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SIDS, suffocation, asphyxia, and sleeping position

Article - June 1998 hd_pedia.gif (3302 bytes)

Sudden Infant Death With External Airways Covered
Case-Comparison Study of 206 Deaths in the United States

N. J. Scheers, PhD; C. Mitchell Dayton, PhD; James S. Kemp, MD

SIDS, suffocation, asphyxia, and sleeping position

Perhaps a bit of clarification of terminology would help here. Death due to overlaying refers to the death of an infant, while co-sleeping with adults, caused by an adult rolling onto the infant and suffocating the child. This was widely believed to be a common cause of infant death for centuries. In centuries past, when parent-infant co-sleeping was the norm, mechanical devices were constructed to prevent this from happening. It is now believed that infant death by overlaying is quite rare.

Infant/parent co-sleeping is a difficult topic and I think it is fair to say that there are data showing 1) that infant/parent co-sleeping is associated with problems and 2) that it is associated with benefits. The existing data are conflicting and there are complex interactions that we don't understand. For example, the problem with infant/parent co-sleeping may be that it is unsafe for an infant to sleep on adult bedding materials and adult mattresses... it may have nothing what so ever to do with overlaying and that sort of thing. There is a lot of work yet to be done in this area before we'll know the answer.

Suffocation refers generally to the death of an infant caused by obstruction of the breathing passages. A classical example is the infant who slips down between the crib rail and mattress and has the face pushed against the mattress leading to suffocation. This is one reason why the CPSC is *so* insistent on using a proper size, shape, and firmness crib mattress. The operative concept here, in suffocation, is mechanical obstruction to breathing due to something blocking air entry/exit, usually at the nose or mouth. It is true that the CPSC reported that some deaths diagnosed as SIDS were likely due to suffocation caused by unsafe cribs and bedding materials and I suspect that they are right. However, as far as I know they are not referring to overlaying, they were referring to unsafe cribs, unsafe mattresses, unsafe bedding material, and other "crib safety" factors.

Positional asphyxia is yet different from any of the above. Asphyxia is a physiology/pathology term referring to breathing insufficiency leading to inadequate intake of oxygen and inadequate exhalation of carbon dioxide. It can be caused by a variety of factors, some of which may be related to sleeping position and/or bedding materials. One theory that has a fair amount of experimental support behind it is that infants sleeping prone with their face in bedding material may actually be not getting enough oxygen and the CO2 (carbon dioxide) the baby is exhaling may be accumulating in the bedding (pillow, mattress, pads, etc.) so that the infant is breathing it back in and therefore can't exhale enough CO2. If this situation persists, the oxygen levels fall to dangerously low levels and the CO2 rises to dangerously high levels. The low oxygen level, in a baby, can actually further impair the ability of the baby to respond to the situation and a vicious circle develops which may lead to death if uninterrupted. It is believed that infants are particularly vulnerable to this sort of asphyxial death at certain times during development after birth, mainly due to immature breathing control systems that take time to mature after birth. This is believed to correspond with the first 4-6 months of life. This is why so much SIDS-related research focuses on development of breathing control after birth. Premature infants are particularly vulnerable to low oxygen levels. This may be part of the reason that the SIDS rate is so much higher in preemies compared to full term infants.

However, this is only one theory of how sleeping position might relate to SIDS; there are others that are also plausible. Of course, more than one may be correct; there could be multiple ways that sleeping position could be related to SIDS. The bottom line is that we just don't know at the present time. It is believed that there is sufficient evidence to recommend non-prone sleeping position for infants, even it we don't yet know for sure the mechanisms involved. If the scientists don't yet know how sleeping position relates to SIDS, how can parents be expected to know? How can anyone act on information they don't have? The most that can be expected of anyone is to do the best one can with the information available at the time.

I hope this helps.

John L. Carroll, M.D.
The Johns Hopkins Children's Center
Baltimore, MD

The physiology of breathing control and it's development after birth is very complex.

I would have to disagree with the statement, "The SIDS baby 'forgets' to breath". There is no evidence in the literature that this is the case. Albert Steinschneider, years ago, reported an apparent correlation between SIDS and apnea and started the research community on years of research looking into the relationship between apnea (stopping breathing) and SIDS. For a long time it was believed that these phenomena were tightly linked. However, we now know, from several large studies, that there is no correlation between apnea recorded on pneumogram type recordings and later SIDS death.

Concerning the possibility that the infant does not respond appropriately to a challenge or stress, this currently *is* believed to be a likely scenario. Perhaps this is what you meant by "forgets" to breathe. In any case, there is evidence indicating that SIDS infants may not respond appropriately to a stressor (such as having the face in a pillow or soft mattress). There are several reports of infants dying while on monitors (memory monitors) and in most cases apnea is the last thing to develop. So, it appears from these recordings that apnea was a final/terminal event and not the initial event.

