Pattern of respiratory-induced changes in fingertip blood volume measured by light transmission

doi:10.4236/jbise.2011.48068

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J. Biomedical Science and Engineering, 2011, 4, 529-534

doi:10.4236/jbise.2011.48068 Published Online August 2011 (http://www.SciRP.org/journal/jbise/ JBiSE

Published Online August 2011 in SciRes. http://www.scirp.org/journal/JBiSE

Pattern of respiratory-induced changes in fingertip blood

volume measured by light transmission

Meir Nitzan1, Daniel Dayan1, Eran Shalom1, Yu val Slovik2, Alan Murray3

1Department of Applied Physics/Medical Engineering, Jerusalem College of Technology, Jerusalem, Israel;

2Department of Otorhinolaringology, Head and Neck Surgery, Soroka University Medical Center, Beer Sheva, Israel;

3Department of Medical Physics Freeman Hospital, Newcastle, UK.

Email: [email protected]

Received 16 May 2011; revised 13 June 2011; accepted 5 July 2011.

ABSTRACT

Respiratory-induced fluctuations in heart rate and

arterial blood pressure have been intensively investi-

gated, but there is little information on the effect of

respiration on peripheral blood volume. In the cur-

rent study, blood volume changes in the finger, ob-

tained by light transmission measurements, were

measured during regular breathing (6 s periods) and

long breathing (12 s periods). Respiratory chest-cir-

cumference changes were simultaneously measured

in order to associate the pattern of tissue blood vol-

ume change with the respiratory cycle. Sixteen sub-

jects were studied, and in fourteen finger blood vol-

ume increased during inspiration and decreased

during expiration in the long-breathing periods. In

all 14 subjects the start of blood volume decrease was

significantly delayed from the start of expiration by

mean ± SD 1.00 ± 0.65 s (p < 0.001, range 0 - 2.3 s).

The start of blood volume increase was significantly

delayed from the end of expiration by 3.45 ± 1.76 s (p <

0.005). In eight, finger blood volume started to in-

crease more than 2 s before the start of inspiration.

For the 6 s breathing period, blood volume decreased

during inspiration in five examinations, and in-

creased in seven. The increase in peripheral blood

volume during inspiration could be attributed to the

higher abdominal pressure during inspiration, and to

the decrease in sympathetic activity during inspira-

tion and the subsequent vasodilatation. The decrease

in peripheral blood volume during inspiration is

probably due to the negative thoracic pressure dur-

ing inspiration and its mechanical effect on thoracic

vessels.

Keywords: Light Absorption; Deep Breathing;

Sympathetic Nervous system; Tissue Blood Volume

1. INTRODUCTION

Respiratory-induced fluctuations in heart rate—respira-

tory sinus arrhythmia—and similar fluctuations in arte-

rial blood pressure have been intensively investigated,

but in only a few studies the pattern of the chang e during

inspiration and during expiration was investigated. Heart

rate increases and systolic blood pressure decreases dur-

ing inspiration [1-3] but there is strong variability in the

relationship between heart rate or systolic blood pressure

and the respiratory changes in chest circumference. The

origin of these respiratory-induced fluctuations is not

decisively known. They have been attributed to fluctua-

tions in central autonomic activity caused by spontane-

ous oscillations in respiratory center activity or by resp i-

ratory-induced mechanical effects on the aortic baro-

receptors and the pulmonary stretch receptors [2-4]. The

direct mechanical effect of respiratory-induced thoracic

pressure changes on arterial blood pressure and on cen-

tral veins may also be significant [5-7].

Respiratory-ind uced flu ctuatio ns hav e also been fo und

in the peripheral circulatory system. Deep inspiration

reduces skin blood flow, measured by skin temperature

[8,9] and by laser Doppler flowmetry [9-12], and this

reduction has been attributed to higher sympathetic ac-

tivity. Peripheral blood volume, as measured by light

transmission through the tissue, has been shown to fluc-

tuate at the respiratory rate [13-18], as it does with each

heart beat, and so induces the photoplethysmographic

(PPG) signal.

