Michaelwicz Etal Ventilatory Metabolic Demands During Aggressive Restraint Jan 2007
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J Forensic Sci, January 2007, Vol. 52, No.1 doi: 10.llllIj.1556-4029.2006.00296.x Available online at: www.blackwell-synergy.com Betty A. Michalewicz, I M.S.; Theodore C. Chan, 2 M.D.; Gary M. Vilke, 2 M.D.; Susan S. Levy, I Ph.D.; Tom S. Neuman, 2 M.D.; and Fred W Kolkhorst,I,3 Ph.D. Ventilatory and Metabolic Demands During Aggressive Physical Restraint in Healthy Adults ABSTRACT: We investigated venti1story and metabolic demands in healthy adults when plsced in the prone maxima1 restraint position (PMRP), i.e., hogtie restraint. Maximal voluntary ventilation (MVV) was measured in seated subjects (n = 30), in the PMRP, and when prone with up to 90.1 or 102.3 kg of weight on the back. MVV with the heaviest weight was 70% of the seatedMVV (122 ± 28 and 156 ± 38Umin, respectively; p<O.OOI). Also, subjects (n~27) were placed in the PMRP and struggled vigorously for 60 sec. During the restrained struggle, ventilatory function (VJ MVV) was 44% of MVV in the resting PMRP. While prone with up to 90.1 or 102.3 kg on the back, the decrease in MVV was of no clinical importance in these subjects. Also, while maximally struggling in the PMRP, VB was still adequate to supply the ventilatory needs. KEYWORDS: forensic sciences, maximal voluntary ventilation, minute ventilation, oxygen consumption, positional asphyxia Law enforcement and prehospital care personnel often confront violent, dangerous individuals who must he physically restrained to ensure safety to themselves and to those around them. Author· ities have developed a numher of physical restraint techniques to control and subdue such individuals in the field (1,2). One of these, the prone maximal restraint position (pMRP), also referred to as the hogtie or hobble restraint, has heen used extensively by field personnel (3,4). When in the PMRP, an individual is prone with hislher wrists secured behind the back, ankles bound together, and wrists and ankles tied together using handcuffs, cords, chains, or hobble devices (3,5). Reports of sudden deaths in individuals in the PMRP have ap· peared at least since the 1980s, which have created controversy regarding the safety of these restraint positions (3,6-8). Some au· thors have suggested that the PMRP may prevent adequate chest and abdominal movement, which places the individual at risk of asphyxiation (7-9). Asphyxiation that is caused by body position has been referred to as 'positioua1 asphyxia" (10). However, a recent study reported that, although PMRP by itself resulted in a small, restrictive ventilatory pattern compared with seated measurements, there was no evidence of hypoventilation, hypercapnia, or hypoxemia (11). Additionally, police officers often apply force to the back to better control an agitated person during the restraint process. This additional force has been hypothesized to constrict the chest and abdomen more than the PMRP alone (12-14) and lead to as· phyxiation (IS). Chan et al. (3) examined the effects on po1mon· ary function from 11.4 to 22.7 kg applied to the back of prone subjects. Although the PMRP with or without force applied to the back led to a restrictive pattern on pulmonary function testing, there was no evidence of hypoxia or hypercapnia. lDepartment of Exercise and Nutritional Sciences, San Diego State University. San Diego, CA 92182. 2Department of Emergency Medicine, Medical Center, University of Cal- ifornia, San Diego, CA 92103. 