There is also evidence that certain groups of infants are very vulnerable to the effects of hypoxia (low oxygen level). An excellent study from Dr. Keen's group showed that the response mechanisms of premature infants to low oxygen levels during sleep are weak and, in a significant proportion of infants, inadequate.

We don't yet know whether most SIDS infants are basically physiologically normal or whether there is some subtle abnormality of breathing control. One theory holds that most SIDS infants could be basically normal but get into unfortunate combinations of circumstances that lead to death. For example, the respiratory control system of infants is immature at birth and takes time to develop. This includes the ability to respond to low oxygen or high CO2 levels. Just after birth and for the first month or so, infants sleep in whatever position they are put in the crib to sleep. They are developmentally not able to change position. At a certain age, say 2-3 months, the infant is able to get into situations that could compromise the airway - for example, role prone, get the face into a soft mattress pad or bumper. However, defense mechanisms are still immature. In the absence of complete nasal/oral obstruction there would be no "suffocation" type response, and the baby could go on breathing into the mattress/pillow. Under these circumstances, the work of Drs. Kemp, Thach, and others tells us that CO2 can accumulate and O2 levels can fall very slowly. When this happens slowly, the response can actually be blunted or reduced. Severe low O2 levels can further depress the ability of the infant to respond - the result is slow asphyxiation *without* a struggle. I apologize if outlining this scenario is painful for some but it is currently thought to be a very plausible explanation. In my opinion, this could happen in a physiologically normal infant.

Dr. Hannah Kinney is a very well respected researcher in the field and it is true that she published a paper in Science showing that a group of SIDS infants had a higher frequency of a particular brainstem abnormality compared with controls. However, the relationship of this finding to the physiology underlying SIDS is unknown. There is no evidence to date, that this abnormality causes SIDS. It would be incorrect to say, at this point, that SIDS is due to a brainstem abnormality.

You are correct that SIDS infants also die without evidence of nasal/oral obstruction being involved. This points out why we have to be very cautious in speculating about the causes of SIDS. My opinion is that 1) all infants are vulnerable during early postnatal development (0-6 months), 2) some infants are more vulnerable due to predisposing factors (such as prematurity, smoking during pregnancy, smoke-exposure postnatally, etc.), and 3) all infants may be challenged by stressors such as colds, getting into potentially dangerous positions (face in pillow or mattress, airway obstruction). I think SIDS is the result of an unfortunate combination of stresses and vulnerability that sometimes is fatal. Whether there is an underlying abnormality (or more than one) associated with SIDS remains to be shown. There are intriguing studies showing that SIDS infants have more obstructive apnea, possible smaller upper airways, and other findings suggesting specific vulnerabilities. Much more work is needed in the area of respiratory control development and how it might relate to SIDS, as well as on potential sources underlying increased vulnerability.

Hope this is helpful.

John L. Carroll, M.D.
The Johns Hopkins Children's Center
Baltimore, MD

I think there are many factors with the supine sleep position studies that influenced the approach in the US.

Some of these include:

1. The SIDS rate in each of the countries where these studies were done is much higher than that of the US. This is despite the fact that almost all the babies in the US slept prone. If sleep position is so critical why is our rate so low ???!!!!

2. There were other factors that contributed to the SIDS rate in other countries including excessive bedding and heating.

3. There were no comparable studies in the US.

4. The other reason is that doctors and health care workers (and your grandmother) all know that sleeping on your back is harmful since the baby can choke and aspirate on the food. This fear has not borne out. There is no increased risk of aspiration that has been reported in the medical literature.

5. Change is very difficult to accept. Difficult for doctors, nurses and other healthcare professionals who have done it this way for 10 or 50 years. Not until the NIH or AAP mandated that the supine sleep position should really be what the medical community recommends did pediatricians consider this sleep position as a possibility. Even after this recommendation many SIDS experts were skeptical and were not recommending this sleep position to their patients.

I am sure there are other factors that I failed to mention, but these few come to mind.

JDDeCristofaro, MD
Dept of Peds
Stony Brook, NY

The most recent study I saw from Montreal was published the May 1996
Journal of Pediatrics (vol 128, number 5, Part 1, pg 616) by K.
Waters etal.

The authors reported on the frequency and duration of face straight
down and face near straight down in normal full term infants in their
cribs at home.

