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the lack s circuit ( or modified co - axial mapleson a breathing system ) is commonly used in spontaneously breathing anesthetized adults .
it does not contain a carbon dioxide absorber ; thus , its performance relies on an adequate fresh gas flow ( fgf ) to prevent rebreathing.1 the lack s circuit is the most efficient and economical system for spontaneous respiration in adults compared to the magill s and bain s circuits.1 it is compact , lightweight and portable , with additional convenient access to the exhaust ( adjustable pressure limiting or apl valve ) , easy scavenging of polluting gases , and low expiratory resistance . according to the philosophy on sustainable economics of thailand s sovereign ( his majesty king bhumibol ) ,  understanding , accessibility and development  represent the three rings for success .
we thus invented the modified lack s circuit for use in adult patients  aka the lack - plus circuit  using inexpensive and readily available materials in the operating room.2 the difference between the lack and lack - plus circuits is that the lack - plus circuit has an additional intraluminal one - way valve on the inner expiratory tube situated proximal to the apl valve , which minimizes fgf , saves anesthetic gas , and decreases operating room pollution .
the components include a 100 ml normal saline solution plastic bottle ; a rubber glove ; 1 , 3 , 10 , and 20 ml plastic syringes ; a 22 mm internal diameter corrugated tube ( outer reservoir tube ) ; a 10 mm internal diameter hollow plastic tube ( inner exhaust tube ) ; a yellow hollow latex rubber tubes with connector for fgf outlet ; and a 22 mm internal diameter three - way plastic connector ( figure 1 ) .
the aim of this clinical trial was to compare the lack - plus and lack s circuit vis -  - vis minimal fgf requirements with no rebreathing in spontaneously breathing anesthetized adults .
the institutional review board of khon kaen university ( kku ) approved the study ( he571249 ) .
the current study was conducted at srinagarind hospital , faculty of medicine , kku , khon kaen , thailand . between september and october 2014
, 24 patients aged between 18 and 60 years with an american society of anesthesiologists ( asa ) physical status i  ii and body mass index ( bmi ) 30 kg / m , scheduled for elective peripheral surgery in supine position under general anesthesia , were included in a randomized crossover study .
patients were excluded if they had a body temperature > 37.8c , a respiratory disease ( copd , asthma , chronic bronchitis ) , a cardiovascular disease , a contraindication to succinylcholine , or they refused surgery .
immediately after the patients entered the operating room , eligible patients were randomly assigned to one of the following two arms : 
 lp - l : the lack - plus circuit was used first and then switched to the lack s circuit orl - lp : the lack s circuit was used first and then switched to the lack - plus circuit lp - l : the lack - plus circuit was used first and then switched to the lack s circuit or l - lp : the lack s circuit was used first and then switched to the lack - plus circuit the multi - gas monitor used in this study was philips intellivue mp30 with m1013a module ( philips medizin - systeme bblingen gmbh , bblingen , germany ) .
all the patients were given diazepam 0.10.2 mg / kg and paracetamol 1015 mg / kg orally 1 h before transferring to the operating room .
baseline blood pressure , pulse , and oxygen saturation ( spo2 ) were obtained by standard monitors .
anesthesia was induced with fentanyl 12 g / kg and propofol 22.5 mg / kg iv .
anesthesia was maintained with fgf containing 50% n2o / o2 in combination with desflurane 4%6% , adjusted to 11.2 mac anesthetic level to maintain normal vital signs .
fentanyl was titrated to ensure an optimal respiratory rate ( rr ; range , 1016/min ) .
all the patients were spontaneously breathing with fgf 4 l / min at the start of each anesthetic breathing circuit .
the authors waited for the depth of surgical anesthesia to be maintained for at least 10 min .
the baseline end - tidal carbon dioxide ( etco2 ) , inspired minimum carbon dioxide ( imco2 ) , rr , mean arterial pressure ( map ) , and spo2 were measured .
