Invasive arterial blood pressure monitoring is employed routinely in high-risk surgery to ensure patient safety. Although the radial artery (RA) is the most common cannulation site, the dorsalis pedis artery (DPA) provides a well tolerated and easily available alternative for patients who have broken skin in their hands and arms because of burns or trauma, or if there is a difficulty with radial arterial cannulation.1,2 Moreover, in neurosurgical operations, the upper part of the body is usually not easily accessible to the anaesthetist and this may cause problems in maintaining a secure arterial line at the wrist. The patient's foot is nearer to the anaesthetists and it is easier to take care of the arterial line and take blood samples for laboratory analysis.3
When blood pressure is monitored at DPA, it is important for the anaesthetist to know about the patterns of the difference between DPA pressure and the more commonly used RA pressure, because overestimation or underestimation of DPA pressure may lead to improper haemodynamic management and therapeutic decisions. It is generally recognised that SBP is significantly higher at the DPA when compared with the RA, and mean blood pressure (MBP) and DBP are similar at the two sites.4,5 Previous research suggested that isoflurane and sevoflurane could affect the DPA-to-RA pressure difference,6,7 but the pattern of change during surgery is unclear. The primary aims of this study were to assess the DPA-to-RA pressure difference under sevoflurane anaesthesia in neurosurgery and the change of pressure gradient with time. The skin temperatures, internal cross-sections and blood flow pattern of DPA and RA were also investigated to establish the causes of any pressure gradient change.
Ethical approval for this study was provided by the Ethics Committee of Southwest Hospital, the Third Military University, Chongqing, China (Chairperson Prof. J. Wu), on 31 October 2013. This study was also registered with ChiCTR (2013: ChiCTR-RNRC-13003853). After obtaining written informed consent from all patients, 37 Chinese patients who underwent neurosurgery were enrolled in this prospective, self-control study from 1 January 2013 to 1 September 2013. Inclusion criteria were age between 18 and 60 years, American Society of Anesthesiologists’ physical status 1–3, a clinical need for invasive arterial pressure monitoring in the supine position and an expected time of surgery more than 3 h. Exclusion criteria were cardiopulmonary disease, aortic coarctation or severe peripheral vascular disease, Raynaud's syndrome, a positive Allen test and emergency surgery.
After admission to the operating theatre, standard monitoring including ECG, noninvasive blood pressure, SpO2 and rectal temperature was applied to all patients. Following preoxygenation, general anaesthesia was induced with intravenous propofol 2 mg kg−1 and fentanyl 5 μg kg−1, and vecuronium 0.1 mg kg−1 was given to facilitate tracheal intubation. The lungs were ventilated using intermittent positive pressure ventilation to maintain SpO2 above 96% and end-tidal carbon dioxide tension between 3.7 and 4.3 kPa (28 and 32 mmHg). Anaesthesia was maintained by inhalation of sevoflurane and the minimum alveolar concentration was kept at 1.2. Remifentanil was infused using a TCI III infusion system (Changfeng Medical Instrument Co., Wuhan, China) and the target effect-site concentration was kept at 3 ng ml−1. Immediately after induction of anaesthesia, the right radial and left dorsalis pedis arteries were cannulated using 20G, 32-mm-long Teflon catheters (B Braun Melsungen AG, Melsungen, Germany). Both catheters were connected to 100-cm-rigid manometer lines filled with 0.9% physiological saline, and connected to identical pressure transducers (B Braun Melsungen AG, Melsungen, Germany). The transducers were positioned at the patient's midaxillary line and zeroed to atmospheric pressure. There were no intravenous infusion catheters in the right arm or left foot. Arterial pressure waveforms and SBP, DBP and MBP from radial and dorsalis pedis arteries were displayed simultaneously on a Solar 8000 M patient monitor (GE Medical Systems Information Technologies, Inc., Milwaukee, Wisconsin, USA). Pressure waveforms were frequently evaluated by the flush method of Gardner8 to rule out catheter malposition or occlusion. Stability of blood pressure was maintained as far as possible during neurosurgery by careful fluid balance and appropriate depth of anaesthesia. Norepinephrine was only used if SBP was less than 90 mmHg. The temperature of the operating room was maintained between 25 and 26°C. A reusable KOALA ΔT Warming Blanket (NOVAMED, Elmsford, New York, USA) is routinely used in our institution during the operative period to maintain normothermia. All patients were prewarmed to keep the body core temperature above 36°C before the skin incision.
