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人体组织中密布着大量的微血管, 包括微动脉、微静脉和毛细血管,人体组织的血氧参量即为上述各种微血管中血液血氧参量的加权平均[8]。因此,通过监测创面周围的组织氧饱和度可以达到反映术后皮瓣血运状况的目的。
局部组织氧饱和度(regional tissue oxygen saturation, rSO2)的定义为局部组织中氧合血红蛋白浓度CHBO2与局部组织中氧合血红蛋白浓度、还原血红蛋白浓度CHB和的比,有:
$ {R_{{\rm{rS}}{{\rm{O}}_2}}} = \frac{{{C_{{\rm{HB}}{{\rm{O}}_2}}}}}{{{C_{{\rm{HB}}}} + {C_{{\rm{HB}}{{\rm{O}}_2}}}}} \times 100\% $
(1) 式中, RrSO2为局部组织氧饱和度的值。在近红外波段(700nm~900nm),组织中还原血红蛋白(HB)和氧合血红蛋白(HBO2)是主要的吸光物质[9],本文中使用两种特定波长的光交替照射组织,在光源同一平面一定距离处接收组织的漫散射光。当波长为λ1,λ2的光穿过组织时,由吸收定律得:
$ \left\{ {\begin{array}{*{20}{l}} {{\mu _{{\rm{a}}, {\lambda _1}}} = {\varepsilon _{{\rm{HB}}, {\lambda _1}}}{C_{{\rm{HB}}}} + {C_{{\rm{HB}}{{\rm{O}}_2}}}{{\cal E}_{{\rm{HB}}{{\rm{O}}_2}, {\lambda _1}}}}\\ {{\mu _{{\rm{a}}, {\lambda _2}}} = {\varepsilon _{{\rm{HB}}, {\lambda _2}}}{C_{{\rm{HB}}}} + {C_{{\rm{HB}}{{\rm{O}}_2}}}{\varepsilon _{{\rm{HB}}{{\rm{O}}_2}, {\lambda _2}}}} \end{array}} \right. $
(2) 式中,μa, λ1是波长为λ1时组织的吸收系数,μa, λ2是波长为λ2时组织的吸收系数; ε是摩尔消光系数,只与吸收物质和波长有关。
由(1)式、(2)式得出局部组织氧饱和度为:
$ {R_{{\rm{rS}}{{\rm{O}}_2}}} = \frac{{{\varepsilon _{{\rm{HB}}, {\lambda _1}}} - {\varepsilon _{{\rm{HB}}, {\lambda _2}}}\frac{{{\mu _{{\rm{a}}, {\lambda _1}}}}}{{{\mu _{{\rm{a}}, {\lambda _2}}}}}}}{{\frac{{{\mu _{a, {\lambda _1}}}}}{{{\mu _{a, {\lambda _2}}}}}\left( {{\varepsilon _{{\rm{HB}}{{\rm{O}}_2}, {\lambda _2}}} - {\varepsilon _{{\rm{HB}}, {\lambda _2}}}} \right) - \left( {{\varepsilon _{{\rm{HB}}{{\rm{O}}_2}, {\lambda _1}}} - {\varepsilon _{{\rm{HB}}, {\lambda _1}}}} \right)}} $
(3) 从(3)式可以看出,求解RrSO2的实质是解出两个光源下被测组织的吸收系数之比。
人体组织对近红外光的吸收远小于散射,尽管单个光子在组织中的传播路径是随机的,但大量光子在组织中依然遵循统计规律,光子在人体组织中的传输路径大致呈月牙状[10-12]。由组织中近红外光谱的漫散射方程[13-14]得:
$ {\mu _{\rm{a}}} = \frac{{{{\left( {\frac{{\partial {T_{{\rm{OD}}}}}}{{\partial d}}\ln 10 - \frac{2}{d}} \right)}^2}}}{{3{\mu _{\rm{s}}}}} $
(4) 式中, ΤOD为光密度(optical density, OD),定义ΤOD=lg(I0/I), I0为入射光强, I为出射光强;d为光源到探测器中心的距离;μs为组织对近红外光的散射系数,与波长无关[15]。
当使用两束不同波长的光照射组织时,组织对两路光的吸收系数比为:
$ \frac{{{\mu _{{\rm{a}}, {\lambda _1}}}}}{{{\mu _{{\rm{a}}, {\lambda _2}}}}} = {\left[ {\frac{{\frac{{\partial {T_{{\rm{OD}}, {\lambda _1}}}}}{{\partial d}}\ln 10 - \frac{2}{d}}}{{\frac{{\partial {T_{{\rm{OD}}, {\lambda _2}}}}}{{\partial d}}\ln 10 - \frac{2}{d}}}} \right]^2} $
(5) 为求出$\partial {T_{{\rm{OD}}}}/\partial d$,在系统中引入第2个光电探测器,探测器于光源同侧一远一近分布,空间分布概念图见图 1。此时两波长的吸收系数比可简化为:
$ \frac{{{\mu _{{\rm{a}}, {\lambda _1}}}}}{{{\mu _{{\rm{a}}, {\lambda _2}}}}} = {\left[ {\frac{{\Delta {T_{{\rm{OD}}, {\lambda _1}}} - \left( {\frac{{2\Delta d}}{{d\ln 10}}} \right)}}{{\Delta {T_{{\rm{OD}}, {\lambda _2}}} - \left( {\frac{{2\Delta d}}{{d\ln 10}}} \right)}}} \right]^2} $
