MIST THERAPY

from many indications, that pediatric edu-cation is assuming that this is the road to be

chosen. There are certain things to

com-mend it-including its potential for

attract-ing into pediatrics a number of young

phy-sicians who might otherwise follow a more

“glamorous” course.

But there is one obvious and overriding itestion. If pediatrics goes the way of

sub-specialization who will take care of the

bulk of the problems of the 76 million

chil-dren coming our way?

Someone must continue to do the job of

general child health care that is being done

now. The task would increase greatly in

magnitude in the years ahead, even if we

confined ourselves to serving the

popula-tion groups we are serving well today. But

neither we nor society will be satisfied with

that goal. We have received and accepted

the challenge to provide the best health

care possible to all children, regardless of

economic or social condition.

In a sense the problem of pediatrics is

analogous to the central problem of

medi-cine as a whole-the management of

spe-cialization, and the organization of health care to serve an entire nation.

In my view pediatrics is in a particularly

strategic position to lead the way. The

pediatrician is a special kind of specialist.

He probably has the broadest range of

con-tact-with children, their parents, and

corn-rnunity institutions-of any medical

disci-pline. As a specialized generalist he is in a

position to influence much more than his

own individual role.

I have said that pediatrics is at a moment

of decision. By whom must the decision

be made?

Clearly, society itself is deeply involved

in the decision-making. The forces

compel-ling the decision are generated within the

population as a whole, and it is these forces

that have brought us to our present

cross-roads. The American people have strong,

though not clearly formulated, opinions as

to what they expect from pediatrics.

WTithin this broad framework, however,

the pediatric profession will be responsible in large measure for shaping its own future. This is not an either-or choice; the future

will undoubtedly involve elements of

spe-cialization and elements of generalized

practice. But determining the balance and

developing the capacity for discharging the

chosen responsibilities can be done only by

the existing body of pediatrics-in practice,

in teaching and in research. Yours must be

the soul-searching, the dialogue, the hard

decisions, and the leadership to carry them

out.

WILLIAM H. STEWART, M.D.

Surgeon General, Public Health Service

U.S. Department of Health, Education,

and Welfare

MIST

THERAPY

N ARTICLE on mist tent therapy in this

issue opens with the remarkable

com-ment that no objective evidence of the

efficacy of mist in patients with cystic

fibro-sis has been published.1 The authors

con-ducted a clinical study which led to the

conclusion that under the conditions of

their study mist was effective as judged by

improvement in some aspects of pulmonary

function. It may be even more remarkable

to note the absence of objective evidence of

the role of mist in the wide variety of

cir-cumstances in which it is commonly used

on most pediatric services.

A review of some physical principles

in-volved in water exchange, of observations

on the deposition of aerosolized particles in

the lung, and of the uses and hazards of

mist therapy seems pertinent.

HEAT AND WATER TRANSFER

The respiratory tract is a conditioner for air. Regardless of its temperature or water

equi-ALVEOLAR’ ENVIRONMENT WATER CONTENT OF AIR (mg/L)

AIR TEMPERATURE #{176}C.

1. The relationship between water content

relative humidity at different temperatures. arrows indicate average room temperature

body temperature (modified from Walker,

et al.’). Ftc. and The and COMMENTARIES

*Data from Radford, E. I

librium with body temperature and to full

saturalion with water vapor by the time it

reaches the carina. Indeed, most of the con-ditioning occurs in nasal passages or phar-vnx, where turbulent convection facilitates

heat and water transfer from the warm and

moist mucosal surfaces to the inspired air.

At the end of inspiration, nasal mucosal

temperatures are lower than at the end of

expiration. Thus during expiration, air tern-perature falls to about 32#{176}Cfrom the 37#{176}C

of alveolar air, and, during this cooling,

water vapor condenses on the mucosal

stir-faces. The release of the latent heat of

va-porization during condensation contributes to the further warming of mucosal surfaces

in preparation for the next inspiration. An

adult dog can warm inspired air of - 100#{176}C

mostly in the tipper w2 Approximately

20 to 25% of the heat and water required

to bring room air to body temperature,

fully saturated with water vapor, are

re-covered; nonetheless, the average human

adult in a temperate climate loses 250 ml

water and 350 kilocalories of heat in his ex-pired air each day.3

Although data on heat transfer in the

lungs of small infants are not available, it is

probable they too achieve conditioning of

their inspired air by the time it reaches the

carina. The proportionately smaller tidal

volumes of the infant and the shape of the

tipper airway should permit adequate

tur-bulence for mixing, warming, and

humid-ification of inspired air.