the amount of fgf was reduced step - by - step by 0.5 l / min every 5 min , waiting for imco2 to be regularly maintained for at least 60 sec ; then etco2 , imco2 , rr , map , and spo2 values were recorded at each step until rebreathing occurred ( imco2 > 0 ) .
the fgf was switched back to 4 l / min for 10 min , after which the procedure was repeated .
the minimal fgf before rebreathing occurred and the imco2 at a fgf of 2.5 l / min were recorded .
a minimum of 20 patients were needed to detect a clinically relevant reduction in fgf of 20% , with a power of 0.90 and a level of significance of 5% .
randomization was achieved using a computer - generated list ( kept in sealed envelopes ) .
statistical analyses were performed with statistical package for the social sciences ( spss ) for windows version 17.0 ( spss inc . ,
continuous data were expressed as meansd and compared using the paired student s t - test or mann - whitney u test where applicable .
categorical data were expressed by number ( % ) , and comparison was made by chi - square test .
a minimum of 20 patients were needed to detect a clinically relevant reduction in fgf of 20% , with a power of 0.90 and a level of significance of 5% .
randomization was achieved using a computer - generated list ( kept in sealed envelopes ) .
statistical analyses were performed with statistical package for the social sciences ( spss ) for windows version 17.0 ( spss inc . ,
continuous data were expressed as meansd and compared using the paired student s t - test or mann - whitney u test where applicable .
categorical data were expressed by number ( % ) , and comparison was made by chi - square test .
twenty - four patients fulfilled the entry criteria and were enrolled in the study . both groups ( lp - l and l - lp ) were comparable with respect to age , sex , bmi , asa physical status , and type of operation ( table 1 ) .
the mean minimal fgf at the point of rebreathing was significantly lower in the lack - plus group than in the lack group ( p<0.001 ) ( table 2 ) .
the etco2 level was significantly higher in the lack - plus group , and the fgf requirement was decreased by 18.7% for no rebreathing .
the mean rr , arterial pressure , and spo2 value between the groups were not significantly different .
the mean imco2 at the fgf of 2.5 l / min was significantly lower in the lack - plus group than in the lack group ( p<0.001 ) ( table 3 ) .
there was no statistically significant difference between the groups vis -  - vis etco2 , map , and spo2 .
the rr was statistically different but was within normal clinical range ( 11.8 and 11.2 tpm ) .
the mean minimal fgf at the point of rebreathing was significantly lower in the lack - plus group than in the lack group ( p<0.001 ) ( table 2 ) .
the etco2 level was significantly higher in the lack - plus group , and the fgf requirement was decreased by 18.7% for no rebreathing .
the mean rr , arterial pressure , and spo2 value between the groups were not significantly different .
the mean imco2 at the fgf of 2.5 l / min was significantly lower in the lack - plus group than in the lack group ( p<0.001 ) ( table 3 ) .
there was no statistically significant difference between the groups vis -  - vis etco2 , map , and spo2 .
the rr was statistically different but was within normal clinical range ( 11.8 and 11.2 tpm ) .
among non - absorber anesthetic breathing circuits , the magill s , lack s , and bain s circuits are commonly used during general anesthesia for spontaneous respiration in adult patients .
humphrey1 demonstrated that the lack s circuit had the lowest imco2 and etco2 compared to the magill s and bain s circuits , by using a fgf of 70 ml / kg / min . additionally , the respective fgf requirement in the lack s , magill s , and bain s circuits that caused rebreathing was 51 , 73 , and 153 ml / kg / min , respectively . the lack s circuit ,
therefore , appears superior in performance to the magill s and bain s circuits for spontaneous respiration in adults , economical use of fgf , convenient exhaust valve , and easy scavenging facilities .
the lack - plus circuit  a modification of the lack s  was invented by theerapongpakdee2,3 for spontaneous breathing anesthesia in adults .
theerapongpakdee devised the lack - plus circuit using materials readily available in the operating room ( figure 1 ) . the intraluminal one - way valve 
situated on the expiratory limb of the non - absorber breathing system  was used in the new enclosed anesthetic circuit such as the enclosed afferent reservoir ( ear),4 baby ear,5,6 and displacement afferent reservoir ( dar ) systems .
