Tuesday, October 16, 2012

Childbirth and Modern Medicine

A fascinating article by Atul Gawande about the history of childbirth and the medical establishment and the ethical considerations facing today's obstetricians.

"The Score"
Published in The New Yorker, Oct 9, 2006


Have you ever wondered why human babies are so much more vulnerable at birth when compared to other newborn mammals? We can't walk, see very far, and our nervous system is still somewhat undeveloped. Turns out it's because of the relatively small size of the human pelvis which is what the baby has to pass through to get born. Since our bodies can't wait for babies to get any bigger, all human babies are effectively born premature and develop the skills most mammalian infants have immediately over the next year or so.

Sunday, October 14, 2012

Losing a Child

An illustrated memoir by Tom Hart and his wife, both artists, about the loss of their two-year-old daughter.

Saturday, October 13, 2012

Oxygenation and Ventilation Are Not Opposites

Breathing has two parts: bringing oxygen into the body, and expelling carbon dioxide from the body. Both are incredibly important.

Why is oxygen important?
With oxygen cut off for a few minutes, the body can't effectively make energy (ATP) and will start shutting down.

Why is getting rid of CO2 important?
If CO2 builds up in the body high enough and for long enough, it can actually put you into a coma. A good way to think about it is that there are actually 3 waste-disposal organs in the body.
1. Intestines - stool
2. Kidneys - urine
3. Lungs - CO2

CO2 is a waste byproduct of the mitochondria doing their work and is just as important to get rid of as what is in urine and what is in stool.

What is Ventilation?
You will often hear people talk about breathing in terms of oxygenation and ventilation. Oxygenation is the breathing in of oxygen, but ventilation is not technically the equivalent word for blowing out carbon dioxide. We actually don't have a good word to specifically mean that. It's a small point, but a conceptually important point:

Ventilation = Air Movement (not the act of blowing off CO2)

Ventilation simply means moving air in and out of the lungs. If this does not happen then yes, you will not be able to get rid of CO2 because air is not moving out of the lungs. But you will also not be able to bring O2 into the lungs if there is no air movement/ventilation so you will not be oxygenating either. 

So ventilation is air movement which is necessary for both oxygenation and blowing off CO2 (we really need a noun specifically for blowing off CO2 . . . expulsion?). Put in another way:

Oxygenation           CO2 Expulsion

Ventilation is the heading under which both oxygenation and blowing off CO2 fall. Again, it may be a small point, but explicitly defining these terms and how they relate to each other makes understanding mechanical ventilation much more conceptually easy when you first start learning about it.

Monday, October 1, 2012

Quick Points on Blood Gases Measurements

This topic has caused me some confusion through my training. Here are some important conceptual points.

There are 3 types of commonly used blood gas tests:

Arterial Blood Gas (ABG)
The best test, taken from an artery presumably before the blood has been used for anything. Tells you the pH, oxygen level (O2), carbon dioxide leve (CO2), and amount of base. Harder to get than other two tests because only a few spots to get arterial blood (usually arteries are deeper, veins are nearer the surface of the skin) or you need a central line to get blood from.

Capillary Blood Gas (CBG)
Blood taken from a capillary. Just as good as as an ABG except you can't trust the oxygen reading. Capillaries are where blood is used by the body, that is, where oxygen is removed from the red blood cells. So the oxygen level is not reliable in this test since you don't know how much has already been removed by the body. The rest of the readings (pH, CO2, base) can be used with confidence.

So what do you do if you really want to know what the oxygen level is? The pulse oximeter (oxygen level reader on patients' fingers) can be used as a stand in in the majority of cases.

CBG + Pulse Oximeter Reading = ABG

Venous Blood Gas (VBG)
Blood taken from the vein. Effectively the same use as a CBG. Good for all the readings except oxygen level so the rules apply.

Obstetrics - Is it PROM, PPROM, or What?

A common point of confusion is what exactly PROM and PPROM mean when talking about a baby being born. When the acronyms are used, there is often some minor doubt as to what exactly we are referring to. There are a few basic concepts here that are straightforward but also important to the health of the baby.

Here are the main concepts:

-What is "rupture of membranes?": The mother's body generally determines when it is time to push out the baby. It does this by having the womb/uterus start to clench (this is what we call Labor) to push the baby out the vagina into the world. When the uterus clenches, it breaks the bag of water inside the uterus (my water broke!) that the baby has been floating in for 9 months. The bag itself is made of two layers which are the membranes that have to break for the water to flow out (the baby isn't going to fit out the vagina if the bag of water is still full). So breaking the water bag that the baby is floating inside = Rupture of Membranes (ROM). So ideally mom's body decides it's time to expel the baby, it starts squeezing, the bag of water protecting the baby breaks, and then some time later the baby is pushed out the vagina. However, it does not always work this way.