3San Diego State Heart Institute, San Diego State University, San Diego, CA 92182. Received II Dec. 2005; and in reWed form 19 June 2006; accepted 1 July 2006; published 8 Dec. 2006. Copyright © 2006 by American Academy of Forensic Sciences Nevertheless, it would be safe to assume that law enforcement officers are able, and do, apply more than 22.7 kg of force to the back of suspects placed in the PMRP, particularly if the individ· uals are violent. Cases of sudden death of restrained individuals often involve those who continued to struggle after heing reo strained (8). This study was undertaken to determine whether the use of force greater than 22.7 kg might inhibit ventilatory function such that it became a clinically important consideration in the analyses of such deaths. Moreover, we were interested in whether individuals stroggling during periods of physical restraint were able to approach the limits of their ventilatory function. This study, which utilized a randomized, cross-over, controlled design, had two parts. For the first part, the effects on maximal voluntary ventilation (MVV) in subjects were examined while in the PMRP and while prone with up to 102.3 kg of weight positioned on their back. The second part of the study was an investigation of max· imally struggling subjects while in the PMRP on cardiopulmonary measurements to determine the effect of PMRP on ventilatory function. Methods Subjects Thirty volunteer male and female subjects were recruited to participate in the study. Exclusionary criteria included any history of pulmonary or cardiac disease (as screened with the Physical Activity Readiness-Questionnaire), current recreational drug use, or other significant illness or disability that would limit the ability to perform the exercise regimen required for the study. Each po. tential participant was screened hefore testing by a physician in· vestigator to ensure that helshe was free of acute illness or injury. Also, urine specimens were collected and tested for the presence of the major metabolites of common drugs of abuse (i.e., phen· cyclidine, benzodiazepines, cocaine, amphetamines, THe, morphine, and barbiturates) and tricyclic antidepressants (Triage@ Dru~s of Abuse Panel plus Tricyclic Antidepressants Test, Bio· site Inc., San Diego, CAl. Individuals were excluded if the im· munoassay detected any of these substances in the urine. Informed consent was obtained from each individual before participation, 171 172 JOIJfIIW. OF FORENSK: SCIENCES .-I aIb;ecb: wbo ~ the Itudy Mft! I i - 6. .1 com~ 'Ibe ~ ...,. cIcaip &lid pcICDooI ""' cP~ by Ibc Dkco SCIlc UrdYC:l:li.ty ad !be Urd'l'Cl'lily of c.liC0!Diiw, Dieso Imtirutioaa1 11m- Bo.oI (IIlB). mel prot' , ... " " ' followed in accmdaDa: with the cdDcaJ.1taDdardr. caNi"''''' br IlIe IllBI. s.a s.... PIUt J-Politicll tlIId WC!i8ht EJfteU 0It Ma:tiIIwl Vollmulry VMrilarIcM (atVV) The finI ~ oflhlll\udy evalllllld the effectJ of IUbjllCtl in the PMRP .Ild .beD prone trirh v.ying trt:i&Ja I:. tile t.:k upoo the MVV. A Ucea.od AmericaD 'JborIc:ie Soclcq (Ai'S) ~ ..... haicD, ,.,."..... od Il11 MVV tcaIiDc (UodGnphice Cl'XJD 5yIU!IIl, MedkaI <lnFIDcI c..p.noaa, St. J>.1. MN). .-I eEb IriaI _ repe&Imd III: leu!: hrEe.. ~ were obbiIB:I ill ac.. CCll'dEI::e wi1tI ATS ilbiDitlidi fm iCfC' dlriNlicy f;Dd ~ (16). bw MVV d.ca ""' zamatm:d h ... bc:ipt. padcl', mel (l7). MVV w.. meuured with rubjrdl Rart.d IIl4 .bile i1I. the PMRP. Ia..sdi!iDD, MVV wulDlCllllled with Plbjoeu pMIC and thme d.i.ft'.nm YmiJhts plIced OIl the back. The onim' of theIe IIlfIUUI'lmlIalta WIll; nndomly choIeD to !IIIpte pouible ordm effcett of rcpcdii'¥e tcltiJI&. For tile ICIlCd potItion, II!bjcetl !1It OIl a chair wiIbout n::IInIird wilh !heir Ccct flat CD IDe azOIaId and tbrtir bad: upript .,.mat Ibe cbir baclL DvriDr Ibe PMRP trW, ~ _ pUud prtlIIC I:. I D - * mat with the bead rumcd II) !be Ddt. 'Ik ~' wriIU IUId IUIkb _ ti&bdJ bowId IopdJcr beblDd !be bid: IIIiD& • maniaIa& faIIric cd (H&. 1). n. __ -' .om _ ~ IMJl.a !up!B. nil poIIitioa. 1n.I Ii.