The water content of air will depend on

its teflhl)erature. One hundred percent

hu-midity at room temperature of 23#{176}Cis the

equivalent in water content of only 50%

humidity at 37#{176}C.The relationships

be-tween water content, temperature, and

hu-midity are shown in Figure 1. Humidity is

related to the water vapor, or water in a

gaseous state. Mist or fog refer to water

suspended in particulate form in air.

EFFECT OF WATER VAPOR AND

MIST

ON

AIRFLOW

The density and viscosity of inspired air

are affected by the presence of water vapor.

It is desirable in the presence of much

tur-bulence, as in croup, to breathe a mixture

of low density; with marked airway

nar-rowing, as in bronchiolitis, it is desirable to

breathe a mixture of low viscosity (see

Table I).

The effect of water vapor is to reduce

both density and viscosity, although the

magnitude of the difference cannot exceed

6%, which is the percent water vapor in

sat-urated air at body temperature. The

addi-tion of particulate matter such as mist to

the air probably has minimal effect on air

flow characteristics, since no change in

air-way resistance in normal adults could

be detected during mistbreathing1 even

though a definite decrease could he

mea-sured during inhalation of a bronchodilator (i.e., isoproteronol). Further studies with newer nebulizers in children are in order.

TABLE I

DENSITY AND VISCOSITY OF GASES AT

0#{176}CAND 1ATMOSPHERES

Gas

Density Jiscosity

(gm/I) (micro poises)

Fog Mist

a1nDrop

Bacteria

es

ee

r

anHai

1#{149}1

Un

DIamet

0.O

0.COf

05

o.boi

0.01

0.1

5

I

1

500

5000

I

10

100

000

PARTICLE

SIZE,

MICRONS

FIG. 3. Illustrative particle sizes (modified from Hatch and Cross7).

Trachea

Nearly

all particles

10 microns

settle

at

level or

Bronchi

Some particles

as

AlveoLi

particles

of

than

2

mi-S

crons

penetrate.

Those

of

less

than

.5 microns

deposit

FIG. 2. Schematic view of principal sites of deposition of particles in the lung (adapted from Mitchell19).

Tobacco

Smoke

C 0 C 4, 4, 0 0 4,) 4, 4, 0. 4, 0 4) 0.

0 4 ₅

C 0 U) 0 0. 4, 0 0 4, > C 4, C.) V ₂

Particle Size, Microns

5

Fic. 4. Percent of alveolar deposition of particles in man and monkey (reproduced with permission;

Palm, et al.’#{176}).

COMMENTARIES

DEPOSITION OF PARTICLES

Particle size is of concern in predicting to what part of the lung the inhaled material

will penetrate (Fig. 2 and 3). Many

mea-surements leave little doubt that about

50% of particles of I will deposit in the

alveoli (Fig. 4). Hardly any particles of

over 4 will reach the alveoli, since 75 to

80% of them settle in the upper airway

(Fig. 5).7 It is relevant to consider the

out-put of commercially available mist or fog

generators with respect to milliliters

aero-solized per minute and particle size. The

values on 15 commercial nebulizers are

given by Mercer, et al. The variation in

output of solution per liter of air flow is

from 6 to 31 tl in small reservoir units, and 26 to 70 p.l in large reservoir units. Particle sizes in both types of nebulizers range from 3 to 9 .t. One ultrasonic nebulizer produced particles of 7.7 to 9.6 t, at an output

equiv-alent to 142 per liter of air flow.

The deposition of particles is affected by

the respiratory rates, in the direction of less

deposition at higher rates.#{176}The percent

re-tention of .5 i. particles rose from 20 to 60%

as the mean time in the lungs rose from 2 to

12 seconds. This fact can be put to use with

cooperative patients who can breath-hold at

RI h(

C

-

Particle Size, Microns

-x- Monkey

FIG. 5. Percent of upper airway retention of

particles of different sizes in monkeys, of similar size to human infants, and adult man (reproduced

with permission; Palni, et al.’).

the end of inspiration. The dependency of

deposition on frequency is apparently not

critical in translating findings in adults to

infants. Monkeys with tidal volumes of 10

to 25 ml and respiratory rates of 40 per

minute, similar to those of infants, had

nearly identical upper respiratory retention

of particles and alveolar deposition to those of adult humansbo (Fig. 4).

