# Can Photoplethysmography Replace Arterial Blood Pressure in the Assessment of Blood Pressure?

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## Abstract

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## 1. Introduction

## 2. Methods

#### 2.1. Hypotheses

- H1: If the PPG amplitude (Linear time domain analysis) is correlated with the ABP amplitude, then the PPG amplitude can replace ABP for measuring BP.
- H2: If the PPG morphology (Linear time domain analysis) is correlated with the ABP morphology, then the PPG waveform morphology holds valuable information that can be used for evaluating BP.
- H3: If the PPG waveform and the ABP have mutual information and coherence (Nonlinear dependency analysis), then the PPG waveform morphology holds valuable information that can be used for evaluating BP.

#### 2.2. In-Phase Analysis

#### 2.3. Out-Phase Analysis

#### 2.4. Statistical Analysis

#### 2.5. Classification Analysis

## 3. Results and Discussion

## 4. Limitation of Study and Future Work

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Simultaneously collected Arterial Blood Pressure (ABP) and Photoplethysmogram (PPG) signals. Note, ABP was measured invasively while PPG was measured non-invasively.

**Figure 2.**Results of in-phase analysis: (

**a**) Analysis results for the group with an amplitude range of 0.6–0.75 of the PPG signal; and (

**b**) analysis results for the group with an amplitude range of 2.2–3 of the PPG signal. The groups are derived from the different gains of the equipment with which the signals were registered. r refers to the morphology correlation between photoplethysogram (PPG) and arterial blood pressure (ABP) waveforms.

**Figure 3.**Examples of in-phase analysis. Note, r refers to the morphology correlation between photoplethysogram (PPG) and arterial blood pressure (ABP) waveforms.

**Figure 4.**Frequency significance in differentiation blood pressure groups: (

**a**) COH $and$ pCOH (

**b**) PDC direction ABP→PPG; and (

**c**) DTF direction ABP→PPG. Abbreviations: NT, normotensive; PHT, Prehypertensive; HT, Hypertensive; COH, Coherence; pCOH, Partial coherence; PDC, Partial directed coherence; DTF, Directed transfer function; dDTF, Direct directed transfer function; ffDTF, Full frequency directed transfer function.

**Figure 5.**Neural networks training, testing, and validation using all causality measures: (

**a**) confusion plot; and (

**b**) receiver operating characteristic (ROC) curve.

**Figure 6.**Examples of different causality measures for normotensive and hypertensive cases. Abbreviations: COH, Coherence; pCOH, Partial coherence; PDC, Partial directed coherence; DTF, Directed transfer function; dDTF, Direct directed transfer function; ffDTF, Full frequency directed transfer function.