-.r iii thI: PMRP t.m ill pre..... IIIIdB &lid _ ftIP(IrtI (11). Far chc prtlIIC tria1a wi.ch wciahta. ali&bt (LW). mocknlll (MW), mil bcayY wQg.ht (HW) in the Corm cC caD"" bq:l. eath &led wtlb. lI.4q aI ieMllbat, were plM:ed 0Il1vbjecu' bac:b. The C&IlTU bqs were eymIy poIitlMod ~ the portarior IbfVIden and mid bIc:1I: lad !ICCarOd by a ~madc 'ICII _ by the lubja:! that had IlnIpI to Imld !ho CIlIa".. hap in place. N we thIDized !hat law .mu.CUlI£Id: af'fIdab vroald .pply leu wclJbt 10 the bIck when rertraiJliD& a llJbIet t.diridual, we \IICd difforiq wei&btI depcading OIl tilt rubject'. -.eiPl SubjrI:b.tao weiJbed" tb. 68q __ 1MIId wilb 22.7:q (LW). 56.1q (MW), .-I !iO..5Iq (HW) wbile IIIlhjec:IIa weipin( IIXft thao 68q 1i'I:n ~ IIriIII lot.lq (LW). 6&.2q (MW). aDd 101311 (HW). Ia &II fM:. .. MVV . . . . 'ed l1ICII _.M., i.........iwJy . . ~ the ~ ~ the Iriah, IUb;cctI aced ia a.-d p;IIitioD. fm c. tSmiD. -,. Ptur l--OvrIl~ 111,_,-. DIulIfI JI4rtoroal .Pot Iht: ~ put of the Itady, oxygca OOZI~ (t'Oil, miIlule VlIDIi1aIiOl1 (li's; ~ rate x tidal vo1ume).1I!Id bHrt rate (HR) 1nIn: IDIWIInd dllriD8 a 6O-111C mnjm.J ItrQQIe while lubjCCU were.ill tho PMRP. Pulmonary Vo,; il a _ of I)'Jtcmie CII;y~ udllatiOD, .hicb:rcl1cctl the mctabalic ~ .., !he muimel VO:t (m-J quaoli1illl the .subic capadIy. Subject. were placed pmt'lII 011 a ~ dr;,Med c. 4Ocm:&om the 1Ioor. A flcc DIiIIIt Wh a 1ow"-t:eU!~ two-WlY brcaIhlJI.a ftlve 1n.I pI.ocd ova the IIIOIIII:h aad - . . aDd Ik..,ja:t _ pnriri .... aacb Iba!: IIiII'I:I« bAd ...... O'IU the atp of!hc II)I!hA 1KJIIld _ arI.l:e . . IIoor Of ~ ~. the IlrUQle. AIr. . . aft ad appliod 10 110 edp ~thI: mMt_ UICd. to preYCDt m kIIb. The .ubjcx:u.' wriltl mil &DkIca .en: tiJhtly ""-' ~ behind the bad: t.hIilK to the PMRP described ahn\tr). Subjed:a then ~(hc4 a mvimal IIIrIIgIc---in an Ittempt 10 eac:ape Cbe IeIIraiJIt- far 60_ wbile cardiopalmnnaty ~U WCft tabm (TrusMaJ. 2400. Parvo Modics IDe., Smdy. IJI). V.t.l encoorapmmIt .... provided IIIrt!qhout eaci1ltrL FIIlIr:Iwioa: Cbe 6O-see ItnIftlc, lubjCCUlaid qgieIIy 011. Iht: !D8llI'e" for 5 miD 10 l'tIOOVC:t W"hiIe ~ IIIir iii be analyzed. Br:foro t«:b lei!, the p i analy.-..-I pncunoI...oo...-_c:alibnud, HR _ moai!arrd a ~ b'wmlitter: Mtvbecl iii !hc cIlIIIt (PolAr 1Dcc!laaict. Eempdc, PiDIMd). All r'll' _ _ _ laxatbi .. zbD ~ uf I.s.- izdcznJa. ftJr ~ !he _ cmtinp'lrnm-y .....-en _ .d !XJQll_""" .au, ~ .-ru... a 'Nn';.w ~ tal 011 a tqW* .,.. I\iIjecU pcdarmed a tradmilI mat at 1.61 mit«. whilD~_ IIlDIW)' ~ wm: IIICUIIlaI. The initial. cIGn&ioa _ 0'J(p aDd .... iDcmued 29&1IYIIrY miaute IIIII.il the mbjlllCt readied YO~ fati&uc. V~ CDeOIID.JC!DCII- Uo provided dllJiaa 1cItiDJ· 'Ibe crlIcWi far reaeI!iDa Vo:-. wczc acIdoviD&: (1) a p1atcmiD ~ (::I: 2m1JkWmiD); (2) II1HR::I: 15bpm oflht: l i p pmdictM mesb".1 HR; mdIm: (3) a ~ em..... n.tIo (RHR) fI 1.l0 or abo'¥e. All Albje<:1:l .meved at leut two fI the a:iIa:i&. """"'" ..... . . Toanalyome. dledl'«:tolthePMRP ad weight brleOD MVV a 1JDD..• •y I , "IJt lklzImmni'II_~ a' L .. ANOVA _ . . . . . Inl I'wt "" were 11-'. iii foUaw up Id",it\cont dl.fI'.erer........ CIt "'.. let 0.05 far .n 1er15 fI signifielnec. All IItItbtital analylel ware pedormfld IIIiDa SPSS v. 11.0 (SPSS (aQ" Cdcqo, a). N'.-y,fi.. pIIlCIII1 confjdrwp.. inllnl1l calculated to ~ MVV v.J.uea ia the vmoos pnrJtI_ ..,jib the: predicted MVV and to MVV mcuurcd in Iht: ICatod politUJa. To analyome. CIrdiopulmaaIry meuarcmtDta dllliq ml.·lm,! ItragIe, we cnmpu.! ~ WriDa; PRMP rtrug\tI with iii aMelI the impIct of rsIniDt I:. OlIJPIl nwmwpiOl'l. lA .dditioa, we CIOCq*od VII aDd Vr/MVV ratio to _ r:apKi1J I11III ~ dIDq: PRMP IInIaJc IIId die mui mel caw ....... ............ --- m.