CAPILLARY BLOOD SAMPLING IN THE INFANT: USEFULNESS AND LIMITATIONS OF TWO METHODS OF SAMPLING, COMPARED WITH ARTERIAL BLOOD

(1)

Abbreviations BD: Base deficit

HC: Histamine capillary

PC: Plain capillary (unwarmed

extremity)

Paco: Arterial carbon dioxide

pressure

Pao5: Arterial oxygen pressure

(Received May 18, 1972; revision accepted for publication October 16, 1972.)

ADDRESS FOR REPRINTS: (C.E.H.) Department of Pediatrics, University of Minnesota, Box 44, Mayo

Memorial Building, Minneapolis, Minnesota 55455.

PEDIATRICS, Vol. 51, No. 3, March 1973

CAPILLARY

BLOOD

SAMPLING

IN THE

INFANT:

USEFULNESS

AND

LIMITATIONS

OF

TWO

METHODS

OF SAMPLING,

COMPARED

WITH

ARTERIAL

BLOOD

Carl E. Hunt, M.D.

From the Department of Pediatrics, University of Minnesota Hospitals, Minneapolis, Minnesota

ABSTRACT. Measurements of simultaneous arterial, histamine capillary (HC), and plain capillary (PC) pH, Pco and Po2 in 46 infants have indicated the clinical usefulness and limita-tions of blood so obtained.

Arterial, HC, and PC pH can be used

inter-changeably for calculation of base deficit (BD).

HC and PC Pc& are identical, but average 10 mm Hg more than Paco2 within the usual clinical range. The resultant error in capillary BD as com-pared to arterial or venous BD can be minimized

by the regression equation: arterial BD = 1.3

(PC BD) + 3.9.

For Po2, the two capillary methods are compa-rable when the Pa&2 is less than 60 to 70 mm Hg. By either capillary method, a Po of 30 represented an average Pao of 30, 50 represented 60 and a capillary Po of 60 indicated a Pao of approxi-mately 90 mm Hg. A PC or HC Po2 greater than 60 to 70 mm Hg did not allow for an accurate and reliable estimation of Pao2. When concerned with a precise quantitation of cardiopulmonary status in any critically ill infant, therefore, there is still no substitute for arterial sampling. Pediatrics, 51:

501, 1973, HISTAMINE IONTOPHORESIS, CAPILLARY

BLOOD SAMPLING, OXYGENATION, ACID-BASE STATUS.

M

EASUHEMENT of arterial blood gases

has become an integral part of

man-agement for the critically ill patient. In the neonatal period this is often accomplished initially by umbilical arterial catheteriza-tion. The need for frequent blood gas moni-toring, however, often does not end with re-moval of the arterial catheter.

Various authors have evaluated the valid-ity of capillary blood gases as a substitute for arterial sampling. Winquist and Stamm1

recently reported arterialized capillary

sampling using histamine iontophoresis to

be a valid substitute for arterial puncture.

Prompted by their report, a study was

un-dertaken in our Infant ICU to compare

si-multaneous arterial (a), histamine capillary

(HC), and plain capillary (PC) blood

gases. Measurements of pH, Pco2, and Po5

were obtained in order to determine which

of the two capillary methods would provide the better approximation of arterial blood.

MATERIALS

All subjects in this study were patients in the Infant ICU. A total of 46 simultaneous

comparisons were obtained in 44 patients.

Twelve of the infants had congenital heart

disease and 22 were neonates with

respira-tory distress syndrome

(

RDS

)

. The other

ten were premature or full-term infants

with noncardiopulmonary problems. Nine of

the patients were less than 5 days of age

but none were under 3.5 days.

Twenty-three of the patients were 5 to 9 days of age and 12 were at least 10 days of age. Except for one 2-year-old child, all were less than 10 weeks of age.

Although all the patients had medical

problems which still required intensive

(2)

C’ I

E

0

0. 0

C)

7.2 73 7.4

Arterial H (mm Hg)

20 30

Arterial pCO2 (mm Hg)

502 CAPILLARY AND ARTERIAL BLOOD

Fic. 1. Comparison of pH regression lines for the two capillary methods. The regression equations are indicated in the lower right corner of graph. The correlation coefficients (r) were .99 for the

HC comparison and .97 for the PC comparison.

blood gas comparisons were obtained. None

of the patients were polycythemic and none

had significant hypotension or evidence of

peripheral vasoconstriction. The blood gas

comparisons were performed at whatever

ambient oxygen concentration was required

0’ I

E E

0 0

C-)

Fic. 2. Comparison of PC& regression lines for the two capillary methods. The correlation coefficients (r) were .82 for the HC method and .78 for the

PC method.

for the patient at that particular moment.

The comparisons were all performed in

pa-tients who had an indwelling arterial

cathe-ter which was no longer considered

essen-tial and which was about to be removed.

