Cyanotic Congenital Cardiac Defects: Physiology of Cyanosis by Tom Kulik for OPENPediatrics

Cyanotic Congenital Cardiac Defects: Physiology of Cyanosis by Tom Kulik for OPENPediatrics

Cyanotic Congenital Cardiac Defects: Physiology of Cyanosis by Tom Kulik for OPENPediatrics

Listen to Dr. Thomas Kulik review the physiology, evaluation, and management of cyanotic congenital cardiac defects.
Direct links to chapters:
3:15 Chapter 2: Pulmonary Cyanosis
4:29 Chapter 3: Persistent Pulmonary Hypertension of the Newborn
7:53 Chapter 4: Congenital Heart Defects
17:29 Chapter 5: Factors Determining Arterial Oxygen Saturation
32:31 Chapter 6: Therapies

Initial publication: September 16, 2013.
Last reviewed: May 16, 2019.

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Content

14.28 -> Cyanotic Congenital Cardiac Defects: Physiology of Cyanosis by Doctor Thomas Kulik.
20.9 -> My name is Tom Kulik.
22.11 -> I'm a Pediatric Cardiologist and Cardiac Intensivist at The Children's Hospital, Boston.
28.95 -> This will be part one of a two part series on the physiology, management, and evaluation
38.44 -> of cyanotic congenital cardiac defects.
41.57 -> We will be speaking today, in this lecture, primarily about physiology of cyanosis and
50.03 -> we'll include a few types of cyanotic defects.
54.32 -> Diagnosis and therapy are primarily discussed in a separate lecture.
62.03 -> Introduction.
65.979 -> I should start the lecture out by saying that Cyanosis is really the bluish color of the
71.14 -> skin and mucous membranes due to approximately five grams or more of deoxygenated hemoglobin
80.35 -> in the capillaries.
82.77 -> Cyanosis can be due to low arterial oxygen saturation.
85.89 -> That's the dangerous form of Cyanosis, you might say, but low cardiac output and venous
90.85 -> stasis can also result in Cyanosis.
94.39 -> We're not so much interested in those today.
97.77 -> So what we're really going to be talking about primarily is arterial hypoxemia.
102.729 -> I put down on the slide this is an arterial saturation, of less than about 95% or so.
110.399 -> But what constitutes hypoxemia is actually pretty variable.
114.58 -> There are some normal babies with a little bit of extra lung water or Atelectasis that
120.17 -> may have low 90 saturation for a period of time.
123.38 -> They're obviously normal for all practical purposes.
127.219 -> On the other hand, there are certain forms of cyanotic heart disease that have a considerable
131.79 -> amount of pulmonary blood flow that may actually have saturations well into the 90s.
137.25 -> So it's hard to define hypoxemia with any one given number when one is trying to discuss
144.61 -> the whole range of causes of Cyanosis, especially with congenital cardiac defects.
151.49 -> There are basically three fundamental causes of cyanosis.
156.129 -> The first is desaturated pulmonary venous blood.
158.95 -> That is to say, lung disease.
160.73 -> I will refer to that as pulmonary cyanosis.
163.8 -> The second cause is increased pulmonary vascular resistance, causing right to left shunting
169.68 -> across patent ductus arteriosus.
172.66 -> Or a patent foramen ovale in the atrial septum.
176.8 -> This is seen in babies, and pretty much only in babies, and is known as persistent pulmonary
181.73 -> hypertension of the newborn.
184.26 -> The third cause of cyanosis, of course, are certain forms of congenital cardiac defects
189.72 -> and we will focus primarily on that for today's lecture.
195.549 -> Pulmonary Cyanosis.
199.78 -> I'd like to briefly say few words about the first two causes of cyanosis.
204.95 -> Pulmonary cyanosis in babies, and to some extent older patients as well, was primarily
210.4 -> caused by RDS, pneumonia, severe atelectasis, although mild atelectasis rarely causes true
219.299 -> cyanosis.