-'*!Cry Thirty.ubjDda (15 mm mil 15 _ ) "II]( ':1 1 the fint part of Iht: study " .... ini ... the dfectI of !be P'MRP and ~Jht 011. MVV. Db from tMl lubjeett 1i'I:n t:lduded from die..". MICHALEWICZ ET AL. • PHYSIOLOGICAL DEMANDS DURING RESTRAINT TABLE l-Subject characteristics (mean ± SD). Total (n Age (yea:r) Weight (kg) Height (m) BM! ~ 30) 24.5 ± 3.5 72.6 ± 14.2 1.72 ± 0.09 24.5 ± 3.5 Men (n ~ 15) 25.0 ± 4.2 81.4 ± 13.9 1.76 ± 0.10 26.1 ± 3.5 Women (n ~ 15) 28.9 ± 6.2 63.9 ± 7.3 1.67 ± 0.06 22.9 ± 2.7 because they were psychologically unable to tolerate restraint (neither subject left the trial for any complaint other than they were frightened of being restrained. Both these subjects elected to withdraw from the study before the exercise period commenced and before any data were collected). Subject characteristics are presented in Table 1. All subjects were healthy and at least moderately active [~ = 50.2 ± 7.8 mIJkglmin; body mass index (BMI) = 24.5 ± 3.4]. Overall, 50% of the subjects weighed less than 68 kg, which included 12 women and three men. Part i-Position and Weight Effects on MW The results of the MVV measurements under various conditions are presented in Table 2. Because there was a significant departure from sphericity (p = 0.001; E = 0.646), Greenhouse-(Jeisser adjusted values were used to evaluate the significance of the main effects. Confidence intervals that included 100% were not considered different from the predicted or the seated MVV. MVV in the seated position was 156 ± 38 Umin, which was 122% higher than predicted (CIg, = 104-140). Conversely, MVV while prone with the HW was lower (85%) than predicted MVV (Clgs = 72-98). All measured MVVs differed from each other (p<0.001), except the comparison of the PMRP and MW trials. MVVs of the treatment trials were also compared with the seated MVV. MVV of the PMRP trial and when prone with MW and HW were significantly less than the seated MVV, although the C1gs from the LW trial suggested no difference from the seated MVV (CI.s = 77-100). Part 2-Cardiopulmonary Measurements During Maximal Struggle Valid data were obtained from only 27 subjects for this part of the study. The struggle was physically difficult for subjects. In spite of continued verbal encouragement, the intensity of movement was visibly waning in all subjects by the end of the 60sec trial. This was supported by the RER---<:alculated as 2: V02-that averaged 1.16 ± 0.14. Only one subject, a female, failed to achieve an RER greater than 1.05 during the maximal veo 173 i ': ;=_E ~ 8070 • • b- -I!l.A, 6", 'A'A, ~ '6 'A, ~ '1 : ~ 40 Z 30 l 20 '5 10 ..J:I ~ 6,~ '6'A'6' Restraint Slru I O~~W~.~ o 60 A HR ' 6 '6--b--6'6'A'6'A va, Passive recovery ____~____~ ____ ~~__~__--, 120 180 240 300 360 Time{s) FIG. 2-Percent differences ofcardiopulmonary valuesfrom peak treadmiU values during and after the restrained struggle trial. restrained struggle. Values in excess of 1.0 indicated that an individual was hyperventilating, and the values observed at the end of the maximal struggle were sintilar to those observed at the end of their maxima1 treadmill test (1.21 ± 0.07). At the end of the struggle, Vo 2 and VE were 40% and 42%, respectively, of the peak values achieved during the maximal treadmill test (Fig. 2). Also, HR at the end of the struggle was 84% of peak HR from the treadmill test. The VEIMVV ratios (using MVV measured in the seated position) of peak VE during the treadmill test and at the end of the restrained struggle were 89% and 44%, respectively (Table 3). Discussion Although sudden death has occurred in individuals placed in the PMRP, the cause of death and whether body position was a factor remain controversial. Some have suggested that PMRP prevents adequate chest wall, abdominal, and diaphragmatic movement, leading to hypoventilatory respiratory compromise and risk for death from positional asphyxia (8,18). Prior studies in healthy subjects have found no evidence of significant hypoventilation when subjects were placed in the PMRP (2,8). Our results in this study appear to support these findings. MVV in all of the treatment conditions remained above 80% of predicted, well within the normal