SBP, DBP and MBP from radial and dorsalis pedis arteries were automatically recorded to computer by an anaesthesia information acquisition system (DoCare; Medical Treatment Technology Co., Ltd., Beijing, China) at 5-min intervals throughout surgery. The arterial pressure waveforms and other vital signs were recorded in real time through a video capture card (EvideoHD; dv2008 Technology Co. Ltd., Beijing, China) for further analysis.
To investigate time-related changes of the DPA-to-RA pressure gradient, we recorded blood pressure values, and the temperatures of wrist and the foot at the following time points:
- T1 (baseline), 15 min after tracheal intubation
- T2, at the beginning of surgery
- T3, 1 h after the start of surgery
- T4, 2 h after the start of surgery
- T5, 15 min before skin suturing was completed.
At each time point, 10 blood pressure readings were obtained simultaneously from radial and dorsalis pedis arteries at 1-min intervals. To minimise the respiratory effect, five consecutive beats were averaged and the mean was considered to be a single reading. The temperatures of wrist and foot were measured twice each using a temporal scanner artery thermometer TAT-2000C (EXERGEN Corporation, Watertown, Massachusetts, USA). One measurement was taken 2 cm proximal to the puncture site and the other 2 cm distal to the puncture site. The mean of these two measurements was considered to be the temperature of the puncture site at this time point. A LOGIQ e colour Doppler ultrasound system (GE Healthcare, Wauwatosa, Wisconsin, USA) with transducer 8L-RS was used to measure the internal cross-sections and systolic blood flow velocities of the DPA and RA at T1 and T5. The measurement position was 4 cm above the puncture site so as to avoid possible turbulence and artefact caused by the arterial cannulae. The angle of incidence to the artery was maintained at about 60°. The internal cross-section of the artery was measured using B-mode ultrasonography. We chose the largest internal cross-section in a cardiac cycle and measured the longest and shortest diameters of the internal cross-section. The area of the internal cross-section was calculated using the formula for an ellipse. The arterial blood flow and systolic blood flow velocity were measured by pulse Doppler sonography at the point of the largest internal cross-section to ensure good signal strength.
Before data analysis, blood pressure artefacts, such as arterial blood sampling and patient repositioning, were detected and removed from the database on the Matlab software (Matlab 7.0; Math Works Inc., Natick, Massachusetts, USA). Absolute differences between RA and DPA pressures of more than 50 mmHg in SBP, DBP or MBP were considered as outliers and excluded from analysis.
The sample size was calculated based on the basis of previous investigations7 and a preliminary experiment which indicated that the systolic pressure difference between DPA and RA pressures (SBP gradient) reduced by 8–12 mmHg during surgery. We anticipated a 10-mmHg reduction of SBP bias and an equal standard deviation (SD) of 7.8 mmHg for the baseline and the end of surgery (derived from the preliminary experiment). Using a two-tailed paired t test, we would have to include at least 28 patients to detect a significant difference with an α of 0.05 and a β error of 0.2.
Statistical analyses were performed using SPSS 11.5.0 (SPSS Inc., Chicago, Illinois, USA) and Graph Pad (Prism v5.0, San Diego, California, USA). Characteristics of patients are expressed as mean ± SD or mean (range). Categorical variables are expressed as numbers. For all data sets, a Gaussian distribution was tested using a Q–Q plot.
To evaluate the change of DPA-to-RA pressure and temperature gradients with time, the pressure or temperature of DPA was compared with those of RA using paired t tests with two-tailed probabilities, and the differences of blood pressure or temperature measured at different time points were analysed using analysis of variance for repeated measures followed by a Bonferroni test.9,10 The percentage error was calculated as follows:13
Bland–Altman plots for repeated measures11,12 were used to analyse SBP, DBP and MBP data collected by the anaesthesia information acquisition system throughout the procedure. Bias was calculated as the mean difference between DPA and RA pressures, and the limits of agreement represented the range within which 95% of the differences between DPA and RA pressures were found.
