Effects of high frequency chest compression on respiratory system mechanics in normal subjects and cystic fibrosis patients

OBJECTIVE: To investigate the short term effects of high frequency chest compression (HFCC) on several indices of respiratory system mechanics in normal subjects and patients with cystic fibrosis (CF).

The patient can either breathe spon taneously during continu -llUS HFCC o r I !FCC can be ac tivated during expiratory hn:athing nKmoeuvrcs.Enhanced mucoci liary cl eara nce has been allributcd.at lc;1st in rart , to high osc illated air flow in the airways ( 1-3 ).
The concept or HFCC was first dc snibed in I LJ66 by Beck (4).He described the use or a belt-like bladder that surro undc<l the upper ,1hdomen and lnwer thorax which was inllatl'll to _1 () cm 1--1:20 and the air osc illatl'd al _,() Hz. lnnea.,L's in mucus expcL•toration.vital eapacity and maximal voluntary ve ntilation ,ve re reported for a patient with d1roniL' obstructiw pulmonary di sease.In, inv or its reported ellcctiveness and the availability or a commercially avai lable dniL•c .the clinical use or HFCC is li kel y to increase.Howevn.st udies investi gating the ,hort term effec ts of HFCC on re spiratory system mechanic s in human subjects haw not bee n rcrortcd.
We studied nmmal su bjects and CF patients to determine the effects of HFCC on external chest wall pressure (Pnv).pl eura l pressure, end-e xpiratory lung volume (EEL V ).oscillated tidal volu me (Vose) and oscillated air flow (Yosc).Some important differences were found between no rmal subjects a nd C F ratients.

PATIENTS AND METHODS
Subjects: This study wus approved by the local ethics commillee and signed in fo rmed conse nt was obtained l'rom all subjects.Tl'11 normal subjects :md I I CF pati e nts were studied.T he normal subj ec ts comprised nonsmoking volunteers 24.2±3.8 years old with normal lung function.The C F patients averaged 23.4±6.7 years.with forced cxriratory volume in I s (FEY t) rang ing from 21 to 11 I 'k of rre<lictcd va lues (7).!\II of th e CF patient s were in a L'iiniL•al ly stable phase or the ir disease an<l daily routine and medical treatme nts were unaltered for this study .
Lung function: Pulmonary runction tests were done within I h before th e HFCC study.Base line lung function, whid1 incl uded spirometry and lung volumes.was detenninc<l in all subjects.For the CT patients.lung volumes were measured with both the he lium <lilution and body rkthysmugraph methods, but onl y helium dilut ion values were obtained for the normals.A Gould 2400 PFf system and a Gould 2800 Autobox were used (Gould, O hio).Baseline heli um dilution funct ional res idual capacity (FRC ) measure me nts were ta ken in triplicate with at least 5 rnins between measurements.T he average of these three values was used to determi ne the EELV during HFCC.Anthropometric and base line lung functi on data fo r the two study groups arc prese nted in Tab le I with normal values fo r spirome try and lung volumes ob -tained from Morri s ct al (7) and Crapo ct al (8) , respec tively.
HFCC system: The T hAIRapy system (America n Biosystems.Minnesota) was used for HFCC.T he system contains an air-pulse generator con sisting of a high vol ume regenerative blower that provides a constant bac kground pressure and a rotary valve that produces alternati ng pos itive and zerogauge pressures.These pressures are applied to a pne umatic vest via two nexible hoses.The det1ated vest covers the ent ire thorax and exte nds to the iliac crest.In this study.the vest was fitted to each subject by having the subject in hale to total lung capacity and then securely fa stening the vest in place.Th is allowed for subjects tu ta ke deep breaths without being restricted by the noncompliant vest.The vest' s background pressure can be regulated and the frequency of the pu lse pressures is controllable bet ween :' i and 25 Hz.Ten of a wide range of pLlssi ble pressure :.ind frequency combinations were investigated: 5.I 0. 15, 20 and 2.5 Hz osc illation frequencies and the lowest ( LP) and hi ghest (HP) background press ure sellings.Activation o f the HFCC sy ste m is controlled by a 'dead man' s' switch desi gned for control by the patient.but in thi s study it was controlled by the investigator.