220.299 -> A normal chest x-ray pretty much rules out pulmonary cyanosis.
227.33 -> There's actually one exception to that.
228.799 -> And that is to say patients with pulmonary arterio-venous malformations can have cyanosis
235.59 -> on the basis of lung disease.
238.09 -> Which is not apparent on chest x-ray.
240.42 -> But that's a very rare entity.
241.541 -> And that's going to almost never show up in the newborn.
248.39 -> Increased inspired oxygen concentration generally much improves or eliminates cyanosis with
254.36 -> lung disease.
255.579 -> I'll say more about that when we get to the sections on the management and evaluation
262 -> in the second part of this lecture.
267.27 -> the Newborn.
273.31 -> Persistent pulmonary hypertension of the newborn causes hypoxemia due to right to left shunting
281.85 -> at the atrial level, or across the PDA, due to very high pulmonary vascular resistance.
288.44 -> It can occur with otherwise normal appearing lungs or in the setting of lung disease, especially
295.889 -> meconium aspiration syndrome.
298.02 -> The increase to pulmonary vascular resistance in PPHN is due to vasoconstriction.
304.669 -> And is, at least with true PPHN-- meaning not another condition, such as alveolar capillary
311.44 -> dysplasia.
312.83 -> But with what we generally refer to as true PPHN, generally is reversible within a few
318.33 -> days.
319.33 -> In most patients after having had this reversed, have no subsequent manifestations of increased
328.65 -> resistance.
329.72 -> This is what I like to refer to as the natural history of the pulmonary circulation or the
336.85 -> pulmonary vascular resistance in pulmonary circulation.
340.56 -> On the y-axis is plotted the ratio of pulmonary to systemic vascular resistance.
347.49 -> And on the x-axis is the age of the person.
351.08 -> As you'll note, the B stands for birth.
354.4 -> Prior to birth, the ratio of pulmonary to systemic vascular resistance is exceedingly
360.27 -> high, somewhere around 10 to 1, depending upon what assumptions you make about pulmonary
365.56 -> blood flow.
367.83 -> After the baby is born and takes the first few breaths, the pulmonary vascular resistance
374.241 -> plummets.
375.241 -> And actually, within the first two or three weeks after birth, the ratio has gone from
381.449 -> 10 to 1 to about 0.2 or 0.3 to one, a massive fall in resistance.
390.47 -> Babies that have PPHN and have a substantial fall in resistance at birth, but not nearly
395.57 -> as much as a normal patient.
397.86 -> As you can see in the pink curve.
402.04 -> And it takes a period of time for this to resolve.
406.08 -> And the babies need to be supported during this period of time.
410.74 -> This is a diagram that shows the levels of right to left shunting in babies with PPHN.
418.419 -> There can be right to left shunting across the patent foramen ovale, which is almost
424.19 -> always present in a newborn.
426.65 -> There can also be a right to left shunting across the ductus arteriosus because pulmonary
432.621 -> resistance is actually higher than aortic resistance.
435.59 -> As you can see, the shunting is predominantly right to left across the ductus.
443.61 -> Because this blood that shunts right to left across the ductus heads south, goes to the
448.699 -> descending aorta.
450.199 -> There may be a differential in saturation, the right arm being considerably higher than
456.349 -> the legs.
458.199 -> And that's characteristic of patients with PPHN.
461.34 -> I should notice that there is also intrapulmonary shunting if the patient has lung disease,
467.56 -> such as meconium aspiration syndrome.
472.199 -> Congenital Heart Defects.
475.91 -> Shunting OK, let's move on now to the main focus of the lecture.
481.699 -> And that's congenital heart defects.
484.3 -> Hypoxemia, or cyanosis, is due to mixing of systemic venous blood, which is blue of course,
491.4 -> with pulmonary venous blood.
493.32 -> That's the fundamental cause of hypoxemia.
495.66 -> And there are two reasons, basically, that red and blue blood can mix.
502.639 -> One is shunting, and the other is simple mixing.
505.52 -> Let's talk about shunting first.