The question of aerosolizing 10%

propyl-ene glycol in water versus water alone is a

matter of debate, advocated by some,1 not

by others.6 Matthews and Doershuk1 advise

its use on the basis that it will retard

evapo-ration of the droplets, stabilize particle size, and increase the amount of water available

for deposition in the peripheral

tracheo-bronchial tree. Another possibility.

sug-gested by Dautrebande, was that the

hy-groscopic nature of propylene glycol could

promote increase in droplet size after

pene-trating the respiratory tract, and hence

limit the extent of penetration. Thus,

propylene glycol could actually lessen the

stability of particles.

The decision to add a hygroscopic agent

such as propylene glycol to increase

particle size would be logical if the aim

it would be illogical if the aim were to de-liver 2 i. particles to the alveoli. Since most

nebulizers produce particles too large to

penetrate alveoli, some evaporation and

re-duction in size could be desirable.

Evapo-ration is enhanced by small droplet size and

retarded by high humidity and solute

con-tent.

EFFECTS OF MIST ON SPUTUM

Objective measurements of sputum

pro-duction, viscosity, and the effects of mist

are very few, even in those conditions

char-acterized by abundant sputum. It has been

shown that sputum produced overnight is

more viscous than that produced during the

day, perhaps because the decreased

cough-ing and removal of sputum at night permits it to accumulate, or the patients are

rela-tivelv dehydrated at night. When fluid

in-take by mouth was tripled, viscosity of

spu-tum was reduced by two-thirds in one

study.1’ Inhalations of mist or steam in

chil-dren w’ith purulent lower respiratory

dis-ease did act as an expectorant and lowered

the viscosity of sputum in the study of

Basch, Holinger, and Poncher.12 No added

effect of mist over full humidity on ciliary

motion has been provided. The studies of

Dalhamn on the isolated rat trachea

es-tablished the efficacy of 70% relative

hu-miditv, and the retarding effect of 30%

hu-midity on ciliary motion. Since air in the

lungs is always humidified by the tipper

air-way, nothing is to be gained by mist,

dis-tinct from humidity, from the aspect of

cili-ary motion. When the nasal cavity is

by-passed, as in patients with tracheostomies

or mouth breathing, inspired air should be

humidified.

The role of mist in bronchiolitis and

bronchopneumonia remains in dispute. The

studies of Kelsch, et ai.14 did not show

any significant benefit as judged by clinical

signs or duration of hospitalization. Mist

would not be expected to be of help if

se-cretions are not a problem. Humidified

oxy-gen, on the other hand, is surely of benefit

to anyone with respiratory insufficiency, if

given with caution and knowledge of its

effects on carbon dioxide retention.

HAZARDS OF MISUSE OF MIST

The distressing habit of ordering mist by

writing the trade name of a commercial

tent has pervaded many services, leaving

the crucial decision of the rate of gas flow,

use of air or oxygen, and temperature

regu-lation to an overworked nursing staff. If ice

cubes are added to cool one surface of the

tent, the internal temperature may fall to

levels which require of the patient

shiver-ing, vasoconstriction, and increased oxygen

consumption. Droplets of water on the skin

or wet clothes aggravate the heat loss by

the patient and the energy expenditure

which accompanies it. Conversely, failure

to watch the tent temperature may lead to

overheating, which is also metabolically

costly. Since any increase in metabolism

re-quires an increase in ventilation, the aim of easing respiratory distress can be defeated

by faulty tent temperature regulation.

Clearly these problems in thermoregulation are critical chiefly for the small infant, most

of whose body is in the tent, who cannot

always help himself to covers, and whose

cries of distress are either not heard or

mis-understood. If the aim is to provide an

opti-mal thermal environment for the infant and

add mist to the inspired air, a tent tempera-ture of ±23 to 25#{176}Cfor the clothed infant is

optimal from the aspect of cardiac work.15

A higher temperature could increase the

water content in the inspired air, but the

cost of greater total energy expenditure in

the hot environment could negate any

benefit from moisture in the lung. If the pa-tient is not in a tent and receiving

humid-ified air or oxygen by mask or tube, the

inspired air could be at body temperature

and fully saturated, with a greater net

amount of water delivered to the lungs.

Any reservoir of water, especially in

ap-paratus which is difficult to clean, may be a

culture medium for the “water bugs,”

pseu-domonas and aerobacter in particular.

proce-165

dures and thorough drying of the

equip-ment between patients are essential.m618

At present, technical advances in the

gen-eration of mists and knowledge of the

de-position of particles exceeds knowledge of

the role of mist in the treatment of

respira-tory disorders. Some evidence exists that

viscous secretions can be thinned by mist;

tipper airway cooling and drying can be

de-creased by added humidity.

