ON THE INDIRECT DETERMINATION OF SYSTOLIC AND DIASTOLIC BLOOD PRESSURE IN THE NEWBORN INFANT

(1)

(Received March 7; revision accepted June 24, 1968.)

Supported in part by a U.S. Public Health Service Grant (HD-00050) from the National Institute of Child

Health and Human Development and by the Association for the Aid of Crippled Children.

N.M.N. is a recipient of a Career Development Award (1-KO3-HD-38867) of the National Institute

of Child Health and Human Development.

ADDRESS: 221 Longwood Avenue, Boston, Massachusetts 02115.

PEDIATRICS, Vol. 42, No. 6, December 1968 934

ON

THE

INDIRECT

DETERMINATION

OF

SYSTOLIC

AND

DIASTOLIC

BLOOD

PRESSURE

IN

THE

NEWBORN

INFANT

A Simple

Bedside

Electronic

Oscillometer

Nicholas M. Nelson, M.D.

The Department of Pediatrics, Harvard Medical School, and the Boston Hospital for Women, Lying-in Division, Boston

ABSTRACT. Classical osciilometr was used to de-termine indirect systolic/diastolic blood pressure in nine newborn infants. The measurement was facili-tated by a relatively simple and inexpensive elec-tronic osciilometer, the construction of which is de-scribed. Comparisons between indirect leg and

di-rect intra-aortic blood pressure revealed a systolic difference of 0 to 5 mm Hg and a diastolic differ-ence of 0 to 10 mm Hg, depending on respiratory variations and accuracy of calibration. Pediatrics,

42:934, 1968, NEWBORN, BLOOD PRESSURE, OSCIL-LOMETBY.

T

HE SIMPLICITY of the auscultatory

method for the indirect estimation of

arterial blood pressure and its widespread

acceptance on that account have obscured

the facts that it is not very precisely

cone-lated with intra-arterial pressures,1’2 that

the criteria for diastolic pressure are still

disputed, and that it can be totally

unreli-able in low flow states.3 Previous to

Korotkoff’s description in 1910 of the

arte-rial sounds during decompression,

oscillom-etry was the standard method for clinical

determinations of blood pressure; the two

designs of mechanical oscillometers

cur-rently available (Pachon and von

Recklin-hausen) stem from that era.4 The method

has been shown to yield accurate estimates

of intra-arterial pressure.4’5 As suprasystolic

tissue pressure around an artery is

de-creased during decompression of the

con-stricting cuff, the artery will expand slightly

as a systolic peak of the pressure pulse

ar-rives at the constriction.4 This is shown in

the second line of Figure 1. (For

illustra-tive purposes in this and the subsequent

two figures, the quiescent middle finger of

the author was substituted for a restless

in-fant extremity in the apparatus to be

de-scribed.

)

As tissue pressure is further

de-creased, a proportionately greater fraction

of the pressure pulse appears until the

di-erotic notch becomes evident. As tissue

pressure reaches diastolic, the oscillations

of the arterial wall become maximal, since

the artery is fully open at systole but fully

closed at diastole. At subdiastolic tissue

pressure (last line, Fig. 1), the artery is

never fully closed so that the oscillations

wane in amplitude (Marey’s principle).

This waxing and waning of arterial wall

os-cillations is what the observer sees

im-pressed on the mercury manometer column

during standard sphygmomanometry and is

illustrated in the upper portion of Figure 2.

Unfortunately, even at suprasystolic cuff

pressures, the centrifugal pressure pulse

impinges on the proximal edge of the

compression cuff and is transmitted to the

system, thereby giving rise to a spurious

in-dication of oscillation of the arterial wall

beneath the cuff. This problem was long

ago solved by placing a second compression

cuff proximal to the main compression and

(2)

pressure at all times but not in pneumatic

continuity, the central pressure pulse is

in-tercepted before reaching the main cuff.

The effect of such a proximal cuff in

lower-ing the apparent indicated systolic pressure

is shown in the lower portion of Figure 2.

In the plethysmographic or palpatory

method some sort of pulse indicator

(fingers, strain gage, plethysmograph at

constant pressure) is placed distal to a

compression cuff. As the proximal artery is

decompressed below systolic, the first crest

of the pulse wave passes distally and is

sensed by the pulse indicator. The distal

pulse subsequently builds up to maximal

(and thereafter constant) amplitude as

compression cuff pressure decreases below

diastolic. This is shown in Figure 3.