507.919 -> You know, these are diagrams of two types of heart.
513.37 -> The heart on the left has a ventricular septal defect.
516.97 -> The heart on the right an atrial septal defect.
519.349 -> And as you can see in the case of the VSD, in some cases patients with VSDs will have
525.25 -> left to right shunting.
526.57 -> That is to say, red blood from the left ventricle will be ejected across the VSD into the lungs.
533.16 -> Where patients with atrial septal defects will have pulmonary venous red blood go across
539.44 -> the atrial septal opening into the right atrium.
542.38 -> That's left to right shunting.
544.53 -> On the other hand, one can have a right to left shunting, where blood flows from the
549.48 -> right ventricle across the VSD into the aorta, or from the right atrium into the left atrium.
555.42 -> So the question is, what determines the direction and magnitude of shunting.
561.64 -> And basically the answer is very simple.
563.73 -> In the case of ventricular septal defects, or patent ductus arteriosus, it's roughly
571.25 -> the ratio of systemic to pulmonary vascular resistance that determines the direction of
576.11 -> shunting.
577.94 -> If the resistance to blood flow in the lungs is higher than the body, then blood will tend
582.78 -> to go right to left.
584.71 -> If on the other hand, as is normally the case, resistance in the body is higher than the
590.01 -> lungs, blood will tend to go left to right.
596.2 -> Simply, pulmonary and systemic vascular resistances that are operative here.
601.6 -> If you take, for example, the case of a heart with tetralogy of Fallot.
607.88 -> Basically with tetralogy, there's a large ventricular septal defect, and then there's
612.22 -> narrowing at the pulmonary valve, and below the pulmonary valve.
616.45 -> The reason that there's right to left shunting in tetralogy of Fallot is not that there's
621.46 -> high pulmonary vascular resistance, in fact, these patients have normal pulmonary vascular
626.71 -> resistance.
628.09 -> But that there is increased resistance to blood flow out the right ventricle and into
633.98 -> the pulmonary artery due to the sub-pulmonary, and the pulmonary narrowing.
638.81 -> So, an additional reason for right to left shunting with a ventricular septal defect
644.41 -> is actually obstruction to outflow of the blood from the ventricle into the pulmonary
650.26 -> artery.
651.63 -> Now, the situation with atrial septal defects is somewhat different.
657.19 -> The reason the shunting pattern with atrial septal defects is due to the relative compliances
665.31 -> of the two ventricles, and not due to pressure differentials, or vascular resistance differentials.
674.25 -> To illustrate the effect of compliance on shunting, I've made a relatively crude diagram
679.5 -> of the heart.
681.45 -> This diagram illustrates the relatively thick walled left ventricle and a much thinner walled
687.65 -> right ventricle.
689 -> And two atria with a large atrial septal opening above the ventricles.
694.31 -> Now, when blood returns from the lungs, and from the body, It has a choice of either stuffing
700.07 -> itself into the relatively thick walled non-compliant left ventricle, or rather moving into the
708.04 -> much more compliant, and more easily filled, right ventricle.
711.82 -> So in a situation like this, the tendency, of course, is to have left to right shunting.
717.5 -> This is not based on the systemic vascular resistances, or pressure gradients, but rather
723.23 -> it's based on the differential in compliances between the two accepting chambers.
732.63 -> Shunting can work the other way, however.
735.35 -> Babies with critical-- so-called critical pulmonary stenosis-- have severe narrowing
740.81 -> of their pulmonary valve, and in fact, they had that in utero.
744.65 -> As a result, the right ventricle worked under very high pressure, and became considerably
749.96 -> thicker and less compliant than normal.
753.52 -> So for these patients-- and this is true even after they have a balloon valvulotomy, and
759.93 -> relief of their severe pulmonary stenosis-- because their right ventricles are substantially
764.88 -> thicker than normal, and non-compliant, when blood enters the right atrium from the two
770.63 -> vena cava, it tends to move right to left across the patent foramen ovale and into the
777.23 -> left atrium.