Two-tailed paired t test were used to compare the internal cross-sections and systolic blood flow velocities of RA and DPA at the baseline and the end of the surgery. Data are expressed as mean ± SD unless designated otherwise. A P value of 0.05 was considered statistically significant.
Thirty-seven patients were enrolled in this study and gave informed consent. Three patients were excluded because their operations were postponed and four patients were excluded due to failures of either radial or dorsalis pedis arterial cannulation. Thus, data from 30 patients were available for analysis. Characteristics of patient are summarised in Table 1. The primary outcome was the time-dependent changes of RA pressure, DPA pressure and the gradient between them, as shown in Table 2. SBP and pulse pressure measured from DPA were significantly higher than those measured from RA at T1 and T2 (P < 0.001), and the differences became smaller during surgery (from T3 to T5). The gradient gradually reduced from the baseline 9.7 ± 8.8 to −1.8 ± 7.6 mmHg for SBP (P < 0.001) and from 11.2 ± 11.6 to 3.6 ± 9.9 mmHg for pulse pressure (P < 0.001). DBP and MBP measured from DPA declined less significantly than SBP and pulse pressure. The gradient between DPA and RA pressures gradually decreased from −2.3 ± 2.7 to −3.7 ± 2.8 mmHg for DBP (P < 0.001) and from −2.1 ± 3.2 to −5.4 ± 3.4 mmHg for MBP (P < 0.001). The percentage errors of SBP were higher than those of DBP and MBP but consistently within 20%.
Figure 1 shows the difference vs. average of SBP, DBP and MBP between RA and DPA with defined limits of agreement during the whole operation. The bias ± SD was 2.2 ± 10.1 mmHg with a percentage error of 19.1% for SBP, −3.1 ± 3.4 mmHg with a percentage error of 11.5% for DBP and −4.3 ± 4.2 mmHg with a percentage error of 11.18% for MBP. In summary, for SBP, DBP and MBP, the biases were all within ±5 mmHg and percentage errors were within 20%, indicating that DPA pressure was sufficiently accurate for routine clinical use. In addition, the bias increased with an increase in the average SBP, and it decreased with increases in the averaged DBP and MBP, which can be observed from the linear regression line.
As shown in Table 3, the temperature at the wrist remained almost unchanged (P > 0.05), but the temperature on the dorsum of the foot gradually increased with time but was always lower than the temperature at the wrist. The absolute RA-to-DPA temperature gradient reduced with surgery time from 3.6 ± 2.4 to 1.1 ± 2.3°C (P < 0.001).
Figure 2 illustrates that the inner cross-section of DPA was significantly smaller than that of the RA just after tracheal intubation (P < 0.01), but there was no significant difference at the end of the surgery. The inner cross-section of RA did not change significantly with time, but DPA expanded distinctly from baseline (P < 0.01). In addition, the blood flow and peak velocity were generally higher in DPA than in RA during sevoflurane anaesthesia, and this phenomenon became more obvious at the end of the surgery (P < 0.001), as shown in Fig. 3.
In this study, we investigated the change of DPA-to-RA pressure gradient with time and the agreement between them during neurosurgery. To elucidate the reasons for the pressure gradient variation, we measured the local temperatures close to cannulation sites at the wrist and dorsum of the foot at five different time points, and the inner cross-sections and blood flow of RA and DPA at the baseline and the end of surgery. The main findings of our study were as follows: DPA pressure gradually declined and more significantly compared with RA, hence the DPA-to-RA amplification effect was negated during surgery; DPA pressure was comparable to RA pressure instead of significantly higher in long-term neurosurgery. The significant increase in the skin temperature, inner cross-section and blood flow of DPA might be the causes of the decrease in DPA-to-RA gradient.