METHODS

All capillary blood gases were obtained

on the unwarmed extremity. The histamine

iontophoresis technique was identical to

that used by \Vinquist and Stamm except

that

(

1

)

the 1% histamine dihydrochloride

(

Imadyl Unction

)

they used was

with-drawn from the market and we had to

sub-stitute a 0.1% histamine dihydrochloride ointment,

(

2

)

electric current was produced

with Electro-Mechanical Instrument

Com-pany#{176} or Simpsont amp-meters rather than

a Gibson-Cooket apparatus, and (3) a

cur-rent of 2.0 rather than 1.5 milliamperes was used.

Immediately following removal of the

histamine ointment, a free-flowing capillary

specimen was obtained from a heel stick

with a No. 1 1 sterile surgical blade. The

foot was stuck in a slow, deliberate manner to an approximate depth of 3 mm in the lat-eral portion of the foot just anterior to the

heel. The foot was elevated to reduce

ye-nous pressure and squeezing was avoided

to prevent venous stasis. W7hile the HC

gases were being obtained, simultaneous

PC gases s’cre obtained in the same

man-ncr from the contralateral foot, which had

had no previous histamine iontophoresis or

warming. Concurrently, an arterial

speci-men of 0.3 cc was drawn into a 1-cc syringe with its dead space filled with heparin. The

arterial, HC, and PC samples were then

promptly analyzed for pH, Pco2, and Po

using an IL pH/gas Analyzer, Model 113

equipped with a microsampling device (IL

34183).

#{176}Electro-Mechanical Instrument Company, 1388 East Walnut, Pasadena, California.

f

Simpson Electric, 5200 \Vest Kinzie Street, Chi-cago, Illinois 60644.

Gibson-Cooke.’

(3)

-.. U

E

0

0.

0

C.)

-iS -12 -6 0 6 2

Base Deficit Arterial (m Eq / I

Fic. 3. Comparison of regression lilies for Base Defi-cit for the two capillary methods, (r) = .69 for

the HC method and .71 for the PC method.

RESU LTS

Separate comparisons of arterial blood

gases with HG and with PC samples were

made for pH, Pco, base deficit, and Po2.

The specific age categories of less than 5

days, 5 to 9 days, and 10 or more days of

age were arbitrarily selected to allow for

comparison with the data reported by

Win-quist and Stamm.1

pH

Thirty-seven simultaneous comparisons

of arterial pH with HG and with PC

sam-ples were obtained over a range of arterial pH of 7.2 to 7.6. There were no differences

between the three age groups or between

the three methods (Fig. 1).

Pco2

Simultaneous comparisons of arterial and

HG Pco were obtained in 37 patients and

of arterial and PG Pco in 36 patients. The

range of Paco was 15 to 60 mm Hg.

No differences were noted between the

three age groups. By either method,

capil-larv Pco2 averaged 15 mm Hg more than

Pa-Co2 at low levels and 10 mm Hg more at

high levels of Paco2

(

Fig. 2) .The difference between capillary and arterial Pco2 was

al-ways greater than the presumed a-v

differ-ence of 4 to 6 mm Hg. Due to the false

elevation of capillary Pco, base deficit esti-mations by either capillary method consis-tently underestimated the true arterial base deficit. This error can be minimized by the calculated regression equation: arterial BD

= 1.3 [PG BD} + 3.9 (Fig. 3).

Po2

Simultaneous comparisons of arterial and

HG Po2 were obtained in 41 patients and of

arterial and PG Po in 38 patients (Fig. 4

and 5). Gomparisons were obtained over a

range of Pao2 from 21 to 355 mm Hg.

By either capillary method, a Po2 of 40 to

50 was never associated with a Pao2 less

than 40 or greater than 90 mm Hg and

could be expected on the average to reflect

a Pao of 45 to 65 mm Hg

(

Fig. 6).

Gone-lation with Pao was best when the patient

was at least 10 days old and the Pao2 less

than 60 mm Hg. HG Po was significantly

superior to PG Po2 only when the Pao2 was

above 90 to 100 mm Hg.

DISCUSSION

The purpose of this study was not to find

an acceptable substitute for arterial blood gases in the management of critically ill

newborn infants, but rather to define the

best alternative to arterial sampling when

continued use of the umbilical arterial

cath-eter is no longer necessary. Although percu-taneous sampling from radial or temporal

arteries is a vell-established technique, these sites are often not adequate for fre-quent determinations in small neonates.

Al-though venous blood is quite adequate for

pH and Pco2 measurements, it is worthless

for Po2 and, furthermore, ordinarily

re-quires that a physician obtain the sample. A valid capillary alternative to arterial punc-ture would, therefore, be helpful in such in-fants.

