measurement with dynamic characteristics including diurnal variation that can be influenced by a number of internal and external factors such as emotions, clinical settings, instruments, and measurement techniques. The measurement of blood pressure in the office after several minutes of quiet rest (casual blood pressure) at different time of the day may be unreliable in up to 20% of patients for the accurate diagnosis of hypertension.1

Blood pressure measurements obtained by a doctor are often greater during the earlier than during the later part of his visit. This response has been ascribed to the occurrence of a transient pressor response triggered by an alarm reaction of the patient to the visit2. The anxiety engendered by the physician measuring the blood pressure may itself produce a substantial increase, which is often referred to as “white coat effect”. If the pressure is measured by a nurse or a technician, it is likely to be lower than when it is measured by a physician and also closer to the patients day time average level of blood pressure3.

Because the level of arterial blood pressure is the basis for major diagnostic and therapeutic decisions in medical practice, the measurement must be correct and maximally, reproducible. Although the occluding cuff technique appears to be simple and easy to learn, there are many possible causes of error and inaccuracy. Therefore, the technique of measurement should be standardized so that the information from different observers is comparable and can readily be used in serial evaluations of an individual or for epidemiological and research studies4.

There are not many studies regarding the observer variability in blood pressure recording, hence this study was undertaken.

Methodology: Two hundred patients of either sex, over 18 years, normotensives or hypertensives, attending the Outpatient Department of the KMC Hospital, Attavar, were selected for the study. The blood pressure was measured by a standard mercury sphygmomanometer, which was recently calibrated. The blood pressure cuff was applied to the right arm and the reading was taken in the sitting position. The readings were taken by 5 observers who included a physician, a postgraduate student, an intern, a medical student and a trained staff nurse. The medical student as well as the staff nurse were carefully trained by the concerned physician and made aware of the potential pitfalls during blood pressure recordings. The readings were taken at 5 minutes intervals by each observer using the same instrument. Each observer’s readings were not known to the other observer.

Table I. Blood pressure recordings by different observers

Characteristic Physician PG Student Intern Student Nurse P value
Mean systolic
blood pressure (± SD)
125.76 (± 20.47) 123.83 (± 20.27) 122.48 (±17.98) 121.67 (±19.29) 122.38 (±16.68) <0.0001*
Mean diastolic
blood pressure (± SD)
80.8 (±13.63) 80.77 (±13.69) 79.72 (±13.24) 80.115 (±13.63) 80.56 (±11.39) 0.5405

By two way ANOVA test; * Very highly significant


Measurements were taken following the WHO criteria which include:
  1. Patient was seated with his arm supported and at the heart level.
  2. Measurements were taken after 5 minutes of rest.
  3. The appropriate cuff size was used.
  4. The sphygmomanometer was recently calibrated.
  5. Both systolic and diastolic blood pressures were recorded and disappearance of sounds (phase V) used for diastolic reading.

The readings obtained by different observer were compared for the variability in readings and its statistical significance studied by use of the “two way analysis of variance”.

Results

Two hundred patients were randomly selected for the study, of whom 170 were males and 30 were females. Variation of systolic and diastolic blood pressure by the 5 observers was separately analysed. Systolic blood pressure shows statistically significant variability among observers (p<0.001). Diastolic blood pressure also shows variability, but it is statistically insignificant (Table 1).

Table II. Comparison of variability among different observer- Systolic BP

Comparison between Mean Diff t P value 95% CI of diff
Physician vs. PG 1.605 1.556 P > 0.05 -1.298 to 4.508
Physician vs. intern 3.255 3.156 P < 0.05 0.3521 to 6.158
Physician vs. student 3.985 3.864 P<0.01 1.082 to 6.888
Physician vs. nurse 3.255 3.156 P < 0.05 0.3521 to 6.158
PG vs. intern 1.650 1.600 P > 0.05 -1.253 to 4.553
PG vs. student 2.380 2.308 P > 0.05 -0.5229 to 5.283
PG vs. nurse 1.650 1.600 P > 0.05 -1.253 to 4.553
Intern vs. student 0.7300 0.7079 P > 0.05 -2.173 to 3.633
Intern vs. nurse 0.0000 0.0000 P > 0.05 -2.903 to 2.903
Student vs. nurse 0.7300 0.7079 P > 0.05 -3.633 to 2.173

By ANOVA followed by multiple comparison test; CI=Confidence interval

Table 2 shows comparison of variability in systolic BP recording among different observers. Statistically significant difference was obtained between physician and postgraduate, physician and student, physician and nurse (P<0.05).

Analysis was also done separately for males and females. Observations were similar but with a greater variability in the systolic blood pressure (Table 3). We could not find any association between observer’s sex and patient’s blood pressure. (Female observer Vs male patient blood pressure studied).