(6) 式中,ΔΤOD为同一波长下不同探测器探测到的光强差的绝对值;Δd为两探测器中心点的距离。结合(3)式,组织氧饱和度计算公式可化简为:
$ \begin{array}{l} {R_{{\rm{rS}}{{\rm{O}}_2}}} = \left\{ {{\varepsilon _{{\rm{HB}}, {\lambda _1}}} - {\varepsilon _{{\rm{HB}}, {\lambda _2}}}{{\left[ {\begin{array}{*{20}{c}} {\lg \frac{{{D_{1, {\lambda _1}}}}}{{{D_{2, {\lambda _1}}}}} - \left( {\frac{{2\Delta d}}{{d \cdot \ln 10}}} \right)}\\ {\lg \frac{{{D_{1, {\lambda _2}}}}}{{{D_{2, {\lambda _2}}}}} - \left( {\frac{{2\Delta d}}{{d \cdot \ln 10}}} \right)} \end{array}} \right]}^2}} \right\} \div \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\left\{ {{{\left[ {\begin{array}{*{20}{c}} {\lg \frac{{{D_{1, {\lambda _1}}}}}{{{D_{2, {\lambda _1}}}}} - \left( {\frac{{2\Delta d}}{{d\ln 10}}} \right)}\\ {\frac{{{D_{1, {\lambda _2}}}}}{{{D_{2, {\lambda _2}}}}} - \left( {\frac{{2\Delta d}}{{d\ln 10}}} \right)} \end{array}} \right]}^2}} \right. \times \\ \left. {\left( {{\varepsilon _{{\rm{HB}}{{\rm{O}}_2}, {\lambda _2}}} - {\varepsilon _{{\rm{HB}}, {\lambda _2}}}} \right) - \left( {{\varepsilon _{{\rm{HB}}{{\rm{O}}_2}, {\lambda _1}}} - {\varepsilon _{{\rm{HB}}, {\lambda _1}}}} \right)} \right\} \end{array} $
(7) 式中,D1, λ1 ,D2, λ1 ,D1, λ2 ,D2, λ2 为远近探测器采集到λ1,λ2的光通过组织后的漫散射光强;摩尔消光系数ε通过查表可得。由此,通过光电探测器采集被测部位的漫散射光,便可进行被测部位RrSO2的估算。
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对受试者进行了前臂rSO2测量验证本系统的稳定性。由于该领域还没有公认的组织氧饱和度黄金标准[17],确定受试者身体健康状态良好的前提下进行测试,比对不同受试者之间的数据确定健康标准;对3名受试者进行长期测量,判断不同个体之间差异。皮瓣内血管栓塞的发生情况可以通过前臂静脉阻断模拟。为了能够无创监测人体皮肤内的血氧输运情况,利用袖带血压计在上臂施加一定的压力实现静脉阻断,从而测量前臂皮肤内的血氧变化情况。
为保证受试者测试时身体状态平稳,让受试对象在测试前放松10min,呼吸均匀、情绪稳定后进行测试。考虑到探测器与人体皮肤接触和光源自身发热导致受测组织局部温度上升,使得微血管内血液流速和血细胞的聚集状态发生改变影响实验的准确性[18]。因此,受试者的测量时间大于3min,等数值稳定后记录实验数据。
在一段时间内对受试者们集中测试,测试部位选择受试者的左前臂内侧、左前臂外侧、右前臂内侧、右前臂外侧。部分结果如表 1和图 6所示。
Table 1. Measurement value of tissue oxygen saturation
ID female(F)/
male(M)left forearm medial lateral left forearm right forearm medial lateral right forearm 1 F 0.536 0.528 0.524 0.527 2 M 0.500 0.505 0.512 0.526 3 M 0.530 0.504 0.545 0.490 4 F 0.517 0.510 0.525 0.492 5 F 0.531 0.505 0.524 0.502 6 M 0.530 0.520 0.527 0.510 7 F 0.538 0.523 0.542 0.520 8 M 0.528 0.519 0.539 0.521 9 M 0.524 0.472 0.532 0.474 10 M 0.508 0.481 0.501 0.502 11 F 0.531 0.515 0.538 0.531 12 F 0.508 0.484 0.526 0.496 13 F 0.520 0.497 0.523 0.517 14 F 0.536 0.533 0.534 0.520 15 M 0.530 0.512 0.509 0.506 16 M 0.543 0.480 0.528 0.515 17 F 0.521 0.525 0.526 0.515 18 F 0.526 0.504 0.516 0.492 19 M 0.510 0.509 0.510 0.524 20 M 0.525 0.493 0.538 0.497 Figure 6. Long-term monitoring of three subjects a—long-term monitoring map of the first subject b—long-term monitoring map of the second subject c—long-term monitoring map of the third subject