778.23 -> And for this reason, these babies can be quite hypoxemic, because of a restriction of filling
784.07 -> of the right ventricle and movement of the atrial blood into the left sided chambers
788.77 -> of the heart.
791.38 -> Simple mixing.
794.7 -> The second reason that red and blue blood can mix is due to simple mixing.
799.17 -> And this is simply due to the anatomic configuration of the heart.
803.35 -> Let me show two examples of that.
806.37 -> Patients with tricuspid atresia have no tricuspid valve, or really no right ventricle, per se.
812.66 -> So all the systemic venous blood that returns to the right atrium has to move across the
818.22 -> patent foramen ovale into the left atrium before entering the left ventricle, because
824.1 -> the systemic venous blood has to come into contact with the pulmonary venous blood.
829.64 -> There is mixing of the two streams.
832.29 -> It's unrelated to vascular resistances, or compliances.
837.23 -> It's simply an anatomic structure of the heart itself that explains it.
840.701 -> In the case of truncus arteriosis, there are two normally formed ventricles, but a large
847.92 -> ventricular septal defect, and a single artery that arises from the two ventricles that gives
853.94 -> rise to both the aorta and pulmonary artery.
856.66 -> In the case of truncus arteriosis, the venous stream does not mix with the pulmonary venous
864.55 -> stream, in the atria or ventricles, but as the blood is ejected into this common outflow
871.37 -> vessel there's mixing of red and blue blood.
874.31 -> So simple mixing at the great vessel level can also result in hypoxemia.
881.69 -> Transposition physiology.
883.64 -> I should mention that there's actually a third reason that patients are cyanotic, and this
892.06 -> is the so-called transposition physiology This is the physiology that occurs in patients
898.65 -> that have D-transposition of the great arteries, sometimes known as D-transposition of the
903.67 -> great vessels.
905.32 -> With D-transposition the aorta arises from the right ventricle instead of the left ventricle,
911.16 -> and the pulmonary artery arises from the left ventricle instead of from the right ventricle.
917.42 -> As a result, the systemic venous blood returns to the right atrium, goes to the right ventricle,
924.26 -> and then is returned immediately to the body without having had a chance to participate
930.21 -> in any gas exchange in the lungs.
933.28 -> Pulmonary venous blood is returned to the left atrium, left ventricle, then immediately
939.11 -> returned to the lungs without having had a chance to participate in any gas exchange
944.54 -> in the peripheral tissues.
946.66 -> So with D-transposition of the great vessels, if that's the anatomy as it strictly exists
951.57 -> at the time of birth, the baby dies very shortly thereafter.
954.121 -> There is obviously no oxygen delivered to the tissues.
957.4 -> In reality, the vast majority of babies with transposition will have some degree of mixing
963.1 -> of blood across the patent foramen ovale and hence will have, at least some degree, of
970.51 -> true gas exchange.
971.78 -> That is to say, there will be some pulmonary venous blood that will find its way to the
976.82 -> aorta.
977.82 -> And some systemic venous blood that will find its way into the lungs.
982.83 -> But the physiology with transposition is characterized primarily by a lack of mixing, not so much
992.07 -> co-mingling of systemic and pulmonary venous blood the way it is with other right to left
996.51 -> shunting lesions.
998.17 -> I should make a final note, and I kind of implied this a bit without saying much about
1005.25 -> this.
1006.36 -> In the case of D-transposition of the great vessels, with transposition physiology, mixing
1013.12 -> of systemic and pulmonary venous blood are key in terms of determining O2 arterial saturation.
1022.59 -> And in fact, babies with transposition can have a huge amount of pulmonary blood flow,
1028.49 -> but if very little of that finds its way to the systemic circulation they can be profoundly
1035.689 -> hypoxemic.
1036.689 -> So, the issues we've just mentioned in terms of QP to QS are much less important with transposition
1043.27 -> than mixing of the systemic and pulmonary venous streams.
1048.8 -> Factors Determining Arterial Oxygen Saturation.
