Method for controlling variables:
- Environmental conditions:
- The aim to control the environmental conditions is to carry out the tests under similar weather conditions. The experimental tests were also run at roughly the same time of day so as not to disadvantage the participants.
- Testing Procedures/Method and Equipment used
- The same method will be followed and same equipment used for each test so that the errors can be minimised. The positioning of the blood pressure device was positioned by the same person, so as to minimise the errors involved with these measurements.
- All of the participants chosen were 16 years old.
- All of the participants chosen were female.
- All of the participants chosen were non-diabetic.
Data Collection:
In every table and graph present in this assignment, the values for Systolic and Diastolic Pressures have been grouped according to the position of the arm – Above Heart, In Line With Heart and Below Heart – to make it easier for interpretation and analysis.
Table 1: The Systolic (SYS) and Diastolic (DIA) Pressures (mmHg) of the subjects’ for varying positions of the monitored arm
Uncertainties: Pressure +/- 3mmHg
Table 2: Qualitative Observations from the experiment trials
Data Processing and Presentation:
Table 3: The Total Average Systolic (SYS) and Diastolic (DIA) Pressures (mmHg) for the respected positions of the monitored arm in relation to the heart
Uncertainties: Pressure +/- 3mmHg
For the following tables and graphs, the average systolic and diastolic pressure (mmHg) values for each of the three positions of the monitored arm have been grouped by these positions and the total average for each position been calculated. This was done so that the tables and graphs can really depict the total averages for the positions of the arm – and thus the resulting difference.
Table 4: The Total Average Systolic (SYS) and Diastolic (DIA) Pressures (mmHg) for the respected positions of the monitored arm in relation to the heart grouped by pressure
Uncertainties: Pressure +/- 3mmHg
Calculations for Descriptive Statistics:
Mean – the average of all data entries
Median – the middle value when data entries are placed in rank order
Variance – A measure of the spread of the values in a distribution. The larger the variance, the larger the distance of the individual cases from the group mean.
The formula says to take a score (X) and subtract the mean (μ), then square this difference (X-μ)2 and sum up all of these squared differences (∑(X- μ)2), and divide the sum by the number of scores(N).
Standard Deviation – a frequently used measure of the variability (spread) in a set of data
Standard Error – an error measurement (reliability of your data)
The 95% Confidence Interval – used to calculate and reveal on average, 95 times out of 100, the limits which usually contain the true mean
P(n – 1) = the value of t at P = 0.05 (from the Critical t-Table) for the appropriate degrees of freedom (df) for your sample (n – 1)
The Confidence Interval reveals the limits which are an interval estimate for the mean. The Confidence Interval combines both the upper and lower limits and gives an indication of how much uncertainty there is in the estimate of the true mean. The narrower the interval, the more precise is the estimate. (adapted from: NIST/SEMATECH e-Handbook of Statistical Methods, http://www.itl.nist.gov/div898/handbook/, 5/05/08.)
Table 4: Descriptive Statistics for the Total Average Systolic (SYS) and Diastolic (DIA) Pressures (mmHg) for the respected positions of the monitored arm in relation to the heart
Uncertainties: Pressure +/- 3mmHg
Table 5: Graphed results for the average Systolic Pressure (SYS mmHg) for the specific positions of the monitored arm in relation to the heart
Uncertainties: Pressure +/- 3mmHg
Graph 2: The Average Systolic Pressure (SYS mmHg) for the specific positions of the monitored arm in relation to the heart
Uncertainties: Pressure +/- 3mmHg
The following graph was created as it clearly illustrates how the different positioning of the monitored arm, affects the systolic pressure (mmHg) readings.
Graph 3: Total Average Systolic Pressure (mmHg) for various positions of the monitored arm in relation to the heart
Uncertainties: Pressure +/- 3mmHg
Table 6: Graphed results for the average Diastolic Pressure (DIA mmHg) for the specific positions of the monitored arm in relation to the heart
Uncertainties: Pressure +/- 3mmHg
Graph 4: The Average Diastolic Pressure DIA (mmHg) for the specific positions of the monitored arm in relation to the heart
Uncertainties: Pressure +/- 3mmHg
The following graph was created as it clearly illustrates how the different positioning of the monitored arm, affects the diastolic pressure (mmHg) readings.
Graph 5: Total Average Diastolic Pressure (mmHg) for various positions of the monitored arm in relation to the heart
Uncertainties: Pressure +/- 3mmHg
Discussion of error bars and descriptive statistics:
As is illustrated by the error bars in Graph 15, the total average Systolic pressure (mmHg) involved very small errors. This is because the values for Systolic Pressure (mmHg) are fairly consistent and there was no significant difference in the values for Systolic Pressure (mmHg).
The calculated values of these errors can be found in Table 15. Under the heading of ‘95% CI’ are the values for the 95% Confidence Interval which is used to calculate and reveal on average – 95 times out of 100 – the limits which will contain the true mean. As can be seen in Graph 15, all errors are relatively small and this can just be attributed to the consistency of the Systolic Pressure (mmHg) values.
As can be seen in Graph 15, there is again relatively small error bars for each position of the monitored arm for the total average Diastolic Pressure values. This can again be attributed to the consistency of the results for Diastolic Pressure (mmHg). The 95% confidence intervals for the Diastolic Pressure (mmHg) at various positions of the monitored arm were consistently very low.
The error bars illustrate the confidence intervals of that group of data. Each of the error bars for these results, are fairly narrow and therefore the estimate of the mean is relatively precise. As the error bars increase in size, this means that the estimate is not as precise as that with narrow error bars.