数据表明不同个体、不同部位之间组织氧饱和度存在微小差异,幅度在0.05左右, 实验验证了本系统具有较高的稳定性, 这是因为不同受测者之间血管分布、脂肪厚度不同造成结果差异,符合客观事实。
对受试者进行静脉阻断实验来模拟皮瓣血管栓塞[19],阻断压力相同,阻断时间3min,测量结果如图 7、图 8所示。静脉阻断时,由于静脉回流受阻,CHBO2, CHB和血容积都迅速增加,此时手臂呈紫红色。一段时间后血液充盈减缓直至消失,此时rSO2才会随着皮肤耗氧开始缓慢减少。解除阻断后,由于血液循环恢复,rSO2快速回升并有过冲现象[20],之后逐步稳定到测量前的值。
基于近红外光谱在皮瓣移植术后的监测系统
Monitoring systems for skin flap transplantation based on near infrared spectroscopy
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摘要: 皮下组织血氧参量是反映皮瓣移植术后皮瓣存活状况的重要依据。为了无创、连续、实时地监测皮肤组织氧饱和度,采用近红外光谱法进行了理论分析和实验验证,提出了一种电流-电压(I-V)转换思路并搭建了测量系统。利用该系统进行了稳定性验证实验、前臂静脉阻断模拟皮瓣血管栓塞实验。结果表明,不同人体、不同部位的组织氧饱和度存在微小差异,差异幅度均在0.05左右;静脉阻断下系统组织氧参量发生显著改变,最大降幅0.25。该系统具有较高灵敏度,能够连续测量组织氧饱和度参量、反映组织氧变化趋势,可以为临床检测术后皮瓣血氧运输状态提供重要参考。Abstract: Oxygen parameters of subcutaneous tissue are the important basis to reflect the survival of skin flaps after skin flap transplantation. In order to monitor the oxygen saturation of skin tissue noninvasively, continuously and in real time, theoretical analysis and experimental verification were carried out by near infrared spectroscopy. A current-voltage (I-V) conversion method was proposed and a measurement system was built. The system was used to validate the stability and simulate the vascular embolization of forearm vein occlusion flaps.The results show that, there is a slight difference in tissue oxygen saturation in different parts of different human bodies, it is about 0.05. Under venous occlusion, oxygen parameters of the system tissue change significantly. The maximum decrease is 0.25. The system has high sensitivity. It can continuously measure tissue oxygen saturation parameters and reflect the trend of tissue oxygen change. It can provide important reference for clinical detection of oxygen transport status of skin flaps after operation.
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Table 1. Measurement value of tissue oxygen saturation
ID female(F)/
male(M)left forearm medial lateral left forearm right forearm medial lateral right forearm 1 F 0.536 0.528 0.524 0.527 2 M 0.500 0.505 0.512 0.526 3 M 0.530 0.504 0.545 0.490 4 F 0.517 0.510 0.525 0.492 5 F 0.531 0.505 0.524 0.502 6 M 0.530 0.520 0.527 0.510 7 F 0.538 0.523 0.542 0.520 8 M 0.528 0.519 0.539 0.521 9 M 0.524 0.472 0.532 0.474 10 M 0.508 0.481 0.501 0.502 11 F 0.531 0.515 0.538 0.531 12 F 0.508 0.484 0.526 0.496 13 F 0.520 0.497 0.523 0.517 14 F 0.536 0.533 0.534 0.520 15 M 0.530 0.512 0.509 0.506 16 M 0.543 0.480 0.528 0.515 17 F 0.521 0.525 0.526 0.515 18 F 0.526 0.504 0.516 0.492 19 M 0.510 0.509 0.510 0.524 20 M 0.525 0.493 0.538 0.497 -
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