Although it has generally been assumed

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Arterial P02 (mm Hg)

FIG. 5. Comparison of simultaneous arterial and PC Po. Note the change in scale. Y = .26x

+

21.71, (r) = .85.

504 CAPILLARY AND ARTERIAL BLOOD

F:c. 4. Comparison of simultaneous arterial and

HC Po. Regression line indicated by dotted line and line of identity by dark line. Note the change in scale for both arterial and capillary Po. Y =

.42x + 21.71, (r) = .86.

more closely approximate true arterial

val-ues than will PC sampling, no study is

available comparing arterial pH, Pco2 and

Po with simultaneous capillary samples

from the warmed and the unwarmed

ex-tremity. Since there is no practical and

reli-able way to precisely control the degree of

warming from sample to sample, Pco2 and

Po2 measurements from the unwarmed

ex-tremity may be more predictable and

repro-ducible even though deviation from the

corresponding arterial value may be

greater. For this reason, plus the greater

time investment required by nursing and

laboratory personnel for “arterialized” cap-illary sampling, it was elected to perform

this study by comparing arterial and HC

blood samples to PG samples obtained from

the unwarmed extremity only.

The validity of any method for measuring

capillary blood gases in infancy has been

thought to be a function of age and clinical

state of the patient. :i, i In any sick newborn

infant, or any patient with shock, the capil-lary pH has been found to be consistently

lower, and capillary Pco2 consistently

higher, than simultaneous arterial samples. This study provides additional verifica-tion that, in “sick” infants, capillary pH and

Pco2 (whether HG, “arterialized,” or PC)

may actually deviate from arterial levels to a greater extent than the usual a-v

differ-ences for pH

(

.02 to .04

)

and Pco2 (4 to 6

mm Hg

)

. The explanation for this greater

deviation must be related to a decrease in

capillary perfusion

(

peripheral vasocon-striction

)

. The obvious conclusion is that in

any “sick” infant the preferred sampling

site for estimation of acid-base status is

(

1) arterial or

(

2

)

venous and that capillary

sampling

(

by skilled laboratory

techni-cians

)

will be helpful as an adjunct to

per-cutaneous arterial or venous sampling

(

by a

physician

)

only when peripheral perfusion can be assumed to be “adequate.”

There is no acceptable substitute for Pao2 in the critically ill newborn infant.3 After the first few days of life in infants who are not critically ill, however, good correlation with Pao has been achieved with capillary sampling from the arteriolar bed of the

dig-ital artery.5 This specific study was

prompted by the encouraging results

re-ported for HG blood gases.’ Our initial ex-perience with the HG method led to defini-tion of three problems: (1) this method

re-quired a greater time investment (per

sam-ple) by nursing and laboratory personnel, (2) blistering of the skin under the positive

(5)

Pa02

(mmHg)

505

70

C’

60

E

C’J 0

0

> 40

a

ci. a C)

50

30

20

Fic. 6. Relationship of HC and PC Po2 to Pao when the Pao is less than 70 mm Hg; (r) = .87 for the HC method and .84 for the PC method.

was more than recommended, and

(

3

)

seri-ous electrical hazards could occur if the

iontophoresis equipment was not properly

grounded.

The HC method did not correlate as well

with Pao5 as was predicted by Winquist

and Stamm.1 All but one of our patients

were young infants, however, whereas their

patients were up to 14 years old. Also, the

necessary substitution of a 0.1% for the

previous 1.0% histamine dihydrochloride may have affected the degree of “arterial-ization” in our HC method. Nevertheless,

even though the HG method did produce

marked local vasodilatation, no significant “arterialization,” beyond that achieved by a

PG Po5, was achieved unless the Pao2 was

approaching a supersaturated level

(

i.e.,

greater than 90 to 100 mm Hg ), where arte-rial sampling is required anyway.

Since the usual a-v difference for Po2 is

large compared to pH and C02, the

con-tamination of arterial with venous blood

which occurs with any capillary method

will of necessity result in a lower capillary

Po5 relative to Pao. However, assuming

the degree of venous contamination to be

constant for any given capillary method, a

given amount of venous blood will

obvi-ously yield a much greater depression of

capillary Po2 when the Pao is 150 than

when it is 40 mm Hg. This relationship

ac-counts for our observation that capillary Po2

will approximate Pao2 at low levels but

has no predictive value at hyperoxic arterial levels.

SUMMARY

The minimal superiority of the HC over

the PC method within the usual clinical

range of Pao is not thought adequate to

justify the increased time and risk of the

HC method. When a substitute to arterial

sampling is appropriate and necessary,

therefore, it has become our practice to

uti-lize PC blood gases, according to the

following limitations:

1. Peripheral perfusion must be

“ade-quate.”