All of these methods have beell

at-tempted in the Ile\vborn and young infant

with varying degrees of success; very few

have been validated by comparisons with

intra-arterial measurements. In view of the

prominence of arterial hypotension in the

respiratory distress syndrome7 and the

re-current clinical problems where accurate

knowledge of pulse pressure would be

helpful, it seemed important to attempt

de-velopment of relatively simple and

econom-ical apparatus for the indirect l)edside de-termination of systolic and diastolic

pres-sure and to validate its performance. The

oscillometric method was selected, and an

electronically amplified oscillometer, whose

accuracy has been validated as satisfactory,

was developed.

MATERIAL AND METHODS

The present apparatus comprises two

cuffs applied to an extremity and most

con-veniently separated by the knee or elbow.

The pneumatic system allows use of the

cuffs separately or together as an effectively

continuous single cuff. Tile two cuffs may

be decompressed together (oscillometry) or

the distal sensing cuff may be held at

con-stant subdiastolic pressure while the

proxi-mal occluding cuff is decompressed

(palpa-tory method). This latter technique is a

convenient and rapidly repeatable

proce-FIG. 1. Oscillometric tracings from an adult finger at occlusion pressures

de-creasing from 130 mm Jig (to1)) to 70 mm Hg in 10 mm steps. Pressure

(3)

936 BLOOD PRESSURES OF NEWBORN

TABLE I

COMPARATIVE BLOOD PRESSURE MEASUHEMENTS

IN NEwB01IN INFANTS

. Subject Number Blood Pressure Aorlzc Blood Difference-indirect rressvre ilznus Aortie

(,,tiit hg) (iiun JIg)

Graphic Recording* 5-6 4.3 35-42 0.3 46-55 5.5 1.8 46-54 6.1 3 2.’2 4-52 7.4 4 10.4 60-70 0 5 -40-45 1 Visual lionitoringt 60 6

-40 47 7 -27 46 8 -‘24 60 9 -38 0 -0 0 -3 2 0 -2 -2

* These were non-distressed infants of diabetic

mothers delivered by elective cesarean section at 37

weeks’ gestation and studied at less than H hours of age.

t These were premature infatits with respiratory

(us-tress syndrome in the first 3 days of life. All survived.

dure for following systolic pressure after

initial determination of systolic/diastolic

pressure by classical oscillometry.

The pulse indicator is a piezoelectric

crystal whose output is amplified by an

in-expensive operational amplifier so that the

arterial pulsations may be transmitted to a

meter needle whose oscillations may be

in-creased to 3 to 4 cm and are hence easily

visible at onset and maximum. Details of

construction and use are included in the

Appendix. Total cost for pneumatic and

electronic parts is less than $100. The

pneu-matic apparatus can be stored inside a

stan-dard sphygmomanometer case. The

elec-tronic box measures 2 X 3 X 6 inches.

Comparisons of indirect estimations of

blood pressure in the extremities were

made with intra-aortic blood pressure in

nine babies whose aortae were cannulated

for diverse therapeutic purposes. The

can-nulae were side-hole polyvinyl feeding

tubes* passed through an umbilical artery to

the approximate level of the diaphragm

and attached to a Statham P23Db strain

gage. After appropriate calibration the

out-put of the strain gage was recorded visuallyt

in Subjects 6 to 9 or on a direct-writing

Sanborn oscil1ograph in Subjects 1 to 6.

The natural frequency of this latter system

was 15 to 30 Hz with a damping ratio of

0.3-0.4 as determined by explosive

de-compression

(

“pop” technique

)

. Usable

fre-quency response thus extended to

approxi-mately 10 Hz. Arterial pulsations were

monitored visually with the electronic

oscil-lometer. In order to obviate human reading

errors in some babies, the piezoelectric

pulse indicator was substituted by a

differ-ential manometer whose oscillations were

recorded along with cuff pressure on a

di-rect writing oscillograph.

