La percepción del turista permite evaluar la calidad de

Introduction

Meaningful in te rp re ta tio n of the re sults presented in th is thesis depends on the v a l i d i t y of study design, equipment, technique and

analysis applied. The following discussion w i l l evaluate each aspect of methodology used in the current in vestigations, in terms o f the

potential influence on measurements made and in te rp re ta tio n o f findings. Population & Recruitment

Since good study design must prevent potential sources o f bias when re c ru itin g subjects, the most ideal recruitment procedure fo r many

physiological studies is randomised selection. However, fo r ethical and p ractical reasons, in th is study recruitment was governed by w ritte n parental consent to p a rtic ip a te . This may have produced an unavoidable bias towards a p a rtic u la r type o f parent who was, fo r instance, in favour o f medical research. However, s t r i c t adherence to selection c r i t e r i a , established before the studies began, ensured that a ll infants recruited compared closely in terms of birthw eight, gestational age and postnatal age at measurement. There was also an equal number of boys and g i r l s recruited in the f i r s t week o f l i f e and w ith in the population who returned fo r fo llo w up (Appendix G; Table 1).

A high proportion o f maternal smoking was reported in the population of infants studied in the f i r s t week o f l i f e (Table 3 .3 .) . However, less infants born to mothers who smoked attended fo llo w up appointments at six weeks. The number of infants in the current studies is too small to discover why there was a greater drop out rate amongst smoking mothers. However, a larger number of infants were reported to have upper

re spira to ry tra c t in fe c tio n (URTI) before the date o f fo llo w up appointment in the parental smoking group compared to non-smokers.

(5 out o f 14, compared with 1 o f 8 respectively. Table 3.4). There was also a larger number o f infants whose parents fa ile d to attend fo llo w up appointments amongst parents who smoked, compared to non-smokers (6 out o f 14, compared with 2 out of 8, Table 3.4) This may have important im plications fo r the planning of epidemiological studies. Recent

evidence suggests that maternal smoking in pregnancy may be associated with impaired lung function in infancy (Hanrahan et a l, 1990) which may

include abnormalities of re spira to ry c o n tro l. Future studies on the development of re spira to ry control may need to consider re c ru itin g large numbers o f infants with f u l l d e ta ils o f the pre and postnatal smoking habits of both parents, so that the effe cts o f parental smoking on the HBR in infants can be established.

For comparisons with and without sedation, infants were not recruited as matched controls. However, since id e n tica l selection and recruitment c r i t e r i a were applied fo r both studies, i t is u n lik e ly th a t re sults were biased by recruitment procedure (Section 3.3, Table 3 .5 ). In addition, a ll infants were recruited from very s im ila r demographic and geographic areas, each comprising a representative cross-section o f social class

(Hackney/Poplar, London). Measurement Conditions

Henderson-Smart (1984) and Hamilton et al (1990) consider i t 'untenable' to perform measurements o f lung function without accounting fo r the potential influences of behavioural state. Examination o f the lit e r a t u r e and experiences from p i l o t investigations confirm th is

b e lie f. Unless s p e c ific a lly examining the influence o f sleep state, a ll studies in th is thesis were confined to periods o f quiet sleep, when behavioural influences on respiratory control are thought to be abolished (Bryan et a l, 1986; Hamilton et a l, 1990). The apparent presence and persistence of the HBR is therefore u n lik e ly to be a ttrib u ta b le to mechanisms o f central o rig in .

The potential contribution o f factors known to influence re spira to ry co n tro l, as outlined in Section 1.4, were acknowledged in these studies. The influence of thermoregulatory and metabolic factors was accounted fo r by performing measurements under c a re fu lly co n trolled lim it s of ambient temperature (21-25°C) and by monitoring in fa n t a x illa r y

temperature, which remained w ith in the normothermic range throughout a ll measurements.

In addition to establishing suitable measurement conditions, p i l o t studies also enabled the optimal number and sequence o f occlusions that could be performed, while achieving maximum re spira to ry s t a b i l i t y , to be determined (Appendix B). Allowing a one minute in terval between

successive occlusions minimised the potential fo r re fle x habituation (Stanley et a l, 1975). This ensured th a t steady-state conditions were re-established p r io r to the next occluded event, thus averting the potential fo r threshold c a p a b ility as described by Younes & Remmers, 1981.