1052.48 -> OK, so we have a pretty good idea now of the physical basis of hypoxemia with congenital
1061.45 -> heart defects in a qualitative way.
1065.08 -> But the real question is, what determines the arterial saturation in congenital heart
1071.63 -> disease.
1072.63 -> It's not enough to simply know why one baby is somewhat bluer than he should be, but you
1078.53 -> want to know exactly what controls how blue that particular patient will be, which is
1084.74 -> critical for both the evaluation of the patient as well as his therapy.
1090.14 -> And I'd like to divide the factors that are important in determining the arterial oxygen
1098.54 -> saturation into what you might call front end factors and back end factors.
1103.87 -> And this is not the typical way this is discussed, but I think that this actually makes a lot
1110.43 -> of sense in understanding the physiology.
1113.86 -> The front end factors are simple.
1116.2 -> One is pulmonary venous saturation.
1119.01 -> And that is pretty obvious.
1120.03 -> If the baby has lung disease with a congenital cyanotic heart defect, he's going to be bluer
1125.41 -> than he otherwise would be, all things put together, all things considered.
1130.94 -> The second is a little bit more complicated, and that is to say the other front end factor
1136.29 -> is the ratio of pulmonary to systemic blood flow.
1140.87 -> Cardiologists abbreviate this Qp to Qs.
1143.25 -> Q, of course, stands for flow.
1146.17 -> And p and s for pulmonary and systemic.
1150.19 -> And let me illustrate this if I can, using some relatively crude, kind of medical folk
1158.12 -> art diagrams you might say, of the heart that I've drawn here.
1162.42 -> And we could look at the left upper panel, a heart consisting on the left hand side of
1169.06 -> the superior vena cava, and the two pulmonary arteries.
1173.02 -> On the right hand side of the diagram, two pulmonary veins and the aorta.
1177.57 -> If you consider the situation in which the amount of blood being returned from the lungs,
1183.54 -> that is to say, fully oxygenated blood is about half that being returned from the body.
1189.89 -> That is to say the pulmonary to systemic flow ratio is 0.5.
1195.78 -> And make the calculations, you will see that - making certain assumptions - the arterial
1202.64 -> saturation is going to be about 50%, and the reason for that's obvious.
1206.1 -> The large volume of blue blood will tend to dilute out the relatively small volume of
1211.68 -> red blood, the saturation is going to be low.
1215.31 -> If you consider on the upper right hand panel, however, a Qp to Qs of 1.0, now there are
1222.15 -> equal volumes of red and blue blood mixing in the heart.
1225.62 -> A patient like that now will have an aortic saturation not at 50% but rather 75%, with
1232.191 -> a pulmonary to systemic ratio of 2:1.
1237.02 -> Now the aortic saturation would be 86%, and with a massive amount of pulmonary blood flow,
1244.89 -> with a Qp to Qs of 3:1.
1247.24 -> The aortic saturations can be about 92%.
1249.63 -> And, in fact, we do see pulmonary to systemic flow ratios fully this high in some patients
1257.76 -> with what are technically cyanotic congenital heart lesions.
1262.25 -> And so the ratio of flow is extremely important in determining arterial saturations.
1269.15 -> And I should say, and this is a little bit of an arcane point, the exact influence of
1275.12 -> Qp to Qs, on arterial saturation, varies a little bit depending upon whether the patient
1282.42 -> has what we call an admixture lesion.
1284.77 -> That is to say, where all pulmonary and systemic blood are mixed together.
1289.42 -> Or isolated right to left shunting as in tetrology of Fallot.
1293.81 -> But the basic principle of Qp to Qs being of prime importance is nevertheless the same.
1300.44 -> So there are two front end factors.
1302.27 -> There are basically three back end factors that relate to one thing.
1310.2 -> And as to say that systemic venous oxygen saturation is actually very important in a
1316.59 -> patient with a cyanotic congenital heart defect in determining arterial oxygen saturation.
1324.25 -> Because you know with a normal heart, that's not the case.