RESU LTS

The results are shown in Table I. The

graphic recordings compared arterial

oscil-lations of the right leg (impressed by the

pneumatic system described below on a

dif-ferential manometer acting as the pulse

transducer) with an intra-aortic pressure

tracing. Cuff pressure and true aortic blood

pressure were recorded from separate strain

gages. Since reflection of standing pressure

waves normally produces some

amplifica-o No. 5 Fr, 15 to :30 cm, Pharmaseal, Glendale, California, and Argyle, St. Louis, Missouri.

f

A-V Monitor, Med-Science, St. Louis, Missouri. Hewlett-Packard, Waltham, Massachusetts.

Sanborn Differential Gas Tranducer,

(4)

I)iastolie

l,1uni1e? Clinical Disadvanhiges Method

Os(illornetric

Ausetiltatory

Palpatory

Pletliysmograpliic

Flush

Yes

Arguable

No

Yes

N

Amplification often necessary.

Very difficult in newborn.

Satisfactory for systolic only. Encessive instrumentation;

serious systolic overestimate.

Crude correlation, rapid

repeatability awkward. FIG. 2. Oscillometric tracings from the adult finger. Top, “single cuff”

sys-tefll showing spuriously early systolic indication (see text). Bottom, isolated

double cuff system-proximal pulse is obliterated when proximal cuff is iso-lated from distal cuff (between large spikes). Note respiratory variations in pulse amplitude. In both tracings the diastolic shift of tile dicrotic notch2’

and decrease in pulse amplitude

(

Marev’s principle) are evident.

tion of pulse pressure in the peripheral

arterial tree, the systolic discrepancy

be-tween central and femoral arterial pressure

was anticipated. Ideal comparisons between

direct femoral pressure and oscillometric

determinations were not attempted in these

infants.

The visual monitoring system employed

the oscillometer precisely as described in

the Appendix and these indirect readings

(also from the right leg) were compared to

direct aortic pressure as monitored by a

very stable strain-gage electronic readout

device. II The apparent improvement in

correlation of central direct and peripheral

indirect arterial blood pressure may

repre-sent more stable calibration or perhaps is

due to visual averaging by the observer

during respiratory variations of the

periph-eral pulsations (only one reading was

re-ported per infant, since repeat

determina-tions varied less than 2 mm Hg).

Alterna-H

Statham P23Db Strain Gage. I lato Rev, Puerto

Rico, and A-V Monitor, Med-Science, St. Louis,

Missouri.

tively, since the cardiovascular status of

in-I ants with respiratory distress often

in-cludes hypotension and poor peripheral

flow, these factors could have served to

abolish the normal peripheral amplification of pulse pressure in these infants who were mildly ill.

DISCUSSION

The available methods for indirect blood

pressure estimation are presented in Table

II. At one time or another all have l)een at-tempted in infants. The flush method is

per-TABLE II

(5)

FIG. 3. “Palpatory” tracing. The distal cuff when held at constant subdiastolic pressure allows registration of the standard plilebogram seen in lower line trace at a higher sweep speed. When proximal occlusion cuff is bled down from supra-systolic levels, the onset of pulsations marks systolic pressure.

Again, note respiratory variations in pulse amplitude.

haps most widely used but it is difficult to

repeat rapidly, gives no indication of pulse

pressure, and is only fitfully correlated with

mean blood pressure.

The plethysmographic method has

en-joyed recent pediatric resurgence in both

standard8 and strain gage forms, despite its

known overestimation of systolic pressure.’#{176}

This is shown in Figure 4 where the

pleth-ysmographic and oscillometric methods are

compared. The increase in limb volume

oc-curring at a suprasystolic pressure and

be-fore the onset of oscillations is probably the

result of arterial inflow to the limbs through

arteries buried within or near bones and

hence incompressible by the occluding cuff.

Since vascular resistance is calculated as

the ratio of blood pressure/blood flow, any

determinations of resistance based on

pleth-ysmographic blood pressure

determina-tions’1’12 must necessarily be overestimates.

The palpatory method for systolic

pres-sure has been notoriously unreliable where

human fingers have been the sensors. But

when the fingers have been supplanted by

a sensitive pulse indicator,13 accuracy,

re-peatability, and simplicity have followed.