**Table 1.**In-phase correlation between photoplthysmogram (PPG) and arterial blood pressure (ABP) signals.

Normotensive | Prehypertensive | Hypertensive | |||
---|---|---|---|---|---|

Subject ID | r | Subject ID | r | Subject ID | r |

’11727_2’ | 0.9073 | ’12531_2’ | 0.8385 | ’10464_2’ | 0.9327 |

’12174_2’ | 0.9142 | ’13600_2’ | 0.916 | ’11187_2’ | 0.906 |

’17848_2’ | 0.9676 | ’15218_2’ | 0.9797 | ’1501_2’ | 0.9023 |

’19700_2’ | 0.951 | ’15716_2’ | 0.9286 | ’15902_2’ | 0.9696 |

’2104_2’ | 0.9459 | ’16129_2’ | 0.9557 | ’18642_2’ | 0.9448 |

’2513_2’ | 0.9201 | ’18970_2’ | 0.9025 | ’19578_2’ | 0.9492 |

’27436_2’ | 0.9806 | ’21730_2’ | 0.7777 | ’20726_2’ | 0.9109 |

’27648_2’ | 0.9689 | ’26897_2’ | 0.9317 | ’22335_2’ | 0.947 |

’27833_2’ | 0.9354 | ’27241_2’ | 0.9359 | ’23201_2’ | 0.9558 |

’27887_2’ | 0.802 | ’27337_2’ | 0.9384 | ’2458_2’ | 0.9744 |

’28077_2’ | 0.9362 | ’27434_2’ | 0.8639 | ’27446_2’ | 0.7908 |

’28187_2’ | 0.9633 | ’27845_2’ | 0.9413 | ’28499_2’ | 0.9787 |

’28813_2’ | 0.9802 | ’28758_2’ | 0.8939 | ’28510_2’ | 0.921 |

’28910_2’ | 0.9699 | ’28882_2’ | 0.9833 | ’28775_2’ | 0.8816 |

’29102_2’ | 0.9178 | ’44088_2’ | 0.8657 | ’29127_2’ | 0.978 |

’29120_2’ | 0.9382 | ’44104_2’ | 0.9413 | ’44118_2’ | 0.9415 |

’3039_2’ | 0.9481 | ’44201_2’ | 0.9773 | ’44171_2’ | 0.9377 |

’44223_2’ | 0.9533 | ’44233_2’ | 0.9592 | ’44173_2’ | 0.969 |

’44409_2’ | 0.9441 | ’44458_2’ | 0.8442 | ’44347_2’ | 0.8966 |

’44422_2’ | 0.9672 | ’44496_2’ | 0.9571 | ’44572_2’ | 0.93 |

’44432_2’ | 0.956 | ’44590_2’ | 0.9745 | ’44615_2’ | 0.8795 |

’44526_2’ | 0.8325 | ’44623_2’ | 0.945 | ’44616_2’ | 0.9505 |

’44598_2’ | 0.9419 | ’44640_2’ | 0.9663 | ’44626_2’ | 0.9526 |

’44601_2’ | 0.9499 | ’44647_2’ | 0.9771 | ’44704_2’ | 0.9689 |

’44629_2’ | 0.9757 | ’44902_2’ | 0.9434 | ’44839_2’ | 0.9742 |

’44671_2’ | 0.9612 | ’45181_2’ | 0.8832 | ’44981_2’ | 0.9583 |

’44758_2’ | 0.9754 | ’45384_2’ | 0.9142 | ’45098_2’ | 0.9663 |

’44763_2’ | 0.9261 | ’45426_2’ | 0.8659 | ’45140_2’ | 0.9561 |

’44783_2’ | 0.9333 | ’45533_2’ | 0.9798 | ’45212_2’ | 0.9223 |

’44810_2’ | 0.8872 | ’45572_2’ | 0.8721 | ’45227_2’ | 0.8929 |

’45049_2’ | 0.9529 | ’45636_2’ | 0.9239 | ’45550_2’ | 0.9355 |

’45186_2’ | 0.9373 | ’45641_2’ | 0.9278 | ’45627_2’ | 0.902 |

’45311_2’ | 0.9615 | ’46138_2’ | 0.9157 | ’46216_2’ | 0.8924 |

’45343_2’ | 0.9688 | ’46297_2’ | 0.8783 | ’46303_2’ | 0.9499 |

’45353_2’ | 0.8794 | ’46416_2’ | 0.7969 | ’801_2’ | 0.9454 |

’45487_2’ | 0.9774 | ’4679_2’ | 0.849 | ’8141_2’ | 0.9484 |

’45556_2’ | 0.9586 | ’6581_2’ | 0.9256 | ’8318_2’ | 0.871 |

’45645_2’ | 0.9703 | ’6692_2’ | 0.9332 | ||

’46122_2’ | 0.981 | ’7614_2’ | 0.908 | ||

’46230_2’ | 0.9661 | ’9124_2’ | 0.9677 | ||

’46424_2’ | 0.9086 | ||||

’5937_2’ | 0.9206 | ||||

’946_2’ | 0.9474 | ||||

Average | 0.9414 | 0.917 | 0.932 |

**Table 2.**Out-phase statistical separability results. Abbreviations: NT, normotensive; PHT, Prehypertensive; HT, Hypertensive; r, Pearson’s correlation coefficient; COH, Coherence; pCOH, Partial coherence; PDC, Partial directed coherence; DTF, Directed transfer function; dDTF, Direct directed transfer function; ffDTF, Full frequency directed transfer function; K-W test, Kruskal–Wallis test.