1327.39 -> One can have a very low systemic venous oxygen saturation.
1331.82 -> But because all the systemic venous blood goes to the lungs before being pumped out
1337.56 -> to the body, it's really irrelevant to how low the systemic venous saturation is relative
1343.77 -> to arterial saturation.
1345.23 -> But that's very different in the case of cyanotic heart defects, because the blue blood obviously
1350.94 -> does mix with the red pulmonary venous blood in order to determine the final arterial saturation.
1359.62 -> Let me illustrate the quantitative importance of this in the following way.
1363.71 -> Let's assume we have two patients with cyanotic heart disease.
1368.7 -> Both of whom had a Qp to Qs ratio of 1.
1372.75 -> Equal volumes of pulmonary and systemic blood flow.
1376.48 -> If in the first patient, the pulmonary venous saturation is at 100%, that is to say normal
1383.49 -> or even supernormal, and the systemic venous saturation is 60%, this will give this patient,
1390.22 -> making certain assumptions, an arterial saturation of 80%.
1394.66 -> If the second patient also has fully saturated pulmonary veins, but the systemic venous saturation
1402.94 -> is 40 rather than 60 percent, now that patient's arterial saturation will be 70%, which is
1409.5 -> substantially less.
1411.84 -> And so, as you can see there is actually considerable influence, again with congenital heart disease,
1420.58 -> on arterial saturations, relative to the systemic venous O2 saturation.
1428.35 -> The explanation for that is actually pretty simple, and I've tried to diagram this here
1433.65 -> in a very, very crude way by indicating on the left side of this diagram in red lines,
1442.02 -> the arterial side of the circulation.
1445.66 -> On the right hand side of this cartoon with the blue lines, the venous side of the circulation.
1452.07 -> And the arrow pointing downward coming off this represents gas exchange in the capillaries.
1458.97 -> Basically, the principle is simple.
1462.07 -> If the capillaries suck out a certain fixed amount of oxygen to supply the needs of the
1468.58 -> tissue, The amount of oxygen that's left over in the systemic venous side is going to be
1475.77 -> determined by how much O2 went into the capillary bed.
1480.46 -> What determines how much O2 is delivered to the capillary bed?
1483.49 -> Well, it's very simple.
1484.75 -> It's two factors.
1485.82 -> One is oxygen content of the blood, which is related to both arterial oxygen saturation
1492.9 -> and hemoglobin.
1493.94 -> And the other, of course, is the amount of blood flow into the capillary bed, which is,
1500.13 -> roughly speaking, cardiac output, or systemic blood flow.
1504.7 -> And so the greater the amount of systemic blood flow, or the higher the amount of O2
1512.059 -> content, all other things being equal in terms of O2 consumption, the higher the systemic
1518.34 -> venous O2 content, also referred to as mixed venous saturation.
1522.45 -> And so as you can see, as cardiac output falls, again given a certain amount of O2 consumption,
1530.05 -> mixed venous saturation will fall.
1533.05 -> Similarly, if arterial saturations or hemoglobin are less, there will also be less oxygen left
1541.63 -> over for the systemic venous circuit.
1544.48 -> Just as importantly, if O2 consumption goes up, and one could imagine this being a clinically
1551.36 -> relevant factor in a patient who's febrile, or in a patient who is very active.
1556.7 -> If the O2 consumption goes up, all other things being equal, mixed venous content will tend
1562.98 -> to go down.
1565.58 -> And so the back end factors consist of systemic blood flow, hemoglobin content of the blood,
1573.3 -> and oxygen consumption, also abbreviated VO2.
1577.91 -> The reason I think that these are important, and I'll say a little bit more about these
1581.91 -> in a moment, is that these are all factors to some extent we can control.
1586.36 -> It can be difficult sometimes to control the pulmonary to systemic flow ratio, or even
1591.88 -> sometimes pulmonary venous saturations.
1593.4 -> But there are ways to modify systemic flow, hematocrit, and O2 consumption that can be
1600.03 -> very helpful.