Table 7: Calculated Difference between positions of the monitored arm for both Systolic and Diastolic Pressure (mmHg)
Uncertainties: Pressure +/- 3mmHg
Graph 6: Calculated Difference for systolic and diastolic pressure (mmHg) values between various positions of the monitored arm
Uncertainties: Pressure +/- 3mmHg
CONCLUSION AND EVALUATION
This experiment set out to investigate the effect of the position of the monitored arm on the blood pressure readings of non-diabetic, female, 16 year olds, in terms of their Systolic and Diastolic Pressure (mmHg).
‘Blood pressure readings may be influenced by body position because of variation in the vertical distance between heart and cuff level.’ (CAVELAARS Marinel; et al. – 2000) As the results show, it was found that both the systolic and diastolic pressure values did change with the position of the monitor.
The values in Table 4 clearly illustrate the respective increase and decrease of blood pressure with respect to the position of the monitored arm and its position in relation to the heart
‘When the blood pressure (BP) is measured, the arm should be at the level of the heart.’ (STEPHEN S.EHRLICH, M.D., - 2004) and that the values thus produced will be the true blood pressure values as this was the controlled and recommended position for blood pressure measurement. From the results of this experiment – as clearly illustrated by graphs 3 and 5 – it can be seen that the blood pressure readings taken in the correct position i.e. with the monitored arm at heart level, the average blood pressure values fell into the middle of the range.
It had been hypothesised the systolic and diastolic pressure values would be lower than the control when the monitored arm was held directly above the head of the subject. This was expected because, past studies into pulse oxygenation had shown that ‘by merely changing the position of the monitored extremity, the SpO2 can change by up to six percent and in this study, the arm was raised 90 degrees from surpine and this resulted in a highly significant decrease in SpO2 with elevation of the monitored arm.’ (Cooke, J., Johansen, J., 2000) As graphs 3 and 5 show, the systolic and diastolic pressures for when the monitored arm was held directly above the head of the subject, were lower than the systolic and diastolic pressure values in the ‘control’ arm position.
It had also been hypothesised that the systolic and diastolic pressure values would be higher than the control when the monitored arm was held parallel to the body – down by the side of the subject. This hypothesis was in line with literature that stated that ‘The systolic and diastolic blood pressure measured with the arm perpendicular to the body was significantly lower than with the arm in a parallel position.’ (UCSD Medical Researchers - 2004) As graphs 3 and 5 illustrate, the systolic and diastolic pressure values were higher significantly higher when the monitored arm was held down by the side of the subject than the systolic and diastolic pressure values of the control.
The differences between the various positions of the monitored arm – in terms of both Systolic and Diastolic Pressure (mmHg) – are presented in Table 6 and Graph 7. These tables and graphs highlight that there are significant consequences of incorrectly positioning and measuring blood pressure. They also show the size of the difference between the various positions in terms of the blood pressure values. The differences for both systolic and diastolic pressure (mmHg) are relatively small between the values for positions of directly above the heart and at heart level. This is because there was only a small difference between the means of these grouped values. These differences are also negative as they are below the values of the control. The larger differences are between the values for the positions of down by side and at heart level for both systolic and diastolic pressure (mmHg). This large difference means that these values are significantly different to each other. These differences further highlight the effect that the position of the monitored arm in relation to the heart, has on the blood pressure readings.
It can be concluded that the results from this experiment do support the original hypothesis: that blood pressure in terms of systolic pressure and diastolic pressure (mmHg) are influenced by body position because of the vertical distance between the heart and the cuff level. The results showed that when the cuff was held directly above the head, it produced the lowest blood pressure values; that when the cuff was held in line with the heart (control) – as is the correct way to monitor blood pressure – it produced the middle value; and that when the cuff is parallel to the body and resting by the side, it results in the highest blood pressure reading.
It can be concluded from these results that to obtain accurate and consistent blood pressure readings, it is important to implement the proper arm position – with the monitored arm perpendicular to the body, with a slightly flexed elbow – to correctly determine blood pressure levels with a wrist pressure monitor.
APPENDIX
CASCADE health
Wrist type
Blood Pressure Monitor
Model CE1541BM
Method of Measurement: Oscillometric
Range of measurement: pressure 0~200mmHg, pulse 40~199beats/minute
Pressure +/- 3mmHg, Pulse +/- 5%
WORKS CITED
CAVELAARS M., TULEN J. H. M., et al. “Assessment of body position to quantify its effect on nocturnal blood pressure under ambulatory conditions.” Journal of hypertension.
vol. 18, no12, (2000) pp. 1737-1743 (29 ref.) < http://cat.inist.fr/?aModele=afficheN&cpsidt=825348>
Emory School of Medicine – Cooke J., Johansen, J., ‘Pulse Oxygenation Falls with Arm Position Change’, (Online), 14 April, 2008 <http://www.econmed.com/pulseox/docs/STA-cooke-armraise.pdf>.
University Of California - San Diego. "Arm Position Matters In Blood Pressure Readings According To UCSD Medical Researchers." ScienceDaily 8 January 2004. 14 April 2008 <http://www.sciencedaily.com /releases/2004/01/040108075421.htm>.
The John Hopkins Hospital and Health System ‘Heart and Circulatory System Glossary’ (Online), 14 April, 2008 <>.
NIST/SEMATECH e-Handbook of Statistical Methods, http://www.itl.nist.gov/div898/handbook/, 5/05/08.
Blood pressure is a measurement of the force of blood flowing against the walls of arteries.
The force of blood against the walls of the arteries when the heart contracts to pump blood to the rest of the body.
The pressure within the arteries as the heart relaxes and refills with blood (which explains why the diastolic number is always lower than the systolic measurement). ()
Cooke, J., Johansen, J., ‘Pulse Oxygenation Falls with Arm Position Change’, (Online),
(14 April, 2008)
< http://www.mnstate.edu/wasson/ed602calcvardevs.htm>