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Quantitative Assessment of the Impact of Blood Pulsation on Intraocular Pressure Measurement Results in Healthy Subjects

View Article: PubMed Central - PubMed

ABSTRACT

Background. Blood pulsation affects the results obtained using various medical devices in many different ways. Method. The paper proves the effect of blood pulsation on intraocular pressure measurements. Six measurements for each of the 10 healthy subjects were performed in various phases of blood pulsation. A total of 8400 corneal deformation images were recorded. The results of intraocular pressure measurements were related to the results of heartbeat phases measured with a pulse oximeter placed on the index finger of the subject's left hand. Results. The correlation between the heartbeat phase measured with a pulse oximeter and intraocular pressure is 0.69 ± 0.26 (p < 0.05). The phase shift calculated for the maximum correlation is equal to 60 ± 40° (p < 0.05). When the moment of measuring intraocular pressure with an air-puff tonometer is not synchronized, the changes in IOP for the analysed group of subjects can vary in the range of ±2.31 mmHg (p < 0.3). Conclusions. Blood pulsation has a statistically significant effect on the results of intraocular pressure measurement. For this reason, in modern ophthalmic devices, the measurement should be synchronized with the heartbeat phases. The paper proposes an additional method for synchronizing the time of pressure measurement with the blood pulsation phase.

No MeSH data available.


Graph of changes in mean correlation rv(j = 5) of the feature wk,v(j = 5) for individual subjects (after removing thick errors, outliers) as a function of phase shift ϕ. Additionally, the graph shows the position of the minimum and maximum IOP values relative to the blood pulsation phase. Minimum and maximum values of IOP are marked with red arrows. Blue lines on the x-axis indicate the arbitrarily adopted measurement times.
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fig4: Graph of changes in mean correlation rv(j = 5) of the feature wk,v(j = 5) for individual subjects (after removing thick errors, outliers) as a function of phase shift ϕ. Additionally, the graph shows the position of the minimum and maximum IOP values relative to the blood pulsation phase. Minimum and maximum values of IOP are marked with red arrows. Blue lines on the x-axis indicate the arbitrarily adopted measurement times.

Mentions: The natural phase shift in blood pulsation between the heart, the index finger (the measurement point), and the eye depends on the distance and subject's anatomical features. For the analysed data, correlation is determined according to formula (1) for subsequent, artificially added, phase shifts ϕ of the feature wk,v(j). The results of changes in correlation for the artificially added phase shifts ϕ are shown in Figure 3. The values of phase shift ϕ for which the correlation rv(j) reaches the maximum and minimum values are given in Table 3 for the feature w(5)  (j = 5). Further calculations were performed under the assumption of the hypothesis Ho that there is a statistical relationship between the features w(1) to w(5) and w(6) and under the assumption of the alternative hypothesis H1 that this relationship does not exist. The calculated correlation for all the analysed subjects indicates statistical significance (p < 0.05 for Student's t-distribution) only for the feature w(5). Therefore, there is a significant correlation between the intraocular pressure and heartbeat phase (between the features w(5) and w(6)). For the measured group of subjects, this correlation is high (it is in the range from 0.48 to 0.99; the measurement for v = 6 was considered a thick error and rejected) and its mean value is 0.78 ± 0.19 (see Table 3). The results presented in Table 3 for the features w(5) and w(6) are extremely important in practice. Low values of mean std of changes in the features from w(1) to w(4) can here result from two elements. The first one is the limited resolution of the analysed image. For the image resolution M × N = 200 × 576 pixels, there is, on average, 20 μm per one pixel. This means that the accuracy of measuring the feature w(1) as well as features w(2), w(3), and w(4) is limited to the resolution of ±20 μm. So, if, for example, the amplitude for the first applanation changes by less than 20 μm for the next i frames (images in a sequence), the measurement error of the feature w(1) will be 231 μs. The other element influencing the low values of mean std of changes in the features from w(1) to w(4) is the lack of correlation with the heartbeat phases. For subjects v = {1,2, 3,4, 5,7, 8,9, 10}, there was a close relationship between the heartbeat phase and IOP. According to the diagram shown in Figure 3, the highest values of IOP are obtained for the phase shift of 60°. Minimum IOP values are obtained for the phase shift of 240° (60° + 180°); see Figure 4. The correlation for these angular values is 0.69 ± 0.26 (p < 0.05). The changes in mean correlation rv(j = 5) for individual subjects (after removing thick errors, outliers) as a function of phase shift ϕ presented in Figure 4 clearly indicate a strong correlation (0.69 ± 0.26) between IOP and the heartbeat phase.