P ilo t studies also provided a useful in d ica tio n o f the v a r i a b i l i t y of measurements, from which prospective estimations o f sample size were made. For a ll protocols except th a t concerning sleep state (Section 3 .5 ), such prospective calculation enabled adequate power o f study to avoid type I I s t a t i s t i c a l errors (Kirkwood, 1989); Limited power of study may in part account fo r the discordance with previously published data (Section 4.3, Table 4.1). In order to adhere to the established protocol (Appendix B) measurements were always performed in quiet sleep f i r s t . Since i t was impossible to estimate the number o f infants who would remain asleep a fte r completion o f QS measurements, the number in whom comparative measurements in AS would be successful could not be predicted in advance. Thus, fo r assessment o f the influence o f sleep state, retrospective sample size calculation was most appropriate. Apparatus

The use of a face mask and pneumotachograph has frequently been reported to produce changes in tid a l volume and re spira to ry frequency. (Fleming et a l, 1982; D olfin et a l, 1982; Haddad et a l, 1988) and/or stimulate trigeminal pathways (Fleming et a l, 1982). I t is therefore l i k e l y that basal v e n tila tio n was increased by the application o f recording

equipment, which had a dead space o f 2.3; 1.6; and 2.6 ml/kg fo r one week, 6 week and year old infants respectively. Without measurements of baseline re spira to ry parameters pre and post apparatus a p p lica tio n , i t is d i f f i c u l t to assess the precise extent to which v e n tila tio n was influenced by the apparatus used.

As discussed in Section 1.3, many slowly-adapting stretch receptors are active throughout the re spira to ry cycle, even in the "uninfla te d state" at FRC. In in fa n ts, stretch receptor discharge, and re fle x a c t i v i t y increases with progressive volume increases (Figs. 3.6a and b). The modest increase in basal v e n tila tio n , associated with facemask

application may r e fle c t some elevation o f COg, with consequent increases in minute v e n tila tio n , and hence FRC throughout the e n tire measurement period. The primary e ffe c t o f modifying COg by facemask application would be an i n i t i a l , s lig h t overestimation o f re fle x response as the applied volume stimulus was increased, and re spira to ry phase-switching altered accordingly (Clark and von Euler, 1972, Gautier et a l, 1973). Thereafter, because the facemask remained in position fo r the e n tire period o f study, one might expect fu rth e r, progressive increases in Vj re s u ltin g from COg elevation and accumulation.

In order to avoid m isinterpretation o f current findings possibly associated with altered chemical drive, retrospective time-trend analysis was performed on data from 10 infants during successive

occlusions. Graphical analysis of mean changes in v e n tila tio n during 5 occlusions in a single measurement period are shown in Fig. 4.1a. No systematic changes in re fle x a c t i v i t y , Vj or T^ occurred throughout any continuous period of data c o lle c tio n , usually la s tin g 10-15 minutes. Although i t is not possible to exclude that some re settin g o f stimulus threshold occurred as a consequence o f facemask a p p lication, i t seems l i k e l y that steady-state conditions were achieved during each

measurement period. Progressive elevation o f COg does not appear to be a primary fa c to r determining the magnitude o f re fle x a c t i v i t y in th is population o f healthy infants. Similar findings have been confirmed by Stahlman and Sexton (1961) and Taeusch et al (1976), both o f whom have demonstrated that despite increases in Vj, as chemical drive increases during COg rebreathing, there are no progressive changes in HBR a c t iv it y during th is time. Furthermore, studies in adult man have also shown no difference in HBR a c t i v i t y during a i r and CO^ rebreathing (Guz et a l, 1966). Any elevation of COg would therefore be u n lik e ly to a ffe c t the v a l i d i t y o f current findings.

HBR

Occlusion number

VT(ml)

Occlusion number

TE (s)

Occlusion number

KEY Infant number

05 10 — ■---- 1 2 ---1--- 27 --- » 43 — ■--- 46 Q-- 61— ^

29 74 Fig.4.1 The influence of occlusion number on HBR response,

tidal volume and expiratory timing in 10 infants during the first week of life.

Although concurrent changes in tid a l volume and frequency occurred

(Section 3.7, Table 3.13), i t is u n lik e ly that in te rp re ta tio n of results regarding the re la tiv e strength of the re fle x between any two occasions was adversely affected by the influence o f apparatus dead space. The

influence of added dead space was s im ila r r e la tiv e to tid a l volume, and body weight throughout the f i r s t year o f l i f e , such that the dead space should have produced an increase in basal v e n tila tio n o f s im ila r

magnitude on a ll occasions. Furthermore, the increase in level of basal v e n tila tio n per se did not a ffe c t the v a l i d i t y o f observations th a t a Hering-Breuer type response was present, since a ll infants

demonstrated marked prolongation o f Tg during occlusion at lung volumes well w ith in the expected basal range f o r a given age (Section 3.7). The resistance of the apparatus (Rapp) used in the current studies was 0.47kPa.1 . s . However, the to ta l airways resistance (Raw) in a new born in fa n t is approximately 3kPa.l~^.s. In healthy infants resistance o f the respiratory system (Rrs) may in fa c t be double t h is . Thus Rapp represented a small component o f the to ta l Resistance, and f e l l below the magnitude suggested by Lopata et al (1977) which influences

occlusion pressures. I t is therefore u n lik e ly to have affected in te rp re ta tio n o f the response to occlusion.