Indeed, the only defect of the

Ashworth-Neligan apparatus has been its inability to

provide a reliable estimate of diastolic

pres-sure. The apparatus described in the

pres-ent report can be used, if desired, as just

such a palpatory sensor with the

advan-tages of rapid repeatability and

considera-bly more visibility than the standard xylol

bead pulse indicator.

Experience with auscultatory

sphygmo-manometry has been uniformly

disappoint-ing in infants. Although extreme observer

dedication and subject cooperation have

oc-casionally combined to produce a reading,

the method has never proven reliable.14

Most probably the critical variables of

arte-rial wall tension, pressure drop across the

occluding cuff, and flow rate through the

compressed artery3 are insufficiently high in

the newborn infant to cause the regular

(6)

culminat-939

ing in Korotkoff’s sounds. Indeed, the poor

correlation of Korotkoff’s sounds with

in-tra-arterial measurements in adults’ ‘‘

bears testimony to their general

unreliabil-ity (especially at diastolic pressure).

De-spite these problems, electronic

amplifica-tion and recording of auscultatory blood

pressure has been used in babies without

the necessary intra-arterial validation.’

The oscillometric method has long been

used in newborns,1619 but persistent defects

have been instrumental insensitivity and

lack of intra-arterial validation. The present

results would seem to establish the

oscillo-metric method’s reliability. In animal and

adult human material,’ intra-arterial

com-parisons have long since demonstrated the

clear superiority of the oscillometric over

other indirect methods. Its total

replace-ment in clinical medicine by the

ausculta-tory method is, of course, the result of the

latter’s extreme simplicity.

This advantage does not apply to the

mm Pig

newborn so that the instrumentation

re-quired in the present method can perhaps

be excused. Indeed, the insufficient

sensitiv-ity (in the author’s hands) of commercially

available mechanical oscillometersl9 has

prompted the present work. Once diastolic

pressure has been determined, the

de-scribed apparatus may be used as a

palpa-tory sensor, for which purpose it offers no

advantage over the Ashworth-Neligan

ap-paratus except readability.

In all methods, proper cuff size is

per-haps the most important factor in achieving

a true approximation of intra-arterial

pres-sure. The standard recommendation for cuff

width in adults is about one-third the limb

circumference, or approximately 2 to 3 cm

for a newborn infant. Many studies have

substantiated this flgure,13 so 2.5 or 3.0 cm

cuffs were used throughout the present

study.

The two main sources of error in the use

of the apparatus are observer error and true

FIG. 4. Comparison of plethysmographic and oscillometric methods of indi-rect blood pressure estimation in the right leg of an infant with a direct aortic blood pressure of 60/35. Top: cuff pressure. Center: mercury-in-rubber plethysmographic tracing. Bottom: oscillonetric tracing. Plethysmo-graphic blood pressure is 81/45 (onset/offset of limb volume increase). Oscillometric blood pressure is 69/31 (appearance/diminishment of limb

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A

C

D

FIG. 5. Pneunlatic circuit (see Appendix for explanation).

10 K

940 BLOOD PRESSURES OF NEWTBORN

Fic. 6. Electronic circuit (see Appendix for explanation).

respiratory variations in peripheral blood

pressure. The observer errors can be held to

less than 2 to 5 mm Hg by using a slow

leak and frequent interruptions of mercury

descent with a check valve (Fig. 5, tap B).

Respiratory variations can amount to about

6 mm Hg. Consistent recording of that

oc-cluding pressure at which any (rather than

all) pulsations show the proper criterion

(appearance of pulse or its waning) will at

least permit consistent determinations of

systolic/diastolic pressure.

It is of perhaps more than simply

nostal-gic interest that in 1913 Balard made

ac-Ima

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APPENDIX

Construction Details

sold for transistor radios. Its output is shunted ELECTRONIC (schematic in Fig. 6): The pulse across an RC low pass filter in order to

attenu-transducer is an inexpensive crystal earphone ate the unwanted higher frequencies which

curate measurements of systolic/ diastolic

blood pressure in normal and low birth

weight infants, including a description of

the slow postnatal rise in blood pressure

only recently accepted as standard.’#{176} Balard

used the oscillometric method, which

unac-countably later fell into clinical disuse. The

present validation and instrumentation will

perhaps encourage and permit the

renais-sance which the oscillometric technique so

richly deserves.

