Erythropoietin in cyanotic heart disease

We hypothesized that children with cyanotic congenital heart disease and moderate hypoxemia, as a result of erythrocytosis, and adequate Iron stores would have low serum erythropoietin titers, low tissue oxygen delivery, and normal red cell 2,3-diphosphoglycerate (DPG) concentrations. We assessed hemoglobin levels, aortic oxygen saturation, iron stores, red cell 2,3-DPG, oxygen consumption, and systemic O2 transport in 19 hypoxemlc patients, aged 3 months to 8 years. Low erythropoietln titers (530 mU/dl) were found in 14 patients. Patients with high erythropoietln titers had lower Pao, (38 f 7 vs 4g k 7 mm Hg, p < O.Ol), lower aortic saturation (88 f 12 vs 81 + g%, p < O.Ol), and higher red cell 2,3-DPG (2.47 ? 0.34 vs 3.23 of: 0.73 Amol/ml, p < 0.01). Aortic oxygen saturation higher than 80% was associated with a low erythropoietin titer and a hemoglobin level below that associated with hyperviscosity. The relatlonship between aortic oxygen saturation and hemoglobin concentration was strong (r = 0.77). These data suggest that for children less than 8 years of age, adequate compensation for moderate hypoxemia can occur with moderate increases in hemoglobin levels. (AM HEART J 1988;118:128.)

Samuel S. Gidding, MD, and James A. Stockman III, MD. Chicago, Ill.

In cyanotic congenital heart disease, erythrocytosis represents a physiologic response to chronic hypox- emia.‘v2 This response is thought to be mediated by increased renal erythropoietin secretion resulting from low Pao,.3 In many hypoxemic patients, normal or near-normal systemic oxygen transport and tissue oxygen delivery can be maintained by an increased hemoglobin concentration and a right-shifted oxy- hemoglobin dissociation curve.4 This physiologic response can be counterproductive when erythrocy- tosis produces hyperviscosity and an overall diminu- tion in systemic oxygen transport, or it can be inadequate when relative iron deficiency is pre- sent.6-g It has been difficult to assess the adequacy of the erythrocytic response, because normal values for hemoglobin concentration at varying levels of oxy- gen saturation are not known.

Observations made in patients with anemia and other forms of chronic hypoxemia suggest that assessment of the serum erythropoietin titer may provide a marker for the appropriateness of the hemoglobin concentration. For example, in patients with nutritional anemias, erythropoietin titers are high and then fall rapidly after initiation of appro-

From the Children’s Memorial Hospital, and the Department of Pediatrics, Northwestern University Medical School.

Supported in part by a Robert and Mary Wood Research Fellowship Grant and by grant HL21676 from the National Institutes of Health.

Received for publication Nov. 12, 1987; accepted Feb. 10, 1988.

Reprint requests: Samuel S. Gidding, MD, Division of Cardiology, The Children’s Memorial Hospital, 2300 Children’s Plaza, Chicago, IL 60614.

priate therapy. lo In subjects with hypoxemia result- ing from lung disease and in subjects moving from a low altitude to a high altitude, increased erythro- poietin titers are found only until compensatory erythrocytosis develops. Erythropoietin titers then decrease while compensatory erythrocytosis is main- tained.“, l2

We hypothesized that a subgroup of patients with cyanotic congenital heart disease and resulting erythrocytosis would have low serum erythropoietin titers if they had sufficient iron, had relatively normal tissue oxygen delivery as assessed by red cell 2,3-diphosphoglycerate (DPG) concentration, and had only mild-to-moderate hypoxemia. To test this hypothesis, we assessed hemoglobin concentration, Paoz, aortic oxygen saturation, iron stores, red cell 2,3-DPG concentration, and systemic oxygen trans- port in clinically stable patients with cyanotic con- genital heart disease. We chose to study the age range of 3 months to 8 years for three reasons: (1) it encompasses the majority of cyanotic patients fol- lowed by pediatric cardiologists, (2) it avoids poten- tial conflicting variables present during the newborn period,13 and (3) it avoids puberty and its associated physiologic increase in the hemoglobin concentra- tion.14