It is important to understand the DPA-to-RA pressure gradient during surgery when therapeutic decisions are made which rely on DPA pressure. Previous research has suggested that SBP and pulse pressure are higher in the DPA than in the RA.4,5,14 This phenomenon is known as systolic and pulse pressure amplification. It has been suggested that the anaesthetist might fail to identify hypotension if the blood pressure is monitored invasively at DPA.6 However, our results indicate that the amplification effect at DPA is negated during the maintenance of anaesthesia for long operations. This may be because DPA is a smaller downstream vessel and its impedance is affected by the larger vascular bed of the lower limb which supplies mainly skeletal muscle and skin.7,15 It has been reported that skeletal muscle and skin blood flow significantly increase after receiving inhalational anaesthetics.16 Thus, the impedance of the lower limb distal to the foot, especially the resistive component, gradually diminishes during the maintenance of anaesthesia with inhalation of sevoflurane. Our finding is consistent with an earlier study, in which the DPA-to-RA pressure gradient reversed during isoflurane-induced hypotension.7
Another possible reason for the gradual and significant decline of the DPA pressure may be the opening of superficial arteriovenous shunts or arteriovenous anastomoses (AVAs). The muscular walls of AVAs are equipped with adrenoceptors and richly innervated by sympathetic and parasympathetic nerves.17 Thus, a decrease in sympathetic activity because of inhalational anaesthetics may lead to passive vasodilatation and AVAs opening. In addition, AVAs play an important role in thermoregulation.18 Clark has stated that if the human body is warmed, AVAs can open and there is a marked increase in peripheral blood flow to enable the body to cool itself.19,20 In our study, the continued use of the warming blanket during surgery may also cause AVAs to open. AVAs were found in both hand and foot as early as 1939.21 However, the relative numbers of AVAs in the hand and foot and the number of adrenoceptors in the muscular walls are still unclear. We speculate that there are more AVAs or adrenoceptors in the foot than in the hand. This hypothesis is based on the gradual increase of temperature of the dorsum of the foot with surgery time, as well as the more obvious vasodilatation and distinct increase in blood flow at DPA at the end of surgery. Negoro et al.22 also reported a gradual increase of skin temperature and blood flow of the foot during isoflurane and sevoflurane anaesthesia, which is consistent with our results. However, further research is needed to verify the opening of AVAs during sevoflurane anaesthesia and investigate the number of AVAs and adrenoceptors in the hand and foot using in-vitro studies.
An opposite pressure gradient, mainly radial-to-femoral, has been reported in some other situations, such as at the end of cardiopulmonary bypass,4,23,24 cardiopulmonary resuscitation,25 deep hypothermic circulatory arrest,26 in septic shock patients receiving high-dose norepinephrine therapy27 and during liver transplantation.28,29 In these situations, the difference between DPA pressure and femoral arterial pressure is expected to be even larger. Consequently, in cases of high-risk, blood loss and redistribution of blood flow, central (femoral artery) pressure monitoring is recommended instead of peripheral (radial or dorsalis pedis) arterial pressure.
Our study has some limitations. First, the changes in inner cross-sections and blood flow of RA and DPA in the process of neurosurgery were not evaluated because of asepsis restriction. Nevertheless, results at the baseline and at the end of surgery can also illustrate the phenomenon. Second, the results of this study only apply to patients without severe cardiopulmonary or peripheral vascular disease of American Society of Anesthesiologists’ physical status 3 or less. The impacts of age, hypertension, atherosclerosis, type of surgery and surgical position on the DPA-to-RA pressure gradient were not investigated. These problems require further studies. Finally, this study was conducted in very stable conditions. How the DPA-to-RA pressure gradient changes during induction of anaesthesia and haemodynamic fluctuation caused by bleeding, shock or vasoactive drugs was not studied.
In conclusion, for patients undergoing neurosurgery, the blood pressure and skin temperature differences between DPA and RA gradually reduce during sevoflurane anaesthesia, and a distinct vasodilatation and increase of blood flow is observed at DPA. A possible reason is that inhalational anaesthetics and intraoperative warming cause AVAs to open in the foot. Thus, if RA monitoring is not available, clinicians may rely on DPA pressure as long as they are aware of the change in DPA-to-RA pressure gradient and make appropriate therapeutic decisions.
Acknowledgements relating to this article
Assistance with the study: the authors would like to thank Ronglong Li from the Department of Ultrasound in Southwest Hospital for his advice on measuring the arterial inner cross-section and blood flow, as well as the technical support from GE Medical system Co., Ltd, Jiangsu, China.
Financial support and sponsorship: this study was supported by the Young Talent Innovation Fund of Southwest Hospital, Third Military Medical University (grant number SWH2013QN05) and the Braun Anaesthesia Scientific Research Fund (grant number BBF-2012-09).
Conflicts of interest: none.
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