Technique

In section 1.4, several methods to assess HBR a c t i v i t y were discussed. From th is i t was apparent that the airway occlusion technique was the simplest and most r e lia b le approach. I t s non-invasive nature was well tolerated by a ll in fa n ts, and i t was both e t h ic a lly and a e s th e tic a lly acceptable to parents, as reflected by the high recruitment rates and subsequent attendance fo r follow up measurements (Section 3.2 ).

Furthermore, the occlusion technique u t ilis e d the natural in f la t io n stimulus of each individual during t id a l breathing, thus acknowledging the important advice of Hering and Breuer (1868) th a t a physiologic stimulus should be applied:

"whenever it is possible to excite a nerve from the periphery, by as it were, natural means, this should be done before any attempt is made to stimulate it artificially."

Translation by Ullmann, 1970

Criticism s levelled at a lte rn a tiv e techniques include the potential fo r in te ra ctio n from chemoreceptors (Younes et a l, 1974), or errors in assessment of timing changes at occlusion (Kosch et al 1985, Section 1.4). Although a lte ra tio n s in blood chemistry cannot be excluded without d ire c t assessment of blood gases during measurements, there is convincing evidence that such do not occur during airway occlusion fo r a single breath. Recent animal investigations have been unable to e l i c i t a lte ra tio n s in blood gases during to ta l airway occlusion (Green &

Kaufman, 1990) or changes in mechano-reflex a c t i v i t y during prolonged v e n tila to ry e la s tic loads, when progressive changes in blood gas

composition may occur. In addition, when an in fa n t's airway is occluded at end-expiration fo r a number o f consecutive breaths, there is l i t t l e change in hypoxic drive during the f i r s t six seconds (Frantz &

M ilic - E m ili, 1975). Occlusion at e n d -inspiration, with a f u l l lung volume reservoir, would be expected to have even less e ffe c t . As each occlusion in th is study was held fo r a single re s p ira to ry e f f o r t which, except during spontaneous sighs, ra re ly exceeded fiv e seconds, i t is u n lik e ly that chemoreceptor a c t i v i t y was altered. Furthermore, measurements during COg re-breathing have been reported to have no

influence on the a c t i v i t y of the HBR in healthy f u l l term infants

(Taeusch et a l, 1976) or adult man (Guz et a l, 1964). COg re-breathing has also been reported to have no e ffe c t on re s p ira to ry frequency in preterm infants ( M ille r et al 1991), or fu llte rm in fa n ts during the f i r s t week of l i f e (Taeusch et a l, 1976).

Reflex e ffe cts evoked appeared to be s e l f - l i m i t i n g in th a t no p h y s io lo g ica lly s ig n ific a n t differences could be determined in

re spira to ry pattern pre and post occlusion, and no progressive response to successive occlusions was observed (Fig 4.1a). Performance of

occlusions per se had minimal e ffe c t on subsequent t id a l breathing (Table B.4). L^drup et a l, (1992) have recently assessed the influence o f occlusion on respiratory pattern in sleeping in fa n ts . They reported a s lig h t , but non s ig n ific a n t increase in re s p ira to ry frequency from 68 to 71 breaths/minute, and a s ig n ific a n t increase in Vt from 20.2 to 22.6 ml. Both of these changes f e l l w ith in the normal range of v a r i a b i l i t y observed w ith in infants on any one occasion in th is study, and were therefore u n lik e ly to have influenced in te rp re ta tio n o f current findings

(see also 4.2). Given that Vj and f remain r e la t i v e l y unchanged when chemical drive is unchanged (M ilic -E m ili et a l, 1979) the independence o f vagal re fle x responses from chemoreceptor a c t i v i t y may be surmised

from the re la tiv e s t a b i l i t y o f the pattern o f re s p ira tio n throughout successive occlusions in quiet sleep (see Appendix B: P ilo t Study, and Section 3.2).

The occurrence of mild hypoxia or hypercapnia cannot be excluded from a ll measurements in th is thesis. However, as these would be expected to have no or minimal influence on re fle x a c t i v i t y assessed from the airway occlusion technique, i t was considered unethical to perform invasive monitoring of blood chemistry in healthy in fa n ts . In the immediate newborn period, i t is l i k e l y that fu rth e r monitoring equipment may have increased the complexity of the study, and hence the lik e lih o o d o f less successful recruitment and more study fa ilu r e s . Although monitoring of Transcutaneous PCOg (TcPCOg) or beat to beat oxygen saturation (SaOg) was p o te n tia lly feasible, equipment fo r such was not available fo r