nevertheless , they propagate the high tie technique since it allows better lymph node retrieval and therefore a more accurate tumour staging . in another review comparing the low tie with the high tie technique , lange et al . nevertheless , they favour the low tie technique since it is less invasive , also with regard to colonic innervation and motility , and it would be beneficial for anastomotic perfusion compared to the high tie technique . performing a high tie ( ht ) technique allows anastomotic perfusion only through the marginal artery , which may lead to a decrease in anastomotic perfusion [ 5 , 6 ] . when a low tie ( lt ) technique is performed , anastomotic perfusion is allowed not only through the marginal artery , but through the left colic artery and its ascending branch as well . this anatomical reality suggests that anastomotic perfusion is higher after low tie ; however , no evidence exists supporting this hypothesis . the aim of this study is to compare the high tie technique to the low tie technique with regard to anastomotic perfusion . patients planned for elective rectal resection for malignancy in four participating hospitals , with nine participating surgeons , were eligible for this non - randomized , prospective study . the o2c system (  oxygen to see  , lea medizin technik , giessen , germany ) is a laser doppler flowmetry system that has often been used to measure intestinal blood flow for research purposes [ 7 , 8 ] . blood flow , expressed in arbitrary units , is determined by analysing the doppler frequency shifts in laser light ( 820  nm ) reflected from moving red blood cells . the laser light is emitted into the tissue , and the backscattered light is detected with a flat probe with a measurement depth of 46  mm ( lea medizin technik , giessen , germany ) . the o2c measurement frequency is 30  hz . measurements were performed at two moments during the operation , being ( a ) right after median laparotomy and ( b ) just before construction of the anastomosis or colostomy , in case of abdominoperineal resection . the measurements were performed on the antimesenterial , serosal side of the colon segment that was to become , or was after resection ( at moment b ) , the proximal loop . for all measurements , after placement of the flat probe , the flow measurement was allowed to stabilize until a constant flow was measured . the mean of these 15 measurements was used to calculate the blood flow ratio ( bfr ) , b / a . during the measurements , the blood pressure was measured as well , and the mean arterial pressure ( map ) was calculated . the high tie technique was defined as ligation of the inferior mesenteric artery ( i m a ) at its origin . the low tie technique was defined as ligation of the superior rectal artery ( sra ) , just below the branching of the left colic artery ( fig .  1 ) . b the dashed line indicates the level of ligature in ht , leaving no flow in the inferior mesenteric artery and its branches . c the dashed line indicates the level of ligature in lt ; grey indicates the flow area of i m a after lt the vasculature of the colon . b the dashed line indicates the level of ligature in ht , leaving no flow in the inferior mesenteric artery and its branches . c the dashed line indicates the level of ligature in lt ; grey indicates the flow area of i m a after lt the bfr distribution was normalised by a logarithmic transformation and compared between the ht and lt groups by means of an unpaired t test . a ht was performed in 16 patients ( 48% ) of whom 12 ( 75% ) received a primary anastomosis . a lt was performed in 17 patients ( 52% ) of whom also 12 ( 71% ) received a primary anastomosis . in all patients receiving a primary anastomosis , nine patients ( 75% ) in the high tie group and ten patients ( 83% ) in the low tie group received a defunctioning stoma . the mean number of lymph nodes harvested in the high tie group was 11 ( range , 623 ) ; in the low tie group , 12 ( range , 633 ) ( p  =  0.35 ) . the mean number of positive lymph nodes harvested in the high tie group was 3 ( 15 ) ; in the low tie group , 4 ( 19 ) ( p  =  0.32 ) . two patients developed anastomotic leakage , one in the ht group and one in the lt group . in the ht group , no significant differences were found in the remaining baseline characteristics ( table  1 ) . table  1patient characteristics of included patientsbaseline characteristicshigh tielow tiep valuegender ( m / f)11/512/51.000age ( years)55    1761    130.363bmi ( kg / m)25    327    70.473operation1.000apr45lar1212neoadjuvant therapy0.024rt148no rt29asa score0.170i75ii86iii16cardiovascular comorbidity2 ( 13%)4 ( 24%)0.656operating time ( minutes)160 ( 100340)145 ( 45225)0.450tumour stage0.250010i44iia24iib10iiia23iiib13iiic01iv40m / f male / female , bmi body mass index ( kilograms per square meter ) , apr abdominoperineal resection , lar low anterior resection , rt radiotherapy , asa score american society of anaesthesiologists score patient characteristics of included patients m / f male / female , bmi body mass index ( kilograms per square meter ) , apr abdominoperineal resection , lar low anterior resection , rt radiotherapy , asa score american society of anaesthesiologists score the mean bfr was significantly higher in the lt group as depicted in table  2 , whereas the blood pressure during measurements was not significantly different as depicted in table  3 . table  2comparison of blood flow ratios between the ht and lt techniquesratioht / ltmean ratiostd . error of the meanp valueb / aht0.910.240.04lt1.480.32table  3map measured during the blood flow measurements at time points a and b , respectivelygroupmap a / bmeanstd . error of the meanp valuehta67.12.20.473b64.23.2lta69.84.40.075b75.72.0 comparison of blood flow ratios between the ht and lt techniques map measured during the blood