1601.56 -> Finally, and we're near the end of this part of the lecture, it's important to note that
1608.27 -> arterial oxygen saturation is really only part of the story, of course.
1614.44 -> Oxygen delivery to the tissues is ultimately what counts.
1618.6 -> One could have a perfectly normal arterial oxygen saturation, but if cardiac output is
1624.47 -> remarkably depressed, a patient can obviously die of tissue dysoxia.
1630.61 -> And so it's very important to think not only in terms of arterial saturation, but O2 delivery
1635.79 -> to the tissues.
1637.57 -> O2 delivery is, the equation that describes oxygen delivery to the tissues is very simple.
1645.07 -> It's basically delivery equals arterial content of oxygen, which is related to both the pulmonary
1653.57 -> venous oxygen saturation, as well as hemoglobin.
1657.82 -> And systemic blood flow, which is in a normal person, a normal heart, cardiac output.
1665.059 -> This is a diagram that roughly illustrates a normal separated four chambered heart, and
1671.28 -> the O2 delivery relationships.
1676.059 -> The situation is considerably more complex in patients with cyanotic heart disease, and
1681.831 -> in particular, patients that have single ventricles.
1685.97 -> And the reason for that isn't that the fundamental O2 delivery equation changes-- it does not--
1692.71 -> delivery of O2 to is identical, in the sense that it's the amount of blood delivered to
1699.679 -> the systemic circulation, times the O2 content of the blood is what delivers, is what determines
1708.02 -> O2 delivery.
1709.14 -> But the reason that things are more complicated in single ventricle patients is that every
1715.57 -> drop of blood that the ventricle pumps out has a choice of either going to the body,
1720.76 -> or to the lungs.
1722.07 -> And the ventricle is not an infinite-- is not a pump of infinite capacity.
1728.72 -> A single ventricle pump has the same limitations as any other ventricle.
1734.9 -> And so once the single ventricle it's pumped as much as it can pump, there will be a limitation
1742.33 -> on systemic blood flow related to the amount of blood flow that finds its way into lungs,
1748.809 -> rather than the body.
1751.39 -> And I've tried to emphasize this in the lower right hand part of the diagram.
1755.75 -> The single ventricle systemic blood flow isn't just the total amount of blood pumped by the
1762.39 -> heart, isn't simply cardiac output.
1764.37 -> But it's cardiac output minus pulmonary blood flow.
1768.64 -> A number of years ago, there was an investigator by the name of Ofer Barnea, who said, listen
1779.04 -> this is a relatively complex relationship we're talking about here.
1783.06 -> I know that as Qp to Qs gets to be higher, oxygen saturations go up.
1789.26 -> But I also know that as Qp to Qs gets to be higher, given a fixed maximum cardiac output,
1795.49 -> there will be less blood going to the tissues and therefore the relationship between Qp
1801.3 -> to Qs and O2 delivery is relatively complex.
1805.5 -> So what Barnea did is he wrote a computer program, and he modeled basically a patient
1811.6 -> with a single ventricle.
1813.88 -> He assumed a certain amount of O2 consumption by the patient that had that single ventricle.
1819.75 -> And he asked the computer to spit out a series of curves, relating the pulmonary to systemic
1828.14 -> flow ratio, to systemic O2 availability, which is the same as O2 delivery, in order to determine
1836.27 -> where the sweet spot is in terms of maximal O2 delivery.
1841.53 -> And this graph actually shows four curves that relate to differing cardiac outputs - assumed
1849.86 -> cardiac outputs - total amount of blood that's pumped by the heart, and relate to availability
1855.84 -> on the y-axis, which again is the same thing as O2 delivery.
1859.71 -> And Qp to Qs.
1861.1 -> And what Barnea found, actually, is that as one increases the Qp to Qs much above 0.5
1871.42 -> to 1, O2 delivery actually tends to fall.
1876.5 -> What I've done is added a red circle to graph.
1880.74 -> In roughly the region of cardiac output, normal cardiac output, for a three kilo baby.