range (19,20). While by itself the PMRP does not appear to compromise ventilatory capacity unduly, the restraining process also frequently includes applying force to the back. Weight applied in the prone position has been hypothesized to further compress the chest and TABLE 2-Measured MVV values (n = 30). MVV(Umin) Position Seated PMRP LW MW HW Mean ±SD 156 ± 128 ± 137 ± 122 ± 109 ± 38 29 27 31 28 Percentage of Seated MVV Range 75-243 65-193 85-189 61-197 57-167 % 82" 88 78" 70" Percentage of Predicted MVV a., % C1" 68-96 77-100 64-93 54-116 122 100 107 95 85' 104-140 99-102 97-117 87-103 72-98 "'Below MVV measured in the seated position. tBclow predicted MVV. Note: All measured MVV values differed from each other except for the PMRP and MW comparison (p< 0.001). PMRP, prone maximal restraint position; LW, prone position with low weight (22.7 or 34.1 kg) applied on subject's back; MW, prone position with moderate weight (56.8 or 68.2 kg); HW, prone position with heavy weight (90.2 or 102.3 kg); MVV, maximal voluntary ventilation. For the three conditions in which weight was placed on the back, the lighter weight was used for subjects who weighed less than 68kg (n = 15) and the heavier weight for those who weighed more than 68kg (n = 15). 174 JOURNAL OF FORENSIC SCIENCES TABLE 3-Cardiopulmonary peak values during maximal treadmill test and at the end of the restrained struggle (n = 27). Restrained Struggle Treadmill Mean± SD liO:z (mI1kgImin) VB (l./min) VT (L) RR (brea1llslmin) IIR (beatslmin) RER 50.2 140.1 2.8 56 190 1.21 ± ± ± ± ± ± 7.8 36.7 0.7 8 12 0.07 Range 29.1-65.8 84.2-206.5 1.8-4.7 39-70 166-221 1.09-1.35 Mean± SD 19.8 ± 57.6 ± 1.0 ± 60± 160 ± 1.16 ± 5.4 23.3 0.4 14 19 0.14 Range 11.6-30.9 29.4-113.3 0.6-2.3 35-1!8 105-196 0.92-1.35 HR., heart rate; RER, respiratory exchange ratio. abdomen, which might lead to hypoventilatory respiratory compromise (IS) and diminished ventilatory function to the point of asphyxiation (20). One of the goals of this paper was to isolate the effects of weight applied in the prone position. Our results indicated that with 90.2-102.3 kg of weight applied to the back of our subjects, MVV was decreased to 8S% (CI.s = 72-98) and 70% (CI.s = S4-4l6) of the predicted and previously measured MVVs, respectively. Despite these decreases, these MVVs are still within published Clos for men (12) and women (14). Even though the decreases in pulmonary function as a result of weight force applied to the back remain with the normal clinical parameters for a healthy person at rest, the circumstances of PMRP-related sudden death cases are very different. Sometimes, the victim has been involved in high-intensity exercise (e.g., running, fighting) before being restrained and, afterwards, will continue to resist the restraint violently. It has been suggested that under these circumstances, oxygen consumption may exceed ventilatory capacity in individuals placing them at risk for respiratory compromise (21). All such, we also measured V0 2 and VB while in the PMRP and compared them with similar measurements from maximal treadmill tests. Our results indicated that with maximal struggle while in the PMRP, V0 2 and VB were less than 42% of peak values obtained from a maximal treadmill test. In general, the most metabolically and ventilatory demanding type of exercise occurs when large muscle groups resulting in the highest work in a rhythmic fashion (e.g., rwming, cycling). It is likely that PRMP limits subjects from using these large muscle groups in rhythmic movements, thus resulting in the low V0 2 and VB we observed. Ventilatory constraint is often determined by measuring how close VB at maximal exercise intensity approaches MVV (18). At V02max, individuals with normal lung function ventilate at 6070% of their MVV (22). Accordingly, in our study, the peak V"; MVV during the maxirual treadotill