Two possible mechanisms have been suggested for

the origin of tissue blood fluctuations with respiration:

mechanical influence of the negative thoracic pressure

during inspiration on the arteries and veins in the thorax

[14-17] and respiratory changes in sympathetic activity

[15-18]. The effect of respiration on sympathetic activity

has been demonstrated by several studies [19-21], which

showed higher muscle sympathetic nerve activity

M. Nitzan et al. / J. Biomedical Science and Engineering 4 (2011) 529-534

530

(MSNA) during expiration and very low MSNA at

end-inspiration, when lung volume is maximal. Since

sympathetic activity generally constricts skin blood ves-

sels, the decrease of sympathetic activity during inspira-

tion is expected to in crease fingertip blood volume.

The mechanical effect of respiration on the tissue

blood volume also depends on the relative contributions

of abdominal respiration and thoracic respiration. The

negative thoracic pressure during inspiration decreases

blood pressure in the arteries and veins in the thorax , and

consequently decreases peripheral tissue blood volume

during inspiration. The higher abdominal pressure dur-

ing inspiration increases blood pressure in the arteries

and veins in the abdomen and consequently increases

peripheral tissue blood volume during inspiration

[22,23]. In particular, finger blood volume is expected to

increase due to the higher blood pressure in the abdomi-

nal aorta, resulting in higher blood flow from the heart to

the upper part of the body.

The respiratory changes in the peripheral blood vol-

ume can therefore exhibit different patterns. In the cur-

rent study we measured the temporal relationship be-

tween changes in finger blood volume and the respira-

tory phase.

2. MATERIALS AND METHODS

2.1. Subjects and Examination

Sixteen non-smoker male subjects aged 21 - 63 years,

with no known cardiovascular or neurological disease

were studied. During the examination the subjects sat

with their right hand comfortably laid on the table, at

about heart level. A PPG probe was attached to the right

index finger for the measurement of light transmission

though the finger tissue, and an op tic-fiber sensor for the

measurement of the respiratory chest-circumference

changes (see later) was applied around their chest. Fin-

gertip skin temperature was measured before the start of

the examination by an alcohol thermometer held by the

index finger and the two adjacent fingers. Room tem-

perature was 21˚C - 24˚C; fingertip temperature was

29˚C - 35˚C.

After a rest period of five min the subjects were asked

to breath three series of 5 regular and 5 long respiration

periods, where inspiration and expiration time were de-

termined by a light point moving on a computer screen

in the form of triangular waves. Regular respiration con-

sisted of inspiration of 2 s and expiration of 2 s followed

by 2 s of no-breath, and long breathing consisted of in-

spiration of 4 s and expiration of 3 s followed by 5 s of

no-breath. It should be noted that in our study there were

no constraints on the tidal volume, and it was allowed to

change between long and regular breathing.

2.2. The Light Transmission and the Chest

Circumference Sensors

The PPG probe consisted of an infrared light-source and

photodetector of a pulse-oximeter probe (Oxisensor N25,

Nelcor), mounted on the same plane (reflection PPG

probe). The probe was attached to the ind ex finger of the

right hand of the subject. A low-pass filter (0 - 40 Hz)

reduced high frequency noise. The signal was inverted

so that a higher signal level corresponded with a higher

blood volume. The signals were sampled at a rate of 500

Hz (16 bit) and digita l l y store d fo r of fl i ne pr ocessi n g.

In order to obtain the relationship between the light

transmission changes and time of inspiration and expira-

tion, we used an optic-fiber sensor previously developed

by us [24] for the measurement of respiratory-induced

changes in chest-circumference. The sensor is based on

the dependence of light transmission through a bent op-

tic-fiber on its radius of curvature, and on the change of

the latter when chest circumference changes. Some light

rays, which are totally reflected by the core-cladding

surface when the fiber is straight or slightly bent, may

escape through the cladding when the fiber bending is

higher, if the angle to the surface normal becomes lower

than the critical angle. The details of the sensor were

described elsewhere [24].