No evidence exists that lower airway

oh-structive disease or bronchopneumonia in

the absence of mtich sputum is improved

by mist. Humidified oxygen can be given

without mist. It would be useful practice

for the physician to consider whether the

aim is to provide oxygen or air, humidity or

mist, and to weigh the hazards of a wet,

tineven thermal environment versus

uncer-tain gain from mist in the treatment of

lower respiratory disease.

MARY ELLEN AVERY, M.D.

MORTON GALINA, M.D.

RICHARD NACHMAN, M.D.

The Harriet Lane Service

Department of Pediatrics

The Johns Hopkins University

School of Medicine

Baltimore, Maryland 21205

REFERENCES

1. Matthews. L. W., and Doershuk, C. F.: Mist

tent therapy in the obstructive pulmonary

le-sion of cystic fibrosis. PEDIATRICS, 39:176,

1967.

2. Moritz, A. R., and Weisiger, J. R.: Effects

of cold air on air passages and lungs:

Experimental investigation. Arch. Intern. Med., 75:233, 1945.

3. Walker, J. E. C., Wells, R. E., and Merrill,

E. \V.: Heat and water exchange in the

respiratory tract. Amer. J. Med., 30:259,

1961.

4. Radford, E. P.: In Fenn, W. 0., and Rahn,

H., ed.: Handbook of Physiology,

Respira-tion I. Washington, D.C.: American

Phys-iological Society, 1964.

5. Butler, J., Caro, C. C., Alcala, R., and

Du-Bois, A. B. : Physiological factors affecting

airway resistance in normal subjects and

in patients with obstructive respiratory disease. J. Clin. Invest., 39:584, 1960.

6. Dautrebande, L. : Physiological and pharmaco-logical characteristics of liquid aerosols.

Plwsiol. Rev., 32:214, 1952.

7. Hatch, T. F., and Gross, P.: Pulmonary

Deposition and Retention of Inhaled

Aero-sols. New York: Academic Press, 1964.

8. Mercer, T. T., Goddard, R. F., and Flores,

R. L. : Output characteristics of several

commercial nebulizers. Ann. Allerg. 23:314, 1965.

9. Altshuler, B., Yarmus, L., Palmes, E. D., and

Nelson. N. : Aerosol-deposition in the human respiratory tract. I. Experimental procedures and total deposition. Arch. Indust. H., 15:

293, 1957.

10. Palm, P. E., McNemev,

J.

Respiratory dust retention in small animals.

Arch. Indust. H., 13:355, 1956.

11. Blanshard, C.: The viscometry of sputum.

Arch. Middlesex Hosp. 5:222, 1955.

12. Basch, F. P., Holinger, P., and Poncher, 11.:

Physical and chemical properties of sputum. II. Influence of drugs, steam, carbon

(lioX-ide and oxygen. Amer. J. Dis. Child., 62:

1] 49, 1941.

13. Dalhamn, T.: Mucous flow and ciliarv activity

in the trachea of healthy rats and rats

exposed to respiratory irritant gases. Acta

Physiol. Scand. (Suppl. 123), 36:13, 1956.

14. Kelsch, R. C., Barr, M., DeMuth, C. R.: Mist

therapy in lower respiratory tract infection.

Amer.

J.

15. Burch, C. E., DePasquale, N. P., and Hvman,

A. L.: Influence of temperature and oxygen

concentrations in oxygen tents. J.A.M.A.,

176:1017, 1961.

16. Reinarz, J. A., Pierce, A. K., Mays, B. B., and

Sanford, J. P.: Potential role of inhalation

therapy equipment in nosocomial pulmon-arv infection. J. Clin. Invest., 44:831, 1965. 17. Edmondson, E. B., Reinarz, J. A., Pierce,

A. K., and Sanford, J. P.: Nebulization

equipment. Amer. J. Dis. Child., 111:357,

1966.

18. Sever, J. L.: Possible role of humidifying

equipment in spread of infections from the

newborn nursery. PEDIATRICS, 24:50, 1959.

19. Mitchell, R. I.: Retention of aerosol particles in the respiratory tract. Amer. Rev. Resp.

1967;39;160

Pediatrics

MARY ELLEN AVERY, MORTON GALINA and RICHARD NACHMAN

MIST THERAPY

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