SUMMARY

Classical oscillometry has been employed

to determine indirect systolic/diastolic

blood pressure in newborn infants. A

rela-tively simple and inexpensive electronic

Os-cillometer, which facilitates the

determina-tion, is described. Comparisons between

in-tra-aortic and (indirect-direct) leg blood

pressure in nine babies revealed a systolic

difference of 0 to 5 mm Hg and a diastolic

difference of 0 to 10 mm Hg, depending on

the accuracy of calibration.

REFERENCES

1. Roberts, L. N., Smiley, B. A., and Manning, C. W.: A comparison of direct and indirect blood pressure determinations. Circulation,

8:232, 1953.

2. Simpson, J. A., Jamieson, G., Dickhaus, D. W., and Grover, R. F.: Effect of cuff bladder on

accuracy of measurement of indirect blood

pressure. Amer. Heart J., 70:208, 1965.

3. Cohn, J. N.: Blood pressure measurement in shock. J. A. M. A., 199:972, 1967.

4. Burch, G. E, and DePasquale, N. P.: Primer

of Clinical Measurement of Blood Pressure. St. Louis, Missouri: C. V. Mosby Co., 1962. 5. Van Bergen, F. H., Watherhead, D. S.,

Tre-bar, A. E., Dobkin, A. B., and Buckles’, J. J.: Comparison of indirect and direct meth-ods of measuring arterial blood pressure. Circulation, 10:481, 1954.

6. Christensen, B. C.: Oscillometric studies. Acta Med. Scand., 120:474, 1945.

7. Rudolph, A. M., Drorbaugh, J. E., Auld, P. A. M., Rudolph, A. J., Nadas, A. S., Smith, C.

A., and Hubbell, J. P.: Studies on the circu-bation in the neonatal period. The circulation in the respiratory distress syndrome. PirnT-IIICS, 27:551, 1961.

8. Celander, 0., and Thunell, G: A plethysmo-graphic method for measuring the systolic and diastolic blood pressure in newborn in-fants. Acta Paediat. Scand., 49:497, 1960. 9. Levison, H., Kidd, B. S. L., Gemmell, P. A.,

and Swyer, P. R. : Blood pressure in normal

full-term and premature infants. Amer. J.

Dis. Child., 111:374, 1966.

10. Landowne, M., and Katz, L. N. : A critique of the plethysmographic method of measuring blood flow in the extremities of man. Amer. Heart J., 23:644, 1942.

11. Kidd, L., Levison, H., Gemmell, P., Aharon, A., and Swyer, P. R. : Limb blood flow in the normal and sick newborn. Amer. J. Dis.

Child., 112:402, 1966.

12. Celander, 0. : Blood flow in the foot and calf of the newborn. Acta. Pediat. Scand., 49:488, 1960.

13. Ashworth, A. M., Neligan, C. A., and Rogers, J. E.: Sphygmomanometer for the newborn.

Lancet, 1:801, 1959.

14. Moss, A. J., and Adams, F. H.: Index of indi-rect estinlation of diastolic blood pressure.

Amer. J. Dis. Child., 106:364, 1963.

15. Morse, R. 0., Brownell, C. L., and Currens,

J.

H.: The blood pressure of newborn infants.

Indirect determination by an automatic

re-corder. PEDIATRICS, 25:50, 1960.

16. Balard, P.: Le pouls et Ia tension arterielle

de l’enfant et du nouveau-ne. Caz. Hop.,

86:837, 1913.

17. Rucker, M. P., and Connell, J. W.: Blood

pres-sure in the newborn. Amer. J. Dis. Child., 27:6, 1924.

18. Bevis, D. C. A., and Schofield, S. F.: The renal

function of infants delivered of toxaemic

mothers. Arch. Dis. Child., 26:109, 1951. 19. Kafka, H.: A simple method for blood pressure

measurements in the premature and

new-born infant. PEDIATRICS, 40:106, 1967. 20. Contis, C., and Lind, J.: Study of systolic

blood pressure, heart rate, body temperature of normal newborn infants through the first week of life. Acta Paediat. Scand. (Suppl.),

146:41, 1963.