3. RESULTS

In 14 out of the 16 long-breathing period examinations

(12 s each, including 5 s pause), tissue blood volume

increased during inspiration and decreased during expi-

ration. Figure 1 shows the tissue blood-volume and

chest-circumference change as a function of time for one

of these examinations. In this examination, finger blood

volume started to decrease about two s after the start of

expiration, while the increase of finger blood volume

started before the start of inspiration, and continued to

increase during inspiration. In two examinations, the

pattern of blood volume change during long breathing

was not in direct relationship with chest-circumference

change. In one of these, an inverse pattern was found for

the long breathing periods: the tissue blood volume in

the finger decreased during inspiration and increased

during expiration. In the other, the finger blood volume

displayed double bloo d volume pulses for each breath.

Figure 2 shows two examples from the 14, showing

the pattern of direct relationship between tissue blood

volume and chest-circumference change for the long-

breathing period examinations. In all those examinations

the finger blood volume started to decrease after the start

of expiration (range 0 - 2.3 s). In ten cases finger blood

volume started to increase before the start of inspiration,

and continued to increase during inspiration, indicating

that the blood volume increase was also related to the

M. Nitzan et al. / J. Biomedical Science and Engineering 4 (2011) 529-534 531

end of the previo us phase of expiration. In 8, the changes

started more than 2 s before inspiration, and in 2, just

before. To allow the relationship between tissue blood

volume and chest-circumference change to be seen more

clearly, the pulses in tissue blood volume were elimi-

nated by depicting only the diastolic (minimal) blood

volume for each pulse, and Figure 3 shows the results

for the same examinations as in Figure 2.

The time of the increase and decrease of the blood

volume pulse relative to respiratory phase was charac-

terized by two parameters: the time-difference TDI from

the end of expiration to the start of blood volume in-

crease, and the time-difference TDD from the start of

expiration to the start of blood volume decrease, as

shown in Figure 1. The time of the start of expiration

was taken as the time of the maximum of the chest-cir-

cumference change curve; the time of the end of expira-

tion was taken as the time of the end of the breathing

instruction triangle (BG in Figure 2).

Figure 1. The changes in tissue blood volume (TBV) and

chest-circumference (CC) as a function of time. TDI is the time

difference between the end of decrease of CC and the start of

increase of TBV; TDD is the time difference between the start

of decrease of CC and the start of decrease of TBV.

(a)

(b)

Figure 2. Two examinations showing, for long breathing peri-

ods, a direct relationship between tissue blood volume (TBV)

increase (upper curve) and chest circumference (CC) increase

(middle curve). TBV changes during regular breathing show an

inverse relationship (a) and a direct relationship (b) with chest

circumference increase. Upward direction indicates increase in

TBV and chest circumference. The lower curve in each ex-

amination (denoted by BG—breathing graph) shows the trian-

gular waves displayed on the computer screen for breathing

instruction.

(a)

(b)

Figure 3. The two examinations of Figure 2, showing the

changes in the minimal (diastolic) blood volume (upper curve)

and chest circumference increas e (lower curve) .

M. Nitzan et al. / J. Biomedical Science and Engineering 4 (2011) 529-534

532

Figure 4. The time of the start of tissue blood volume increase,

TDI, relative to the end of expiration (empty circles) and the

time of the start of tissue blood volume decrease, TDD, relative

to the start of expiration (full circles) for the long breathing

periods. Each pair of circles presents the values of TDI and

TDD for one of the 14 subjects, displaying a direct relationship

between tissue blood volume change and chest circumference

change.

Figure 4 presents the detailed results for the mean

values of TDI and TDD for each of the 14 subject ex-

aminations. These mean values were obtained by aver-

aging, for each subject, the values for all the long-

breathing pulses, not including the first in each group.

The increase in blood volume was significantly delayed

from the end of expiration by TDI 3.45 ± 1.76 s (mean ±

SD, p < 0.005) and the decrease in blood volume was

significantly delayed from the start of expiration by

TDD 1.00 ± 0.65 s (p < 0.001).