-Why do we care about membranes rupturing? The main reason we care is that for it's entire growth in the womb, the baby is immersed in this fluid filled sack. This sack is what protects the baby from bacteria which can cause infection. If the sack wasn't there, bacteria and viruses from places like the vagina could enter the womb and infect the baby. The sack is a barrier for this happening (not perfect, but still very important). So when the sack breaks, the baby loses it's defense barrier. Also, around this time the womb is starting to open up the passage to the vagina to get ready to start pushing the baby out. When the passage starts opening, it creates a direct path for bacteria from the vagina to go into the womb and infect the baby. So having the sack intact, is very important to the baby not getting an infection.

-What is Premature Rupture of Membranes?: So normally the bag of fluid only breaks after it has been squeezed a bit by the uterus during contractions. Sometimes though the bag will break on its own without the uterus making it happen. If the baby is old enough, this doesn't mean that the delivery will not go well, but it does beg the question, "why did this happen?" The main thing we worry about is infection. An infection involving the baby or mother can trigger the water breakage. So if the water breaks without the uterus breaking it itself, you gotta be sure there's no infection going on. Because this is a concerning event, the term Premature Rupture of Membranes is used to emphasize that this is an important event.

Literal Definition:

Premature Rupture of Membranes (PROM) = fluid sack breaks before labor (i.e. the sack breaks on its own with no help from the squeezing of uterus)

-What is PPROM?: this stands for Pre-Term, Premature Rupture of Membranes. This is basically the same thing as PROM except when it happens to a baby less than 37 weeks along. The distinction is made because since the fetus is not old enough to truly be ready to come out, there is a greater risk for problems for the baby once it's been born. 

-What is Prolonged Rupture of Membranes?: this term goes back to the sack being a protective barrier for the baby against infection. It doesn't necessarily have anything to do with PROM either. It just means that there was a longer time between the time the sack breaks and when the baby was fully delivered than is hoped for. This is usually considered more than 1 day. So the baby has had a longer time hanging out in the womb without the protection of the sack so has had more time possibly exposed to bacteria which can cause infections. People often confuse this term with Premature Rupture of Membranes because the acronym would be the same. The key is to remember that there is no acronym for Prolonged Rupture of Membranes. PROM never equals prolonged rupture of membranes.

To review the three terms:
  • PROM: Premature Rupture of Membranes (sack breaks before labor/contractions start; be worried about infection)
  • PPROM: Preterm PROM (same as PROM except when baby is younger than 37 weeks; alerts doctors that there is a higher health risk for the baby since it is not fully developed yet)
  • Prolonged Rupture of Membranes: NO ACRONYM! The time from when the baby loses the sack as an infection barrier to the time the baby is born was long (>1 day; greater time exposed to bacteria in the mother so greater risk of having an infection)

Fetal Circulation & Ductal Dependent Lesions


One thing that is usually not made clear when talking about fetal circulation is why it is specifically set up the way it is. We know that the placenta, interfacing with mom’s circulation, is acting as a substitute for the lungs (gas exchange), and the kidneys and intestines (nutrient and waste exchange). The lungs are full of fluid and consequently have high vascular resistance, so they receive very little of the total amount of blood pumped out by the heart, while the placenta has low vascular resistance to encourage more blood to flow through it. Fetal hemoglobin also plays a part, as it has a greater affinity for oxygen, allowing it to load oxygen from the placenta at the same low O2 saturation that in mom’s adult hemoglobin causes the unloading of oxygen.

Theoretically, the placenta could perform molecule exchange much like a dialysis machine does, by simply taking blood from a blood vessel, performing the exchange, and then delivering the blood back to the systemic circulation via the same or a different vessel. The rest of the circulation would not need to be any different from an adult’s. But it is. Because what’s left out of most discussions is that in addition to gas and nutrient exchange, the fetal circulation is responsible for the preferential delivery of oxygenated blood to the most important organs; the brain, heart and liver. Fetal circulation first drops off a significant portion of oxygenated blood straight to the liver, and then shunts the remainder of oxygenated blood directly to the brain and heart, while it shunts deoxygenated blood past these organs. And this requires three modifications:

1. Ductus venosus (connects umbilical vein directly to the IVC)
2. Foramen ovale (an opening between the shared wall of the left and right atria)
3. Ductus arteriosus (connecting the pulmonary artery to the descending aorta)

Fresh, oxygenated, nutrient-rich blood coming from the placenta via the umbilical vein is divided up between the developing liver and the ductus venosus, which connects to the IVC. This blood from the IVC streams across the right atrium, and is shunted straight through the foramen ovale to the left atrium, where it ends being pumped by the left ventricle to the aortic arch, directly perfusing the brain and heart.