Lung mechanics measurements during HFCC :
To study the short term e ffects of HFCC, a system capable of simu ltaneous ly measuring several indices of respiratory syste m mechan ics was de vi eel (Figure 1 ).These included Pew, esophageal pressure (Pes), changes in EEL V, spontaneous tidal volu me (V1-).spontaneous breath ing frequency Pew was meas ured via a 120 cm long and 0.20 cm inner diamete r catheter wi th a latex balloon covering its tip.The balloon was 4 cm long and 2 cm in d ia meter and partially in fl ated with 1.5 mL o f air.The cathete r was taped to the external chest wall so that the balloon was 2 to 4 cm below the lc rt pec tora l muscle and under the vest.A Validync MP 45 (±50 cm H20) (Validyne Eng ineering Corp, Cali fo rnia) pressure transd uce r was used and the frequency response of the chest wall ball oon system was flat through 25 Hz.
Pes was measured using the method described by Milic Emil i et al ( 9).An esophageal balloon was connected lo a Validyne MP 45 (±50 cm H20) transducer via a lJ() cm long and 0.15 cm inner diameter catheter.T his was then cunncctL'd to a 60 cm long 0.50 cm inner diameter catheter.The response of the Pes meas ure ment syste m was frequency dependent and correction factors were used to account for this.T he correction fal'lors ranged from 1.00 to I. I :' i at S H/ and 15 Hz.respec ti ve ly.
T he immediate HFCC induced changes in EEL V, Y.r and f were measured by a rebreathing system (   T he subjects brealhed for I min from the closed ci rcui t without .superimposedHFCC to obta in the baseline spirn .. gram.Then.w hen HFCC wa s sta rted.th e increased exlernal chest wall pressure caused a shirt in the spirogram 10 a ne w and stable position which was measured as EEL V. H FCC conlinucd ror 2 mins.EELV du ring HFCC (exp1nscd as r;cFRC ) was ohlamed by use of the baseline helium dilutilln FRC and the H FCC induced change in EELV.
The e xhal ation pathway ,1l'ted as a low pass filter so lhal rapid volume changes at th e mouth cau sed by HFCC (Vo.,cl could be recorded a.s pressure changes in a 20 L isothermic chamber located nl:.ar the mouth (Figu1t• I).The Vose measuremen t was frequency dependent and correction l'actllrs were used to obtain lhe true Vose.These correction faclors Can Respir J Vol 2 No 1 Spring 1995 ranged from 1.00 to I .'i'ial 5 Hz a nd 25 Hz.respcl'livcly.The correct inn J'al'lors wne nb1aincd hy osL•illaling the isolhcrmic chamber with a Malrix mcillatm urnnected L'.ilhn al the mouth piece or dircc lly lo lhe L •lwmlw r wilh a shorl piL' L' C or wide bore tubin g.Airflow throu g h llll' brca1hi11 g system.eilher inspiration or L'xpiralion.had no ellccl on VosL• measureme nt.
Since Vose apprnximaled a sinusoidal wave.th e mean Vose durin g HFCC was cakulaled hy multiplying lhe Vose by half the oscillalion rrequency ,11 whil'l1 it was measured.

RESULTS
Esophageal pressure was ohtai1ll'd for ;tll or lhL' normal subjects hul rm onl y seven or till' .l l CF palie nt s.Four patients were unable lo tolerate the esophageal balloun.In this study.a wide ra nge o r pres., urcs was delivered 10 the che st \Vall due 10 the differe nt backgrnuncl pressure .,ettings (LP and HP) and the five different oscillation frequencies used.Pes-pp (cm H20) Figure 3 The transmission or pressure from outside the chest tu the pleural space is shown in Fi g ure 2A.At FRC, without HFCC, Pes-m ave raged-4.8cm H20 in the normal suhjccts and -2.7 cm H20 in the CF patients.During HFCC , the changes in Pes-111 were sim ila r for th e normal s and C F patients, a ve raging 0.092 cm H20/cm H20 and 0. 103 cn1 H20/cm H20. respectively.
As expected, increasing Pcw-m decreased EELV (Figure 28 ).EELV decreased s imilarly for both normal subjects and C F patie nts.T he lowest EELV seen in the norm al group was 66% of the pre-H FCC FRC a nd in the CF group it was 50%.In both instances the lowest EELV ocrnrrcd at !he HP 25 Hz setting.The lowest six values for EELV, seen in Figure 28.came from the same CF patient.
The ahility lo transmit HFCC pulse pressure~ through the Oscillation Frequency (Hz) Figure 4) chest wall to the pl eu ral space is indicated in Figure 3A.Increasing Pew-pp was associated with higher Pes-pp in both the no rmal and CF groups .On average for the IO experimental HFCC settings, Pes-pp was 50:±69'0 of Pew-pp in norma ls and 50±5% of Pew-pp in CF patients.Each of the regressions in Figure 3A was si gni ficant (P<0.00I) but they were not significantly different from each othn.The effects of Pcs-pp 011 V osc are shown in Figure 38.Although there was a large scatter of points for both the normal subjects and C F palirnts the linear regressions were signilicanl (r=0.48 and r=0.29.respectively).For a given Pcs-pp the Vose.measurL'.dduring spontaneou s expiration.was approximately three times greater in normal s than in CF patients.
The effects or oscillation fre4uem:y on oscillated tidal llow rate in normals and CF pal ienls during spontaneous expiration arc shown in Figure 4.Although the normals had much hi gher Vose than the CF patie nts, the highest Vose occurred at 10 H1 in both groups.A similar pattern was seen for Vose measured during spontaneous inspiration except that Vose peaked at 15 Hz and that Vose.like V0sc. was higher during spontaneous inspiration than during expiration.especially for the CF patients.
Fi gure 5 shows the effect of airway obstruction, as assessed by FEV1, on Vose at HP and 15 Hz.Regardless of whether Vose was measured during spontaneous in spiration or spontaneous expiration.decreasing FEY 1 decreased Vose.T hb e ffect was larger during spontaneous expiration even tho ugh the measure ments were made al the same lung volume as during spontaneous inspiration.A similar effect of FEY I on Vose was observed for all or the olhL'r HFCC   pressure and frequency comhinalions investigated m this study.