Quantitative Assessment of the Impact of Blood Pulsation on Intraocular Pressure Measurement Results in Healthy Subjects
Graph of changes in mean correlation rv(j = 5) of the feature wk,v(j = 5) for individual subjects (after removing thick errors, outliers) as a function of phase shift ϕ. Additionally, the graph shows the position of the minimum and maximum IOP values relative to the blood pulsation phase. Minimum and maximum values of IOP are marked with red arrows. Blue lines on the x-axis indicate the arbitrarily adopted measurement times.
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fig4: Graph of changes in mean correlation rv(j = 5) of the feature wk,v(j = 5) for individual subjects (after removing thick errors, outliers) as a function of phase shift ϕ. Additionally, the graph shows the position of the minimum and maximum IOP values relative to the blood pulsation phase. Minimum and maximum values of IOP are marked with red arrows. Blue lines on the x-axis indicate the arbitrarily adopted measurement times.
Mentions: The natural phase shift in blood pulsation between the heart, the index finger (the measurement point), and the eye depends on the distance and subject's anatomical features. For the analysed data, correlation is determined according to formula (1) for subsequent, artificially added, phase shifts ϕ of the feature wk,v(j). The results of changes in correlation for the artificially added phase shifts ϕ are shown in Figure 3. The values of phase shift ϕ for which the correlation rv(j) reaches the maximum and minimum values are given in Table 3 for the feature w(5)  (j = 5). Further calculations were performed under the assumption of the hypothesis Ho that there is a statistical relationship between the features w(1) to w(5) and w(6) and under the assumption of the alternative hypothesis H1 that this relationship does not exist. The calculated correlation for all the analysed subjects indicates statistical significance (p < 0.05 for Student's t-distribution) only for the feature w(5). Therefore, there is a significant correlation between the intraocular pressure and heartbeat phase (between the features w(5) and w(6)). For the measured group of subjects, this correlation is high (it is in the range from 0.48 to 0.99; the measurement for v = 6 was considered a thick error and rejected) and its mean value is 0.78 ± 0.19 (see Table 3). The results presented in Table 3 for the features w(5) and w(6) are extremely important in practice. Low values of mean std of changes in the features from w(1) to w(4) can here result from two elements. The first one is the limited resolution of the analysed image. For the image resolution M × N = 200 × 576 pixels, there is, on average, 20 μm per one pixel. This means that the accuracy of measuring the feature w(1) as well as features w(2), w(3), and w(4) is limited to the resolution of ±20 μm. So, if, for example, the amplitude for the first applanation changes by less than 20 μm for the next i frames (images in a sequence), the measurement error of the feature w(1) will be 231 μs. The other element influencing the low values of mean std of changes in the features from w(1) to w(4) is the lack of correlation with the heartbeat phases. For subjects v = {1,2, 3,4, 5,7, 8,9, 10}, there was a close relationship between the heartbeat phase and IOP. According to the diagram shown in Figure 3, the highest values of IOP are obtained for the phase shift of 60°. Minimum IOP values are obtained for the phase shift of 240° (60° + 180°); see Figure 4. The correlation for these angular values is 0.69 ± 0.26 (p < 0.05). The changes in mean correlation rv(j = 5) for individual subjects (after removing thick errors, outliers) as a function of phase shift ϕ presented in Figure 4 clearly indicate a strong correlation (0.69 ± 0.26) between IOP and the heartbeat phase.

View Article: PubMed Central - PubMed

ABSTRACT

Background. Blood pulsation affects the results obtained using various medical devices in many different ways. Method. The paper proves the effect of blood pulsation on intraocular pressure measurements. Six measurements for each of the 10 healthy subjects were performed in various phases of blood pulsation. A total of 8400 corneal deformation images were recorded. The results of intraocular pressure measurements were related to the results of heartbeat phases measured with a pulse oximeter placed on the index finger of the subject's left hand. Results. The correlation between the heartbeat phase measured with a pulse oximeter and intraocular pressure is 0.69 &plusmn; 0.26 (p &lt; 0.05). The phase shift calculated for the maximum correlation is equal to 60 &plusmn; 40&deg; (p &lt; 0.05). When the moment of measuring intraocular pressure with an air-puff tonometer is not synchronized, the changes in IOP for the analysed group of subjects can vary in the range of &plusmn;2.31&thinsp;mmHg (p &lt; 0.3). Conclusions. Blood pulsation has a statistically significant effect on the results of intraocular pressure measurement. For this reason, in modern ophthalmic devices, the measurement should be synchronized with the heartbeat phases. The paper proposes an additional method for synchronizing the time of pressure measurement with the blood pulsation phase.

No MeSH data available.