test was 72% of the measured MVV. On the other hand, during PRMP struggle, VB IMVV was ouly 36% of the measured MVV. Our findings of clinically normal MVVs with PMRP and prone weight in phase I, as well as the lower and VpfMVV during PRMP struggle suggest that our subjects appeared to have adequate ventilatory reserve when struggling while restrained. Furthermore, the extremely low V"; MVV ratio at the end of maximal struggle, compared with the actual MVV measured with weight on the back, suggests that should weight be applied while individuals were in the PMRP it would be well tolerated as well. Clearly, this remains to be proven in future studies. Based on these findings, as well as previously published studies, we suggest that factors other than ventilatory failure associated with the restraining process may be responsible for the sudden unexpected deaths of restrained individuals. Although autopsy va, va, evidence is often unrevealing as to the cause of the death, those individuals who die at times seem to succumb suddeuly (23), which is a pattern generally inconsistent with a respiratory death. Some individuals have been repurted to die suddenly while restrained without force applied to the back (6,7,10,23), restrained in a supine, sitting, or side position (9, 17), or even without being restrained (24). Other factors, such as excited delirium, drug intoxication, stress, trauma, and catecholamine hyperstimulation. are considered to be the most likely factors in these sudden deaths (2,3,19). In addition, studies indicate that many of these individuals have an abnormally enlarged heart on autopsy, likely related to chronic stimulant drug abuse (S). Not only is there a greater risk for cardiac dysrhythmias and sudden death in those with cardiomyopathy, but recent investigations suggest that individuals with this condition have decreased capillary density in their endocardium, placing them at risk for chronic and perhaps acute cardiac ischemia (2S). Our results, as well as those of others (2,3,IS), suggest that in deaths associated with the PMRP, factors other than ventilatory compromise may playa more important role. Clearly, this study has a number of limitations. First, our subjects were young and generally healthy and may not reflect the population of individuals who are restrained in the field sening. It should be noted that the baseline-measured MVVs of subjects were 122% of predicted, suggesting that our subjects were both highly motivated, and had a high aerobic fitness level (26). In the actual field setting, underlying medical conditions and other differences from our subject population (e.g., age, weight, etc.) might theoretically influence the outcome. Second, we could not reproduce all conditions during which this type of restraint method is used in the field. In particular, while we had subjects restrained and maximally exening themselves, we could not reproduce the psychological or other physiologic stresses associated with a field pursuit, struggle, or trauma). During the trials with weight applied to the back, the weight was disttibuted evenly over the back, unlike in a field situation in which force is applied to the back frequently with a knee that focuses the force over a smaller area. In addition, a small number of our subjects did opt out of the study out of fear of the restraint Clearly, in the field setting, individuals are unlikely to have such a choice as to whether they are restrained or not; however, it is difficult to understand how such factors might affect ventilation. Therefore, how such factors may or may not contribute to these deaths will require future study; however, an animal model suggests that restraint alone (without affecting an animal's ability to breathe) increases the death rate in animals treated with cocaine (27). This implies that the physiologic effects of restraint