DISCUSSION
We found that both normal subjects and CF patients had decreases in EEL V during HFCC.At HP and al the hi gher oscillation fre q uencies.EELV decreased to less than 80% of the baseline FRC.We also found that transmi ssion of HFCC induced pulse pressure across the chest wall to the pleural space was simil ar for normals and CF patie nts.However.conversion of Pes-pp to Vose and lo Vmc was less efficient in CF patients than in normal suhjccts, especially during spontaneous expirali<lll.
Patients with airway obstruction tend lo hreathe al an elevated FRC, which minimizes the degree of airway closure ( 10) and it is generally considered undesirable rm them to decrease EELV .Since HFCC decreases EEL V one might expect widespread airway closure in CF patients who have significant airway obstruction.Although we did not observe a decrease in arterial oxygen saturation during short term HFCC. it should be recall ed that F102 was kepi al approximately 0.5 during the studies.Typically.HFCC treatments arc conducted over 30 mins ,vhile the patients breathe room air so it is possible that hypoxcmia could develop in that instance.On the other hand .Vr increased during HFCC and it is possible that.at end-tidal inspiration, any airway closure is overcome and gas exchange may be reasonably well preserved .Also.HFCC, especi ally al the lower rrequencies where Vose i.~ highest.may be beneficial lo gas exchange ( 11,12 ).I lowcvcr.animal studies sugges t that any bcndicial effects or chest wall oscillation on gas L'xchange require larger, anu nol srnaller.lung volumes ( 11 ).Studies uuring HFCC sho uld be done to determine whether the decreased EELV causes any signiric1nt e!Tecls on gas exchange .
We shmVL'J ( FigurL' 4) that Vose, measured during spon-1ancm1s expiration, was consiue rabl y lower in CF patients than in normal subjects and there was a iL'ss well defined •best frequency" ror Vose in the CF patients.We be lieve this difference in Vose bt:tween th e two groups is related to the higher airway resistance in CF patients.Increased airway rL'.,istancc causes a larger time constant and.at a given Pespp , Vose will be lower.The decrease in EELY would be cxpeclL'U to cause ;1 i'urther increase in time constant ( 13) and this m;1y contrihutL' to the lown Vose that occurs with increasing HFCC rrL'LJUL'ncy in both normals and CF patients (Table 3).
Figure 5 .shows that Vose change s with FEY I anu with thL' phase or spontaneous breathing.During inspiration Vose was , without exception , higher than during expiration for both norma ls and CF patients .Also. the lower the FEY I the greater was the disparity in Vose bL'tWeL'n inspiration and expiration.We bclil'.vethis illustrates the influence or ple ural pressure on airway resi stance.'fhe measurements in Figure 5 were obtained at the midpoint, or inspiration and ex piration when lu ng volume and elasti c pressure were similar.Therefore.diffen:nces in pkural press ure between midinspiralion and midexpiralion represent the pressure required to overcome resistance ( I..J.).For inst;1nce, at high background lll'L'Ssurc and at 15 Ht, this resisti ve pressure.obtained from Pes -111.aVL'ragcd 5.l) cm H20 in the rwrmal subj e cts and 1 • 2.2 L'Jll H20 for thl' CF patirnh.The higher airway res is-lilnL'L'.augme nted hy a more positive pkural pressu re, cau ses Vose to be lowLT uuring expiration in CF patients than in nonnals.T his effect is g reater when till' dcgrL'l' or airway obstruction is greater.
If HFCC-inuuced increases in murns cb1ranc(' dcpl'nd 011 \lose.then l)lir results suggest that Cl; palil'nts with 111odcr;1tc or seve re airway obst ruction may Ix-kss hc11c!'icially af'f"cclcu by HFCC treatment than patients with less airw:,y o bst ruction .However. this 111ay be an ovnsimplistic view of' the factors that affect murns ckar;1nn'.It is possible that the lower Vose in CF patients with scvcrl' airway obstruction.when accompanied by the dcncased LEI ,V that should further narrow the small airways ( 13 ).ma y incrcasl'.air!low velocity more than Wlluld be predicted from till'.low Vose.Recently.Arens et al ( l 6) showed that daily HFCC thcrapeu-tiL' sessions g iven over a two-week period did improve lung function in CF patients with sl'.vc re a irway obstruction.
A lthough we did not assess the effects of HFCC on mucus clearance , the highest Vose occurred ;1t e ither 10 or 15 Hi in the no rmal subjects and CF patients.Th is is similar to the l'rcqucncy found most effective in enhancing mucus clc;,rancc in normal ca nine lungs (I).
Figure I) with separate inspiration and exhalation pathways (O hio 840, Can Resp1r J Vol 2 No 1 Spring 1995