What these researchers did was to first select full term "normal"
infants that usually slept on their bellies. Ten infants ages 10-22
weeks were monitored in their sleep. They were placed to sleep on
their belly the first night, side the second night, and belly the
third night. They measured many different physiologic parameters
including oxygen saturation, capillary tissue carbon dioxide levels,
chest and abdominal wall movements, EKG and heart rate, and
videotaped the infant. They found that normal term infants that
sleep on their belly will place their face straight down into the
bedding 0.6 times per night (range of 0-4) for an median duration of
3.3 minutes. Additionally these 10 babies put their face in the near
straight down position 5.3 times per night (1-10 range) for 22.4 min.
None of these events resulted in a prolonged apnea, bradycardia,
carbon dioxide retention or desaturation. All infants self aroused
when in these positions.

JD DeCristofaro, MD
Dept of Peds
Stony Brook, NY

Dr. Hoffman was in the Biometry Branch at NICHD and played an important role in the largest study of SIDS ever conducted. This was the "National Institute of Child Health and Human Development SIDS Cooperative Epidemiological Study"(ref 1),which involved 838 SIDS cases, of which, after extensive review of the data, 757 were classified as being probably SIDS. For each case there were several controls (~ 1600 total). This was a *huge* undertaking involving not only the principal investigators but data coordinators,pathology lab coordinators, a pathology study panel, a large advisory committee, numerous consultants, and NICHD staff members. Sleeping position was one of many factors investigated by this study. It must be remembered that this study was actually begun in 1978, when the sleeping position issue was not so well known as it is now. It is my understanding that Dr. Hoffman played a significant role in this study and I'm not aware that the quality of the study or these data have been called into question.

The sleeping position data were discussed by Dr. Hoffman in a 1992 publication in Clinics in Perinatology (ref 2). In the NICHD study they asked the "usual" sleeping position within the 2 weeks prior to death. 81% of SIDS, 72% of control A, and 77% of control B patients answered yes to 'usually slept prone'. The difference was, in fact, statistically significant. Dr. Hoffman acknowledges that because the prone sleeping position was so common, it may not emerge as a strong risk factor. Recent publications have made this point more strongly... in a population in which prone sleeping is very common (eg,. 80%) epidemiological studies may miss an association even if one exists. He acknowledged this and goes onto say that within the next 2-3 years (from 1992) much stronger evidence should become available. He then reviews New Zealand, Tasmanian, and other data and points on how SIDS rates have fallen "substantially" since changing to the non-prone sleeping position for infants (ref 2).

I would add that Dr. Hoffman's work on the large NICHD epidemiological study was very instrumental in moving the research community forward on the SIDS/smoking link, and a great deal of other very interesting data came from that study.

Research grant money is *not* under the control of the staff at NICHD. NIH grant funds are distributed by a peer-review system that is designed to avoid just such potential problems. The process is very rigidly controlled and involves review of every grant proposal by a panel of scientists who know the field. Each grant proposal is judged by the peer-review panel based on it's scientific merit and the track record of the investigator.

The recommendation that healthy infants be placed non-prone for sleep came from the American Academy of Pediatrics (AAP), not NIH. For reasons they outlined in their statements, they were being cautious. The issue of sleeping position and SIDS had been discussed for years in various forums and it was felt by many researchers that the data were insufficient to come out earlier with such recommendations. At the time that statement was written (1991) the available data on sleeping position were all from other countries, many of which had very high SIDS rates(much higher than in the US - New Zealand for example). Many researchers were concerned that sleeping position may not play the same role in SIDS in the US as it appeared to in other countries. In addition, at that time, there were prominent researchers who were strongly opposed to the AAP recommendation (published in 1992). Some of these researchers voiced strong opposition and continued to publish opposing viewpoints and express concern about the potential dangers of the supine sleeping position after the statement came out.

Many researchers were concerned that there were significant dangers possible with a nationwide switch to supine infant sleeping and they didn't feel the data warranted a nationwide recommendation for the USA at that time. The basic idea was that in New Zealand, for example, they started with a SIDS rate of 4-6/1000 live births (depending on whether you include the Maori population) and after they switched to non-prone sleeping position for infants the SIDS rate dropped to 2.5 (in 1991, when the statement was being written). This rate of 2.5/1000 was *still*higher than the USA rate. Some researchers feared that the New Zealand and other data (eg., from Tasmania, Norway, other countries) didn't represent the USA, where the SIDS rate was already lower than in those countries. So, the picture was not so clear in 1991-1992, when the AAP statement was being written. In fact, many felt at that time that the AAP statement was premature.

I am not defending NIH or NICHD management. I just want to point out that it wasn't as clear 5 years ago as it may seem now and that, in any case, NICHD staff don't directly allocate research funds nor do they directly set funding priorities. The current 'management' of the SIDS program at NICHD is, in my opinion, excellent. I can provide you with more information about the present NICHD SIDS program if that would be of interest. Some of this information is available on the NIH website.

