The PUlse oximetry in Limb SalvagE (The PULSE Study) was a proof-of-concept study. The aim of the study was to investigate how the pulse oximeter trace, the perfusion index and oxygen saturations varied with tourniquet inflation. The PULSE Study was approved by Northwest/Haydock national research committee. IRAS project ID: 269444, protocol number JS 442219, REC reference 19/NW/0528, Sponsor UHCW.
The clinical scenario of reduced blood flow to the limb was simulated by applying and inflating a tourniquet (Brand Name: ANETIC AID AT4 Electronic Tourniquet System 40080, Company Name: Anetic Aid Ltd, Havant, Hampshire UK) and occluding the blood flow for a short (90 second) interval. The pulse oximeter trace, perfusion index and saturations were recorded simultaneously on both the Masimo (Brand Name: MASIMO Rad-97 Model: 9738, Company Name: MASIMO CORPORATION Irvine CA 92618 USA) and Phillips pulse oximeter probes (Brand Name: PHILIPS Intellivue Pulse Oximetry Module M1020B #A01 SpO2 Module with Model: 862112, Company Name: Philips Medizin Systeme Böblingen Gmbh.) attached to the relevant display systems (Brand Name: PHILIPS Intellivue MX800 Patient Monitor, Model: 865240, PHILIPS Intellivue Multi Measurement Server X2, Model: 865039, Company Name: Philips Medizin Systeme Böblingen Gmbh). Masimo and Philips probes were both used in the limbs without the tourniquet (control limb) and with the tourniquet (experimental limb) to allow comparison / equivalence between the 2 testing systems simultaneously.
Participants were volunteer members of staff at a single centre. They were invited into the test room and eligibility assessed and participant information sheet provided. Inclusion criteria: volunteer members of staff, all ethnicities, aged 18 years or above, male and female, able and willing to provide informed consent. Exclusion criteria: peripheral vascular disease, diabetes, heart disease, kidney disease, previous venous thromboembolism, lower limb injury, pregnancy, under the age of 18 years, unable to provide informed consent.
There were 22 participants and each participant contributed to both control and experimental sides (left and right sides) and hence provided both control and experimental outcomes. (See Appendix for flow chart of study).
Volunteers were warned of transient discomfort associated with the tourniquet inflation and for the period of the time the tourniquet was inflated and that they may experience a ‘rush of blood’ feeling back into the limb after the tourniquet was released. However, the same tourniquets are used routinely in surgery for more than one hour, without any long-term consequences.
The volunteer lay on the couch and a baseline pulse and blood pressure were recorded. Hand dominance and gender were recorded. A tourniquet was applied to one thigh and the Masimo and Phillips probes applied to the second and third toes of both the experimental limb and the control limb. The pulse oximeter trace, perfusion index and saturations were recorded. The tourniquet was inflated to 300 mmHg (which exceeded the systolic blood pressure by > 100 mmHg in all volunteers). The pulse oximeter trace was recorded continuously. The perfusion index was recorded before inflation (timepoint 0 = baseline), at the point the oximeter waveform lost its amplitude (timepoint 1 = during inflation, trace lost), during inflation (minimum value) before release of the tourniquet (timepoint 2) and after deflation (timepoint 3).
This procedure was repeated on the upper limbs with the inflation pressure set to 250mmHg. All equipment was carefully cleaned between participants. All investigations were carried out before COVID-19 arrived in the UK (January 2020).
At the end of the data collection, data were downloaded (pulse rate, saturations, and perfusion index) for the whole time period from the central workstation in Intensive care, including a print off of the pulse oximeter trace and perfusion indices and oxygen saturations. Perfusion index was calculated as the ratio of the pulsatile blood flow to the non-pulsatile blood in peripheral tissue.
Statistical analysis
We summarised gender composition and other categorical participants’ characteristics using count and percentages. Pulse, systolic blood pressure (SBP) and diastolic blood pressure (DBP) were summarised by reporting appropriate measures for continuous data such as mean, median and interquartile range.
The pulse oximeter traces were examined for changes in response to tourniquet inflation and deflation. We explored perfusion index values by plotting individual participant profiles across timepoints 0 to 3. We also reported count and percentages of perfusion index values equal to zero.
At inflation we compared control and experimental sides perfusion index data using nonparametric Wilcoxon signed-rank tests. Nonparametric tests were used because during inflation, perfusion index values were mostly equal or close to zero and so they were not normally distributed. To assess whether at deflation perfusion index values revert to baseline values, we fitted linear mixed models that included baseline and deflation data. The models included a random effects term for participant and fixed effects terms for group (control or experimental), time point (baseline or deflation) and group-time point interaction.
A scatter plot was used to assess visually whether Phillips and Masimo values were similar. For the control sides at all time points and experimental sides at baseline and at deflation, we compared the means for the Phillips and Masimo perfusion index values using paired t-tests. We quantified the associations using Pearson’s correlation together with 95% confidence interval (CI) and intraclass correlation coefficient (ICC). For experimental sides, at inflation when trace was lost (timepoint 1) and during inflation (timepoint 2), because most values were close to zero, we compared Phillips and Masimo perfusion index values using Wilcoxon signed-rank tests. We quantified the correlation using the Spearman’s and Kendall’s correlations as there did not seem to be a linear relationship.
We analysed arms’ and legs’ data separately. Also, except when comparing Phillips and Masimo perfusion index values, we analysed Phillips and Masimo data separately.
All hypothesis tests were performed at 5% significance level. The figures were produced in R statistical program while models used to test hypotheses were fitted in SPSS (IBM, Chicago IL, USA).
Data were secured on a secure computer on the trust server. This was password protected and the excel spreadsheet (Microsoft corporation, Redmond, Washington, USA) was password protected. The dataset supporting the conclusions of this article is not included within the article as this was not approved by the ethical committee for the study and consent was not obtained by the participants.