Figures 2 and 3 also present results of blood volume

measurements with regular breathing of the 6 s period. A

pattern of direct relationship between tissue blood vol-

ume change and chest-circumference change (i.e. in-

crease of tissue blood volume during inspiration and

decrease during expiration) was found in some subjects

as shown in Figure 2(b) for one examination, while for

other subjects it was an inverse relationship (as in Fig-

ure 2(a)). For the 14 examinations, seven showed a di-

rect relationship, five an inverse relationship and in two

the relationship was not clearly defined. No quantitative

results could be derived from the examinations of regu-

lar breathing due to the low signal-to-noise ratio in the

blood volume measurement.

4. DISCUSSION

In the current study we measured the blood volume

changes in the finger, as obtained by light transmission

measurements, during regular breathing (6 s periods) and

during long breathing (12 s periods). The examinations

were performed simultaneously with measurement of

chest-circumference change, in order to see whether in-

spiration was associated with an increase or decrease in

tissue blood volume. The main finding of the study was

that in all except two long-breathing examinations of

finger blood volume changes, tissue blood volume in-

creased during inspiration and decreased during expira-

tion (direct relationship). In only one examination did

tissue blood volume decrease during inspiration and in-

crease during expiration (inverse relationship). This in-

verse relationship was also found in some regular-

breathing examinations, even though the long-breathing

examinations in these subjects were associated with a

direct relationship pattern.

An important finding was the relative timing of blood

volume change. The start of the decrease in finger blood

volume followed the start of expiration, suggesting that

the former was induced by the latter. On the other hand

the start of the increase in finger blood volume preceded

the start of inspiration in the majority of the examina-

tions. Though the increase in finger blood volume re-

lated to inspiration, it is likely to have been initiated by

the end of previous expirat i o n .

As presented in the Background Section, two possible

mechanisms have been suggested as the origin of the

tissue blood volume fluctuations with respiration: me-

chanical influence of the thoracic pressure on the tho-

racic blood vessels [14-17] and sympathetic activity os-

cillations [15-18]. The sympathetic activity has been

shown to decrease during inspiration [19-21], which can

explain the increase in fingertip blood volume during

inspiration, found in almost all of our long-breathing

results. Though those studies on sympathetic activity

(measured by MSNA) were performed with spontaneous

breathing, with a respiration frequency which was even

higher than our regu lar breathing, it seems reasonable to

suggest that sympathetic activity is similarly modulated

by the respiratory pattern of our long-breathing exami-

nations.

In former publications the mechanical effect of

breathing on the peripheral blood volume was attributed

to the negative internal thoracic pressure during inspira-

tion which decreases blood pressure in the thoracic ar-

teries and increases blood volume in the thoracic veins.

These two effects are expected to decrease peripheral

tissue blood volume during inspiration. However, inspi-

ration is generally also accompanied with increased ab-

dominal pressure, which increases the blood pressure in

the abdominal arteries and veins and diverts blood to the

peripheral tissue [22,23]. This effect increases the blood

volume in the peripheral blood vessels during inspiration

and can explain the increase in tissue blood volume dur-

M. Nitzan et al. / J. Biomedical Science and Engineering 4 (2011) 529-534 533

ing inspiration and its decrease during expiration (direct

relationship) in long breathing. In addition to the effect

of sympathetic activity decrease during inspiration, the

respiratory change in tissue blood volume is probably

affected by the two opposing effects of thoracic and ab-

dominal breathing on the peripheral tissue blood volume.

5. CONCLUSIONS

While in most long-breathing examinations finger blood

volume increased when chest-circumference increased,

an inverse relationship between the two parameters was

found in one long-breathing examination and also in

several examinations of regular breathing. It is likely

that different mechanisms are involved in the effect of

respiration on peripheral blood volume, probably in-

cluding lower thoracic pressure, higher abdominal pres-

sure and lower sympathetic activity during inspiration.

6. ACKNOWLEDGEMENTS

The study was supported by E.W. Joseph Fun d.

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