Time Domain | NT vs. PHT | NT vs. HT | PHT vs. HT | p-Value (K–W Test)NT vs. PHT vs. HT |

$\mathit{p}$-value (Wilcoxon Test) | ||||

r | 0.4466 | 0.4552 | 0.9600 | 0.6726 |

Frequency domain | $\mathit{p}$-value (Wilcoxon Test) | $\mathit{p}$-value (K–W test)NT vs. PHT vs. HT | ||

COH & pCOH | 0.2355 | 0.0069 | 0.1630 | 0.0281 |

PPG → ABP | $\mathit{p}$-value (Wilcoxon Test) | $\mathit{p}$-value (K–W test)NT vs. PHT vs. HT | ||

PDC | 0.6258 | 0.1861 | 0.4536 | 0.4248 |

DTF | 0.5324 | 0.2061 | 0.4475 | 0.4219 |

ffDTF | 0.2527 | 0.2928 | 0.9715 | 0.4352 |

dDTF | 0.2681 | 0.2973 | 0.9552 | 0.4506 |

ABP → PPG | $\mathit{p}$-value (Wilcoxon Test) | $\mathit{p}$-value (K–W test)NT vs. PHT vs. HT | ||

PDC | 0.0264 | 0.0102 | 0.7250 | 0.0188 |

DTF | 0.1788 | 0.0479 | 0.5995 | 0.1304 |

ffDTF | 0.0762 | 0.0022 | 0.1128 | 0.0064 |

dDTF | 0.0718 | 0.0020 | 0.1249 | 0.0061 |

**Table 3.**Classification accuracy results. Abbreviations: COH, Coherence; pCOH, Partial coherence; PDC, Partial directed coherence; DTF, Directed transfer function; dDTF, Direct directed transfer function; ffDTF, Full frequency directed transfer function.

# | Classifier | COH | PDC | DTF | ffDTF | dDTF | All Features |
---|---|---|---|---|---|---|---|

1.1 | Linear | 66.3% | 51.3% | 53.8% | 65.0% | 62.5% | 70.0% |

1.2 | Diaglinear | 66.3% | 51.3% | 53.8% | 65.0% | 62.5% | 70.0% |

1.3 | Quadratic | 60.0% | 52.5% | 50.0% | 62.5% | 62.5% | 58.8% |

1.4 | Diagquadratic | 60.0% | 52.5% | 50.0% | 62.5% | 62.5% | 65.0% |

1.5 | Mahalanobis | 70.0% | 52.5% | 48.8% | 61.3% | 61.3% | 65.0% |

1.6 | SVM | 66.3% | 55.0% | 53.7% | 56.2% | 56.2% | 67.5% |

1.7 | KNN | 63.8% | 45.0% | 56.2% | 47.5% | 47.5% | 71.3% |

1.8 | Tree | 71.3% | 57.5% | 65.0% | 52.5% | 57.5% | 60.0% |

1.9 | Naive Bayes | 62.5% | 52.5% | 50.0% | 62.5% | 62.5% | 65.0% |

1.10 | Ecoc | 53.7% | 53.7% | 53.7% | 53.7% | 53.7% | 60.0% |

1.11 | Esemble | 63.8% | 51.2% | 50.0% | 53.7% | 56.2% | 62.5% |

1.12 | Two-layer feed forward Neural Network | 77.5% | 66.3% | 63.7% | 78.8% | 67.5% | 87.5% |

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**MDPI and ACS Style**

Martínez, G.; Howard, N.; Abbott, D.; Lim, K.; Ward, R.; Elgendi, M.
Can Photoplethysmography Replace Arterial Blood Pressure in the Assessment of Blood Pressure? *J. Clin. Med.* **2018**, *7*, 316.
https://doi.org/10.3390/jcm7100316

**AMA Style**

Martínez G, Howard N, Abbott D, Lim K, Ward R, Elgendi M.
Can Photoplethysmography Replace Arterial Blood Pressure in the Assessment of Blood Pressure? *Journal of Clinical Medicine*. 2018; 7(10):316.
https://doi.org/10.3390/jcm7100316

**Chicago/Turabian Style**

Martínez, Gloria, Newton Howard, Derek Abbott, Kenneth Lim, Rabab Ward, and Mohamed Elgendi.
2018. "Can Photoplethysmography Replace Arterial Blood Pressure in the Assessment of Blood Pressure?" *Journal of Clinical Medicine* 7, no. 10: 316.
https://doi.org/10.3390/jcm7100316