2. The patient must be at least 3, but

preferably 5 to 10 days of age. 3. The error in capillary BD is minimized

according to the regression equation:

arterial BD = 1.3 [PC BDI + 3.9.

4. The lower the Pao2, the more reliable

the PC Po2 will be. When the PC Po2

is greater than 60 to 70 mm Hg., there

can be no guarantee that the Pao2 will be below hyperoxic levels.

REFERENCES

1. Winquist, R. A., and Stamm, S. J.: Arterialized capillary sampling using histamine iontopho-resis. J. Pediat., 76:455, 1970.

2. Candy, C., Grann, L., Cunningham, N.,

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THE ALLEGED DELETERIOUS EFFECTS ATTENDANCE AT VASSAR COLLEGE

HAD ON A YOUNG GIRL’S HEALTH A CENTURY AGO

506

pH and Pco, measurements in capillary sam-ples in sick and healthy new born infants.

PE-DIATRICS, 34:192, 1964.

3. Banister, A.: Comparison of arterial and arteri-alized capillary blood in infants with respira-tory distress. Arch. Dis. Child., 44:726, 1969. 4. Siggaard-Andersen, 0.: Acid-base and blood gas

parameters-arterial or capillary blood? Scand. j. Clin. Lab. Invest., 21:289, 1968.

5. Corbet, A. J. S.: Oxygen tension measurements on digital blood in the newborn. PEDIATRICS,

46:780, 1970.

Acknowledgment

To the technicians in the microchemistry labora-tory and nursing staff in the Infant ICU who helped to obtain the simultaneous blood samples.

Many n ineteenth ceo turv physicians believed that a girl’s health would be adversely affected if she were to devote as much time at college

to scholarly pursuits as was required of boys. The passage below, written in 1873 by a well-known Boston physician, is an excellent exam-pie of this opinion.1

Miss D_-_ entered Vassar College at the age of fourteen. Up to that age, she had been a healthy girl, judged by the standard of American girls. Her parents were apparently strong enough to yield her a fair dower of force. The catamenial function first showed signs of activity in her Sophomore Year, when she was fifteen ears old. Its appear-ance at this age is confirmatory evidence of the

normal state of her health at that period of her college career. Its commencement was normal, without pain or excess. She performed all her col-lege duties regularly and steadily. She studied, re-cited, stood at the blackboard, walked, and went through her gymnastic exercises, from the beginning

to the end of the term, just as boys do. Her account of her regimen there was so nearly that of a boy’s

regimen, that it would puzzle a physiologist to

determine, from the account alone, whether the

subject of it was male or female. She was an aver-age scholar, who maintained a fair position in her class, not one of the anxious sort, that are ambitious of leading all the rest. Her first warning was faint-ing away, while exercising in the gymnasium, at a time when she should have been comparatively quiet, both mentally and physically.

This warning was repeated several times, under the same circumstances. Finally she was compelled to renounce gymnastic exercises altogether. In her Junior Year, the organism’s periodical function be-gan to be performed with pain, moderate at first, but thore and more severe with each returning

month. When between seventeen and eighteen

years old, dysmenorrhea was established as the

order of that function. Coincident with the appear-ance of pain, there was a diminution of excretion;

and, as the former increased, the latter became more marked. In other respects she was well; and, in all respects, she appeared to be well to her companions and to the faculty of the college. She graduated before nineteen, with fair honors and a poor

phy-sique. The year succeeding her graduation was one of steadily-advancing invalidism. She was tor-tured for two or three days out of every month; and, for two or three days after each season of

torture, was weak and miserable, so that about one sixth or fifth of her time was consumed in this way. The excretion from the blood, which had been gradually lessening, after a time substantially stopped, though a periodical effort to keep it up was made. She now suffered from what is called

amenorrhea. At the same time she became pale,

hysterical, nervous in the ordinary sense, and

al-most constantly complained of headache. Physicians were applied to for aid: drugs were administered; travelling, with consequent change of air and scene, was undertaken; and all with little apparent avail. After this experience, she was brought to Boston for advice, when the writer first saw her, and learned all these details. She presented no evidence of local uterine congestion, inflammation, ulceration, or dis-placement. The evidence was altogether in favor of an arrest of the development of the reproductive apparatus, at a stage when the development was nearly complete. Confirmatory proof of such an arrest was found in examining her breast, where the milliner had supplied the organs Nature should have grown. It is unnecessary for our present

pur-pose to detail what treatment was advised. It is sufficient to say, that she probably never will be-come physically what she would have been had her education been physiologically guided.

NOTED BY T. E. C., JR., M.D.

REFERENCE

1. Clarke, E. H.: Sex in Education; or, A Fair

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1973;51;501

Pediatrics

Carl E. Hunt

ARTERIAL BLOOD

LIMITATIONS OF TWO METHODS OF SAMPLING, COMPARED WITH