flow measurements at time points a and b , respectively to date , the discussion on the matter of high tie versus low tie continues . this study focuses on the colorectal vasculature and the flow change after ht or lt . found the colonic blood flow to vastly decrease after ligation of i m a or sra , with the subsequent conclusion that this could be an unavoidable factor in the pathophysiology of colorectal anastomotic leakage . only a slightly decreased blood flow was observed at the end of the operation ( bfr 0 , 91 ) , whereas an increased blood flow was measured after lt ( bfr 1 , 48 ) . the blood flow changes occurred independently from the systemic blood pressure ( table  3 ) . these different findings may be explained by the time interval between arterial ligation and measurement . study the first measurement was performed immediately after laparotomy , and the second measurement , just before construction of the anastomosis or colostomy , i.e. at the end of the operation . therefore , the interval between ligation and measurement is much longer in this study compared to the aforementioned study . this suggests that over time , a recruitment of colonic arteries occurs , allowing recovery of blood flow . in order to study whether these blood flow changes are permanent or not , blood flow measurements in the postoperative period would be interesting . in addition , since anastomotic leakage is generally detected around the eighth postoperative day , it could provide important information on the pathophysiological processes concerning blood flow leading to al . the bfr was significantly higher after lt which means lt allows better perfusion of the proximal anastomotic loop at the end of the operation . most likely , this is due to the preservation of the left colic artery and its ascending branch . in addition to the marginal artery , these arteries allow a second pathway for blood supply and faster and/or a more extensive recruitment of colonic arteries . therefore , since good perfusion is essential for proper anastomotic healing , lt would be the preferred technique for this aspect of the high tie  low tie comparison . the average bfr after lt shows an increase in blood flow compared to the initial value at the end of the operation . this has been described before by karlicek et al .   and could be due to reactive hyperaemia as a result of colon manipulation . however , it could also be due to a variety of ischaemia reperfusion injuries ( iri ) . these injuries have been well described in animal models in which an iri leads to visible hyperaemia and decreased anastomotic strength . this response is probably also present after ht ; however , it is more outspoken after lt most likely due to preservation of the left colic artery . whether these findings have an impact on the incidence of anastomotic leakage should be evaluated by analysing the blood flow during the postoperative period or in a similar but larger study . the o2c allows non - invasive measurement of blood flow ; however , the measurements are sensitive to several variables . since it is virtually impossible to mark a spot on the colon without influencing the local blood flow or without hindering the progress of the operation , placement of the probe will be slightly variable . second , the measurements are sensitive to different pressures applied on the probe when holding it in the right position . higher pressures are likely to lead to more compressed arteries and a lower blood flow . in order to limit the influence of these variables on the outcome in addition , the blood flow ratio was calculated for which the first measurement served as a control . the use of a ratio also allowed standardizing intrinsic , patient - related differences like microangiopathy due to atherosclerosis and diabetes mellitus . table  1 shows baseline characteristics to be comparable between ht and lt except for radiotherapy . this is , however , unlikely to have an effect on the blood flow in the proximal anastomotic loop since this loop is located outside the radiation field . in addition , the high tie group contained a higher number of patients with metastasized disease ( stage 4 present in 25% in the ht group vs. 0% in the low tie group ) . this difference most likely illustrates the participating surgeons having preferred to perform a high tie technique in patients with metastasized disease . when comparing high tie ligation to low tie ligation , this study shows the perfusion of the proximal loop of the anastomosis to be better after low tie ligation . considering neither of both techniques is favourable on the oncological or technical aspect , low tie ligation may be the technique of choice in patients undergoing rectum resection .
purposeboth  high tie  ( ht ) and  low tie  ( lt ) are well - known strategies in rectal surgery . 
 the aim of this study was to compare colonic perfusion after ht to colonic perfusion after lt.methodspatients undergoing rectal resection for malignancy were included . 
 colonic perfusion was measured with laser doppler flowmetry , immediately after laparotomy on the antimesenterial side of the colon segment that was to become the afferent loop ( measurement a ) . 
 this measurement was repeated after rectal resection ( measurement b ) . 
 the blood flow ratios ( b / a ) were compared between the ht group and the lt group.resultsblood flow was measured in 33 patients , 16 undergoing ht and 17 undergoing lt . 
 colonic blood flow slightly decreased in the ht group whereas the flow increased in the lt group . 
 the blood flow ratio was significantly higher in the lt group ( 1.48 vs. 0.91 ; p  =  0.04 ) , independent of the blood pressure.conclusionthis study shows the blood flow ratio to be higher in the lt group . 
 this suggests that anastomoses may benefit from better perfusion when lt is performed .