the advantage of such a position was more effective prevention of rebreathing during the inspiration phase .
our hypothesis is that the lack - plus circuit  with the addition of an intraluminal one - way valve  would be more efficient than the lack s circuit in that it requires less fgf to prevent rebreathing , reduces loss of heat and humidity , and causes less pollution in the operating room .
the minimal fgf at the point of rebreathing for the lack - plus circuit in the current report was significantly less than for the lack s circuit ( 2.70.8 and 3.30.5 l / min , respectively , p<0.001 ) .
this result agrees with kain and nunn8,9 who demonstrated that the minimal adequate fgf to prevent rebreathing of the magill s circuit was 3.14.6 l / min . at a fgf of 2.5 l / min ,
the imco2 of the lack - plus circuit was significantly lower than the lack s circuit ( 1.52.0 and 4.22.6 mmhg respectively , p<0.001 ) , demonstrating that the intraluminal one - way valve situated proximal to the apl valve effectively helps to prevent rebreathing .
notwithstanding , there were no significant differences in clinical practice owing to the etco2 value of both the systems , which were within the normal , clinical , acceptable range ( 46.97.9 and 44.88.1 mmhg , respectively , p=0.025 ) .
since we reduced fgf at a step of 0.5 l / min , it might be not absolutely accurate to detect the actual fgf at each end point of the study .
since we reduced fgf at a step of 0.5 l / min , it might be not absolutely accurate to detect the actual fgf at each end point of the study .
the lack - plus circuit is a modified lack s circuit ( i.e. , a modified co - axial mapleson a ) , which is a non - carbon dioxide absorber anesthetic circuit with an additional intra - luminal one - way valve and a gas scavenging port .
it can be used safely , effectively , and requires less fgf than the lack s circuit in spontaneously breathing anesthetized adults ( bmi 30 kg / m ) .
the optimal fgf requirement for the lack - plus circuit to prevent rebreathing is 2.70.8 l / min .
backgroundthe lack s circuit is a co - axial mapleson a breathing system commonly used in spontaneously breathing anesthetized adults but still requires high fresh gas flow ( fgf ) . 
 the lack - plus circuit was invented with the advantage of lower fgf requirement . 
 the authors compared the lack - plus and lack s circuit for the minimal fgf requirement with no rebreathing in spontaneously breathing anesthetized adults.methodsthis was a randomized crossover study . 
 we enrolled 24 adult patients undergoing supine elective surgery , with a body mass index 30 kg / m2 and an american society of anesthesiologists physical status i  ii . 
 they were randomly allocated to group 1 ( lp - l ) starting with lack - plus then switching to lack s circuit or group 2 ( l - lp ) ( with the reverse pattern ) . 
 after induction and intubation , anesthesia was maintained with 50% n2o / o2 and desflurane ( 4%6% ) plus fentanyl titration to maintain an optimal respiratory rate between 10 and 16/min . 
 starting with the first circuit , all the patients were spontaneously breathing with a fgf of 4 l / min for 10 min , gradually decreased by 0.5 l / min every 5 min until fgf was 2.5 l / min . 
 end - tidal co2 , inspired minimum co2 ( imco2 ) , mean arterial pressure , and oxygen saturation were recorded until rebreathing ( imco2 > 0 mmhg ) occurred . 
 the alternate anesthesia breathing circuit was used and the measurements were repeated.resultsthe respective minimal fgf at the point of rebreathing for the lack - plus and lack s circuit was 2.70.8 and 3.30.5 l / min , respectively , p<0.001 . at an fgf of 2.5 l / min , the respective imco2 was 1.52.0 and 4.22.6 mmhg , respectively , p<0.001.conclusionthe lack - plus circuit can be used safely and effectively , and it requires less fgf than lack s circuit in spontaneously breathing anesthetized adults .