METHODS

iu’ineieen patients undergoing eiectlve cardiac catheter- ization for cyanotic congenital heart disease were studied. At the time of study no patient was receiving iron supplementation, and none was receiving medications such as salicylates, beta blockers, or prostaglandin. Par-

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Number 1, Part 1 Erythropoietin in cyanotic heart disease 129

Table I. Clinical data -__

Patient Age EPO Ao Sat SVC Sat Pao, Hb 2,3-DPG vo2 SOT NO. (yr) (mUld1) (%) (%) (mm Hg) (gmldl) (~mollml) (mllmin/m2) @./m%m’) (mllminlmz)

1 3.0 2000 50 39 30 16.7 4.25 169 7.3 768 2 1.1 160 74 52 39 15.3 3.21 211 4.6 711

3 0.3 85 66 46 33 22.1 2.65 114 1.9 380 4 0.3 72 68 48 32 19.0 2.76 145 2.8 493

5 5.8 50 81 63 48 18.1 - 132 3.1 580 6 1.4 23 70 55 38 17.4 2.80 133 3.8 621 7 1.2 16 86 60 58 14.5 1.85 143 2.8 473 8 0.4 15 79 60 45 15.2 1.90 155 4.0 644 9 4.5 12 86 64 54 15.5 2.52 -

10 7.2 11 87 65 58 14.2 2.45 146 3.4 575 11 1.1 8.5 76 58 43 19.7 2.65 157 3.3 633

12 4.4 7.5 83 61 49 15.1 2.36 163 3.6 612 13 1.3 7.1 87 62 58 16.1 2.05 137 2.5 477 14 1.9 5.5 83 68 50 16.1 - 184 5.6 1019

15 0.5 5.1 81 51 50 14.6 2.60 153 2.6 404 16 2.5 4.9 78 64 47 19.9 - 136 3.6 759 17 4.0 3.6 87 64 54 13.7 2.80 - - -

18 0.5 3.5 68 53 35 18.9 2.60 150 3.8 680 19 0.9 0.1 79 61 45 14.8 2.70 - -

Mean 2.2 131 77 63 49 16.7 2.63 152 3.7 614 SD 2.0 454 10 12 9 2.3 0.56 23 1.3 159

AO Sat = Aortic saturation; CI = cardiac index; EPO = erythropoietin; Hb = hemoglobin; SOT = systemic oxygen transport; SVC Sat = superior vana cava saturation; VO, = oxygen consumption.

ents gave informed consent according to an institutionally approved protocol.

A complete blood count was obtained from all patients, including measurement of red blood cell indices (Coulter counter) obtained by venipuncture on the day before cardiac catheterization. Drugs used for sedation during cardiac catheterization included fentanyl, droperidol, and promethazine. During catheterization, immediately after vascular access was achieved, 15 ml of blood was drawn through indwelling catheters for measurement of arterial and superior vena cava blood gases, aortic and superior vena cava oxygen saturation, serum erythropoietin, red cell 2,3-DPG, serum ferritin, serum iron, and total iron binding capacity.

Laboratory measurements. Serum erythropoietin titer was measured by means of a radioimmunoassay technique described in detail elsewhere.15 The red cell 2,3-DPG determinat.ion was performed by means of a commercially available technique (Abbott Laboratories, North Chicago, Iii., normal values = 1.6 to 2.6 mol/ml). Oxygen consump- tion was measured in 16 patients by means of a commer- cially available flow-through hood system (Waters Instru- ments, Inc, Rochester, Minn.) and normalized to body surface area. Cardiac index was calculated by means of the Fick principle. Systemic oxygen transport was calculated as the product of cardiac index and arterial oxygen content. Transferrin saturation was calculated as serum iron/total iron binding capacity x 100 and expressed as a percentage.