1888.42 -> And as you can see, the maximal O2 delivery actually occurs with what we would consider
1893.29 -> to be a relatively low Qp to Qs.
1896.5 -> I think a clinical experience would indicate that this curve may be a little bit left shifted.
1901.9 -> That is to say that Barnea might suggest that a Qp to Qs as low as 0.5 to 1 would represent
1908.41 -> maximum O2 delivery.
1910.059 -> I think most people's sense is that it's probably somewhat greater than 0.5, maybe closer to
1915.34 -> one to one.
1916.34 -> But the point I'm trying to make here is that as pulmonary flow goes up, substantially above
1923.45 -> one, although the arterial saturation gets better for the baby, the O2 delivery actually
1929.54 -> tends to fall.
1930.67 -> And this is really the explanation for why patients with single ventricle lesions that
1936.54 -> have unrestricted pulmonary blood flow need to be treated in a way that tends to restrict,
1942.23 -> rather than encourage, pulmonary blood flow.
1944.34 -> I'll say more about that in the second part of this series.
1949.679 -> Treatment Therapies-- So what are the therapeutic implications of
1958.13 -> the physiology we have just discussed?
1960.77 -> Well, the first is that one has to have a pretty accurate diagnosis and understanding
1966.09 -> of what the patient has.
1967.52 -> For example, a patient with persistent pulmonary hypertension as a newborn will be treated
1972.6 -> with a pulmonary vasodilator, for example, inhaled nitric oxide.
1976.85 -> In general, supportive measures, may be up to, and including, extra corporeal membrane
1982.679 -> oxygenation if required, in order to allow the baby to basically recover from the high
1988.18 -> pulmonary resistance.
1990.41 -> On the other hand, a patient with D-transposition of the great vessels who requires atrial-level
1995.97 -> mixing in order to have adequate arterial saturations will often times require a balloon
2002.53 -> atrial septostomy.
2003.55 -> Again, we will discuss this subsequently.
2007.51 -> Sometimes prostaglandin E1 in order to maintain ductal opening can be helpful in these patients,
2013.88 -> but not always.
2015.29 -> And a balloon septostomy is a really definitive therapy, a definitive palliation for many
2020.679 -> patients.
2021.679 -> On the other hand, a baby with tetralogy of Fallot with relatively mild tetralogy of Fallot
2027.45 -> and only modest arterial hypoxia may require no therapy as a newborn.
2033.16 -> If the degree of right ventricular outflow tract obstruction is more severe, it may be
2039.89 -> palliated by keeping the ductus open with prostaglandin E1.
2043.6 -> Or alternatively, if the patient doesn't have a ductus or the ductus is closed and can't
2048.409 -> be opened again, increasing systemic vascular resistance in a patient like that to force
2053.829 -> blood essentially across the restrictive opening between the ventricle and pulmonary artery
2059.169 -> may be useful.
2060.389 -> Finally, a patient with a single ventricle lesion without pulmonary stenosis, as I mentioned
2066.55 -> a few minutes ago, in a patient like that you may want to avoid maneuvers that decrease
2072.39 -> pulmonary vascular resistance as an important part of this therapeutic regiment.
2078.46 -> The second therapeutic implication is that pretty much regardless of the defect, systemic
2085.46 -> blood flow, hemoglobin levels, and O2 consumption are really critical in determining whether
2092.53 -> or not the baby has adequate O2 delivery.
2096.45 -> These are variables, as I mentioned before, that we have ways of modifying.
2100.8 -> And as long as we keep this in the back of our mind when dealing with a severely hypoxemic
2105.3 -> patient, it will give us things that we can do, even if they don't address the root cause
2112.28 -> of the hypoxemia, that can be extremely useful in maintaining an adequate O2 delivery.
2118.84 -> So this concludes the lecture on the physiology of cyanosis, and especially related to cyanotic
2125.67 -> congenital heart defects.
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Source: https://www.youtube.com/watch?v=0Dz1xhMvlQk