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abound in most any clinical acoustic-electronic

environment. Generator impedence is kept

mod-erately low in order to minimize error voltages.

The conditioned signal is then amplified by a

battery-driven operational amplifier0 connected

as a voltage follower with a closed-up gain

capability of 1,000. This gain has proven to be

more than ample to produce full-scale

deflec-tion with the smallest pulses. The output signal is rectified in order to simplify interpretation of

the otherwise complicated wave form as seen

on the meter needle. Because the dicrotic notch

so prominent in the record from an adult finger

(Fig. 1 and 3) is not seen in the infant leg (Fig.

4), rectification of the pulse wave-form was felt to result in no serious loss of information. Input

impedance of the amplifier in this configuration

is approximately 5 megohms and expected

bat-tery life is 1,000 hours.

PNEUMATIC: The design is depicted in Fig.

5

and is an adaptation of that given by

Chris-tensen.6 The polyvinyl cuffs are commercially

available.

f

Rubber cuffs may well be too

corn-pliant for accuracy in oscillometry. Tubing is of

plastic and latex leftovers from intravenous ap-paratus. The manometer is the standard clinical portable variety. T-joints are small-bore plastic.t

The three-way tap is of disposable plastic, and

the inexpensive “needle valve” is reclaimed from an exchange transfusion set. The crystal

microphone (earphone) is sealed with epoxy

resin on all seams because as purchased it is

insufficiently robust to withstand the required

pressures. The four-way stopcock is machined

from a standard metal three-way stopcock and

a stop is attached so that only a 45#{176}rotation is

possible (i.e., all four ports are in mutual

com-munication or are totally isolated). However, a

less accurate version which is simpler to use substitutes two equal lengths No. 21 gage

cap-illary needles for the four-way tap B. The cuffs

are inflated by the bulb inserted via a T-tube

between the capillaries. The capillaries thus

act as a low pass filter between the cuffs, which

allows a continuous bleed through C (which in

this version must be moved to a position

be-tween the capillaries and the inflating bulb).

#{176}No. 2LV-1, Nexus, Dedham, Massachusetts.

Sage, White Plains, New York. Bolab, Reading, Massachusetts.

Use

PRELIMINARY ADJUSTMENTS: Cuffs are care-fully applied above and below the knee or el-bow and are connected as indicated in Figure

5. With both cuffs in communication (through

taps A and B) tap C is closed, the cuffs are in-flated, tap C is then opened, and the minor

leak at C is adjusted carefully for a bleed of

about 2 to 3 mm Hg per second. This should

rarely need readjustment. The transducer is

plugged into the amplifier, which is then

switched on. With the amplifier at approxi-mately half-gain, the meter needle is brought to slightly above zero with the zero control. The gain control is then turned down to zero. The cuffs are then inflated to approximate mean pressure between expected systolic and diastolic pressures, and the gain is advanced until the needle pulsations are brought to nearly full

scale. This should need only minor

readjust-ment.

OSCILLOMETRIC BLOOD PRESSURE (systolic/ diastolic) : The cuffs are in communication through taps A and B. Tap D is closed and the cuffs are inflated above systolic. By alternately

opening and closing tap B while observing the

meter needle, the cuffs are deflated in 2 to 5 mm steps, and systolic pressure is taken as that at which first pulsations are seen with B closed. Respiratory variations will often be encoun-tered.

After noting the systolic pressure, B is opened,

and the cuffs are bled continuously until pulse

amplitude first begins to wane, at which point

tap B is again closed (if desired) while the dia-stolic pressure is noted. Tap B is again opened and the cuffs are rapidly deflated tllrough tap D.

PALPATORY BLOOD PRESSURE (systolic) :‘

The cuffs are in communication through taps A and B. The cuffs are inflated to a pressure

be-tween systolic and diastolic (or even slightly

below diastolic) and tap A is turned to isolate the distal sensing cuff. The proximal occluding

cuff is deflated through D and B, which latter

is then left in open position. Further rapid sys-tolic readings are made by inflating the

proxi-mal cuff and deflating through D again, noting

onset of pulsations as the systolic pressure

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1968;42;934

Pediatrics

Nicholas M. Nelson

Oscillometer

BLOOD PRESSURE IN THE NEWBORN INFANT: A Simple Bedside Electronic