involve more than ventilation alone, which is consistent with the results of this and prior studies (3,11). Second, we placed weight on sub- jects' back when MVV was measured in the prone position, but no force was applied on subjects when positioned in the PMRP. Such a model has its limitations and does not necessati1y duplicate the sequence of events that may take place in any given field situation; however, we do feel this represents a first step in investigating a very complex arena. Third, the exertion and struggle of our subjects in PMRP was of short duration and also may not reflect a field situation where prolonged struggles can occur. Furthermore, our subjects exerted themselves on a voluntary basis, although they were verbally encouraged by the investigators to struggle as much as possible throughout the I-min periud. This voluntary nature may not exactly reflect field situations where individuals are often under the influence of drugs or are mentally incapacitated. On the other hand, our subjects did exercise to exhaustion or near MICHALEWICZ ET AL. • PHYSIOLOGICAL DEMANDS DURING RESTRAINT exhaustion and their significantly increased HR suggests high levels of exertion on their part. Finally, of course, none of our subjects used illicit drugs. However, none of the illicit drugs frequently used in the setting we are trying to simulate have an effect upnn ventilation and therefore it appears to be more likely that the role such drugs play in these deaths is through some other mechanism than their effect upon ventilation. There are other factors in the field that that are also dil'ferent from our serting that might theoretically affect our results (e.g., a gymnastic mat rather than the actual outside surface, and the even distribution of the weight force we used across the back rather than it being localized to a smaller area); however, it will await future studies to determine wbether these factors play any role in such deaths. In sururuary, this study attempted to investigate the impact of varying weight force upon the back in healthy individuals in the prone position. We recognize the dil'ferences between the laboratory setting and actual field conditions; nonetheless, we found no clinically important restriction of ventilatory reserve when subjects were placed in the PMRP or when prone with up to 90.2 or 102.3 kg of weight on their back. Likewise, when subjects were maximally struggling for 60 sec while in the PMRP, there were no clinically important limitations of metabolic or ventilatory functions. Based on these observations in healthy subjects, we conclude that PMRP and prone pnsitioning with moderate weight force on the back do not in and of themselves restrict metabolic or ventilatory demands to any clinically important degree. As such, factors other than isolated ventilatory failure should be considered wben evaluating deaths occurring in the serting of restraint in the field. Acknowledgments The authors wish to acknowledge the contribution of Mohammed Najeed, RT, UCSD Medical Center, for performing the MVV measurements in this study. This study was funded in part by an unrestricted grant from the University of California, San Diego, Department of Emergency Medicine Research Fund. Portions of this study were presented at the Society for Academic Emergency Medicine, New York, NY, May 2005. References 1. Chan TC, Vilke GMt Neuman T. Reexamination of custody restraint position and positional asphyxia. Am J Forensic Med Pathol1998;19:201-5. 2. Chan Te, Vilke OM, Neuman T. Restraint position and positional asphyxia. Am J Forensic Med Pathol2000;21:3. 3. Chan TC, Neuman T, Clausen I, Eisele I, Vi!ke GM. Weight force during prone restraint and respiratory function. Am J Forensic Med Pathol 2004;25:185-9. 4. Eisele J, Chan T. Vilke G. Neuman T. Clausen J. 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