1). Hoffman HJ, Damus K, Hillmand L, and Krongrad E: Risk factors for
SIDS: Results of the NICHD coorperative epidemiological study. Ann NY
Acad Sci, 533:13-30, 1988.

2). Hoffman HJ and Hillman LS: Epidemiology of the sudden infant death
syndrome: Maternal, neonatal, and postneonatal risk factors. Clinics in
Perinatology, 19(4):717-737, 1992.

John L. Carroll, M.D.
The Johns Hopkins Children's Center
Baltimore, MD

Date: Sat, 11 May 1996 12:21:54 -0700
From: Charles Peterson
Subject: SIDS,Suffocation

Digging through my research archives I come upon a few more references that may be of interest to you, or to anyone else interested in the subject of suffocation verses SIDS. In short, while there are some indications at autopsy that are similar between the two causes of death they are definitely different and should not be confused.

Richard L. Naeye
Science 186:837, 1974

Richard L. Naeye
Am J Clin Pathol 66:526, 1976

Richard L. Naeye
New England Journal of Medicine Vol. 289, No. 22, November 29, 1973 pp 1167 -

Richard L. Naeye, Russell Fisher
Science (Vol. 19?) February 13, 1976

Richard L. Naeye
Pediatrics Vol. 63, No. 1, 1979 pp 8 - 12

D.P. Southerly, J. Richards...
Archives of Disease in Childhood, 1980, Vol. 55, pp 7 - 10

Joan E. Hodgman, Toke Hoppenbrouwers...
Pediatrics (ISSN 0031 4005) 1982, pp 785 - 792. American Academy of

Alfred Steinschneider, Steven L. Weinstein...
Pediatrics Vol. 70, No. 6, December 1982 pp 858 - 863

David P. White, Neil J. Douglas...
Am -Rev Respir Dis 128: 984, 1983 pp 984 - 986

D.G. Fagan, A.D. Milner
Archives of Disease in Childhood, 1985, 60 pp 471 - 485

William P. Potsic, Ralph F. Wetmore
Otolaryngologic Clinics of North America, Vol. 23, No. 4, August 1990 pp 651 -

Vicki L. Schechtman, Ronald M. Harper...
Pediatrics Vol. 87, No. 6, June 1991 pp 841 - 845

Sally L. Davidson Ward, Daisy B. Bautista...
Pediatrics Vol. 89, No. 5, May 1992 pp 860 - 864

Frederick Mandell
Arch Dis Child 56:312, 1981

Dorthy H. Kelly, Howard Golub...
The Journal of Pediatrics Vol. 109, No. 2, August 1986 pp 249 - 254

D.F.N. Harrison
The Journal of Laryngology and Otology August 1991, Vol. 105 pp 646 - 650

Richard L. Naeye
Sci American 242:56, 1980

M. A. Valdes-Dapena
In: SUDDEN INFANT DEATH SYNDROME (Tildon, et al., 1983), p. 169

H. F. Krous, A. C. Catron, J. P. Farber
Arch Pathol Lab Med 108:77, 1984


Is a child who died from breathing their own co2 without struggling properly considered SIDS because he failed to struggle, or is it normal for an infant to not struggle when slowly deprived of oxygen?

The normal response of an infant deprived of oxygen is to arouse. All the babies in this study self-aroused from an obstructed sleep position and began to breath again.

Infants who are deprived of oxygen do not necessarily build up carbon dioxide. Carbon dioxide goes up when we don't breath (apnea) or if we don't breath enough (hypoventilate). In the movie Apollo 13 we saw the crew get exposed to dangerously high levels of carbon dioxide. In normal infants and people who are exposed to extra carbon dioxide (from apnea or hypoventilation or if we increase the CO2 they are breathing) the normal physiologic response is to increase breathing (deeper breaths and more rapid breaths). However, if the CO2 levels climb to dangerously high levels, our normal brain responds by stopping breathing, we go into a trance like state and die.

An infant who failed to respond to this co2 build up would die. It would be considered SIDS if this happened. The major hypothesis for SIDS currently is this failure to arouse from a hypoxic (low oxygen) and/or hypercapneic (high CO2) state.

The point of the above article was that normal full term infants frequently find themselves in this stressed situation and adapt to this stress by arousal. "Vulnerable" infants are exposed to this stress and fail to adapt or respond.

I hope this helps.

JDDeCristofaro, MD
Assistant Professor of Pediatrics
Medical Director, Infant Apnea Program
UMC Stony Brook, NY

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