At the same time, the deoxygenated blood from the rest of the body sluggishly enters the right atrium, via the SVC and the IVC distal to the ductus venosus. This blood ends up getting pumped by the right ventricle into the pulmonary artery. Most of it bypasses the lungs and the aortic arch via the ductus arteriosus, and then mixes with the highly oxygenated blood from the aortic arch at the descending aorta, to perfuse the rest of the body.

To recap, despite the fact that highly oxygenated blood from the placenta enters the the right atrium via the IVC, the same place where the rest of the systemic circulation ALSO enters via the SVC and IVC, the anatomy actually encourages ‘preferential streaming’ of the highly oxygenated blood through the foramen ovale into the left atrium and consequently to the brain and myocardium. The deoxygenated blood from the systemic circulation bypasses the aortic arch, therefore never reaching the brain. They only come together at the descending aorta after the brain and heart have received the most oxygen-rich blood, perfusing the rest of the body and flowing via the umbilical arteries back to the placenta, where CO2 and waste products are removed, and O2 and nutrients are picked up.


After birth, fluid in the lungs is cleared and placental circulation is clamped off. Pulmonary vascular resistance decreases, and blood starts flowing into the pulmonary artery, causing a decrease in RA pressure. Blood from the lungs returns to the LA via the pulmonary vein, increasing pressure there. As RA pressures decrease and LA pressures increase, the right to left flow across the foramen ovale and ductus arteriosus decreases, and they both close off soon after birth. The right side of the heart pumps blood to the lungs, the left side to the rest of the body, and congenital heart lesions (structural/anatomic defects of the heart and major blood vessels) involve problems with how these two parts of the circulatory system, driven by different sides of the heart, connect. These lesions can be broken down into three functional categories:

1. circulation bypasses the lungs
            (pulmonary stenosis, pulmonary atresia, tricuspid atresia, tetralogy of fallot)
2. circulation bypasses the body
(aortic stenosis, aortic coarct, hypoplastic left heart syndrome)
3. circulation between the lungs and the body is completely disconnected
            (transposition of the great arteries)

Lesions in all three of these categories can cause parts of the body to not receive any oxygenated blood, evident as cyanosis, and some of these lesions are termed ‘ductal dependent’. This means that the effects of the lesion (poor or no mixing of the two parts of the circulatory system) are mitigated while the ductus arteriosus remains open, (maintaining some mixing of the pulmonary and systemic circulation).*

If a neonate develops cyanosis or dyspnea that is not responsive to supplemental oxygen, then the differential includes problems with oxygen delivery (i.e. lung issue) or problems with circulation itself. This is when it becomes important to assess whether the infant has a ductal dependent heart lesion. Aside from getting an echocardiogram to evaluate the lesion, this is done by monitoring preductal and postductal oxygen saturations, i.e. pulse oximetry of a preductal extremity (one that is supplied by the aorta proximal to where the ductus arteriosus inserts, classically the right arm but also the left) and a postductal extremity (one of the lower legs). A difference of >10% O2 saturation between the two extremities indicates that they are likely getting blood from different parts of the circulation, and the ductus arteriosus is still patent. If there is cyanosia AND the test is positive (i.e. there is an actual difference in oxygen saturations between the extremities), there is a high chance that they have a ductal-dependent cardiac lesion. This means that any worsening cyanosis signifies relative hypoxia of those parts of the body that will soon be getting only deoxygenated blood if the ductus arteriosus closes off completely.

Identifying a ductal dependent lesion is important because we can do something about it: we can medically prevent the ductus from closing using a continuous IV infusion of Prostaglandin E, which relaxes the smooth muscle in the walls of the vasculature. Consequently one of the major side effects of PGE is hypotension, and the infant has to be observed closely while on the drip, with resuscitative fluids and inotropes available. Usually they will require intubation and ventilation as the other major side effect is apnea. (NSAIDS inhibit the COX enzymes that make prostaglandins, so they are absolutely contraindicated.)

*Note that just as lesions are ductal dependent, many lesions are only compatible with life due to additional shunts between the right/pulmonary and left/systemic circulation, e.g. atrial and ventricular septal defects.