Among the 200 patients 30 were known hypertensives and 170 were normotensives. Variability of blood pressure is more with systolic blood pressure in both the groups and there was no significant increase in variability among hypertensives (Table 4). Two way Anova was used to check the variation between the observers.

Table III. Blood pressure variability in males and females.

Blood Pressure Males Females
F value P value F value P value
Systolic 4.3217 0.0127* 3.4395 0.0107*
Diastolic 3.2138 0.0234* 2.5815 0.048*

Table IV. Blood pressure variability in normotensives and hypertensives.

Blood Pressure Normotensives Hypertensives
F value P value F value P value
Systolic 3.0069 <0.05* 2.8924 <0.05*
Diastolic 0.3226 0.8629 0.684 0.5432
* Significant

Discussion

Direct measurement of blood pressure during routine clinical evaluation of a patient is not practical in the usual office or clinical settings. Thus the physician must rely on indirect assessment of blood pressure for diagnosis of hypertension. The standard method is casual office measurement of blood pressure using a sphygmomanometer and stethoscope.

Our study shows that the usual procedure of measuring blood pressure with sphygmomanometer is fraught with considerable inter-observer variations. This variation is more statistically significant in systolic blood pressure than in the diastolic blood pressure. However, since the variations were only around up to 4mm, these cannot be considered clinically significant.

The rise in blood pressure was not the consequence of cuff application or inflation. It usually began when the doctor appeared at the patient’s bedside well before the procedure for blood pressure measurements was initiated. Doctor’s appearance at the patient’s bedside itself may create an alarm reaction in the patient that will result in a rise of blood pressure. This can lead to over estimation of blood pressure and over diagnosis of hypertension with unnecessary medication.3

The 24-hour variability in systolic, diastolic and mean blood pressure was studied in 89 subjects by Girseppe Mancia5 and his colleagues. The inconsistency was significantly greater for systolic than for the diastolic blood pressure. Litter6 also reported significantly higher variation in systolic than in diastolic blood pressure. Litter’s study also suggested that the higher the systolic blood pressure, the greater was the variability. This phenomenon may reflect the poorer baroreflex control found in hypertension7. But in our study, we could not find a statistically significant variation in either systolic or diastolic blood pressure among the hypertensives when compared to the normotensives. Our study population had only 30 hypertensives (out of 200 patients), which could have been the reason for this nonsignificance.

A recent report of Framingham study appears to confirm the importance of systolic hypertension as a risk factor in cardiovascular disease, systolic pressure being a stronger determinant for the risk of coronary heart disease than either diastolic or mean blood pressure in women and in men over 45 years of age. This stresses the importance of accurate determination of blood pressure especially systolic blood pressure.

Armitage and Rose8, in a study of normotensive subjects who had repeated blood pressure measurements taken by a single trained observer over a 6 week period, demonstrated a highly significant difference in blood pressure both at each visit and between visits.

M Carry and colleagues9 studied a large population in Virginia, USA; those found to have high blood pressure in the initial survey had repeated blood pressure measurements at a specialized centre. It was found that the prevalence of sustained hypertension decreased from 54% to only 9%.

To conclude, there is a significant variability of blood pressure among different observers. Physician-measured blood pressure was higher in our study, probably because of the anxiety produced in the patient when a physician measures blood pressure. This illustrates the importance of repeated measurement by the physician, after the patient is sufficiently relaxed and comfortable, before labelling him as hypertensive.

References

  1. Peter B, Albert A, Robert BR, Michal P. Variability of indirect methods used to determine blood pressure. Arch. Intern. Med. 1992; 152; 139-44.
  2. Giuseppe M. Guido G, Guild P et al. Effects of blood pressure measurements by the doctor on blood pressure and heart rate. Lancet. 1983; 695-7.
  3. Thomas GP. Blood pressure measurements and detection of hypertension. Lancet 1994; 344; 31-4.
  4. Petrie JC. Brien ET, Litter WA, Swiet MD. Recommendations on blood pressure measurement. Br. Med. J. 1986; 293; 611-4.
  5. Giuseppe Mancia, Albert F, Luisa G et al. Blood pressure and heart rate variabilities in normotensive and hypertensive human being. Circ. Res. 1983; 53; 96-104.
  6. Litter WA, West MJ, Honour AJ, Sleight P. The variability of arterial pressure. Am. Heart J. 1978; 95; 180-5.
  7. Giuseppe M. Blood pressure variability at normal and high blood pressure. Chest. 1963; 82; 317-21.
  8. Armitage P, Rose GA. The variability of measurements of casual blood pressure. Clin. Sci. 1966; 30; 325-35.
  9. Robert MCarry, Robert AR, Carlor et al. The Charlottesville blood pressure survey value of repeated blood pressure measurements. JAMA, 1996; 236; 847-51.
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