Analysis of data. Patients were considered iron defi- cient if they met one of the following criteria: serum

ferritin <12 rig/ml, transferrin saturation <IS% (ages 2 to 8 years) or <7 % (less than 2 years), serum iron concentra- tion <50 mg/dl if iron binding capacity had not been measured, or red cell mean corpuscular volume less than the fifth percentile for age and sex in the absence of thalassemia trait.16

Means and standard deviations were calculated for all measured and calculated variables. For statistical analy- sis, patients were divided into two groups, those with low serum erythropoietin titers (130 mu/ml) and those with high erythropoietin titers. Groups were compared by means of an unpaired t test, and a p value <0.05 for a two-tailed test was considered statistically significant. After patients with iron deficiency were excluded, regres- sion analysis was used to establish a relationship between aortic oxygen saturation and hemoglobin concentration.

RESULTS

The patients ranged in age from 3 months to 7 years, with a mean of 2.2 -t 2.0 years. There were 14 boys and five girls. Cardiovascular diagnoses in- cluded d-transposition in eight, tetralogy of Fallot in six, severe Ebstein’s anomaly in one, truncus arteri- osus in one, and other complex cyanotic lesions in three. Baseline clinical data are shown in Table I. Erythropoietin titers 530 mu/ml were found in 14 patients (mean 9.3 + 6.4 mu/ml), and five patients had high erythropoietin titers (range 50 to 2000 mu/ml). All patients had aortic oxygen saturation below 90% (range 50% to 87%). Polycythemia with

130 Gidding and Stockman July 1988

American Heart Journal

EW

peo.01

. 2 l

HIGH EPO

~~ p! i’it=~ p-y; LOW MGH

EPO EPO EPO EPO

Fig. 1. Comparison of patients with serum erythropoietin (EPO) titers <30 mu/ml and >30 mu/ml, with respect to Pao, (n = 19), aortic oxygen (0,) saturation (n = 191, and red cell 2,3-DPG concentration (n = 16). Low titers were associated with less severe hypoxemia and lower (high-normal) red cell 2,3-DPG values.

Table II. Iron stores

Patient Fer Fe TIBC

No. fngldl) (gmldl) (gmldl) % Sat MCV

1 20 29 537 5 64

2 27 27 468 6 79

3 53 55 - 88

4 32 64 426 15 88 5 63 166 432 38 98

6 18 38 432 9 80

7 46 42 324 13 86

8 54 63 330 13 86 9 46 106 330 32 88

10 49 116 420 28 90

11 13 72 387 19 86

12 7.6 112 - - a6 13 15 29 324 9 84 14 51 173 - - a7

15 23 66 300 22 80 16 51 161 390 41 92 17 61 104 - 90

18 14.5 36 324 11 86 19 55 131 291 45 83

Mean 37 84 381 20 85

SD 19 49 71 13 7

Fe = Iron; Fer = ferritin; MCV = mean corpuscular volume; % Sat = transferrin saturation; TIBC = total iron binding capacity.

hemoglobin concentrations above the ninety-sev- enth percentile for age and sex was present in 17 of 19 patients. Hemoglobin concentrations between the ninetieth and ninety-seventh percentiles in grams per deciliter were present in two patients; both had aortic oxygen saturation of 87% .14

Fig. 1 compares Paoz, aortic oxygen saturation, and red cell 2,3-DPG concentrations in patients with low and high erythropoietin titers. Patients with high erythropoietin titers had lower Paoz (p < O.Ol), iuwer aortic saturation (p < O.Olj, and higher red

cell 2,3-DPG values @ < 0.01). In addition, superior vena cava oxygen saturation was significantly lower in the group with high erythropoietin titers (50 f 9

vs 60 + 5%) p < 0.01). Hemoglobin concentrations showed a trend toward being higher in the five patients with high erythropoietin titers than in the group with low erythropoietin titers (18.2 k 2.6 gm/dl vs 16.1 + 2.1 gm/dl, 0.1 > p > 0.05), despite the presence of iron deficiency in two patients. Oxygen consumption (154 * 38 vs 151 * 15 ml/ min/m2), cardiac index (3.9 f 2.1 vs 3.5 f 0.9 L/ min/m2), and systemic oxygen transport (628 f 165 vs 586 k 158 ml/min/m2) were similar in the high and low erythropoietin groups, respectively. These studies suggest that a high erythropoietin titer is associated with severity of hypoxemia and abnormai tissue oxygen delivery, as assessed by an increased red cell 2,3-DPG concentration.

Three patients met the criteria for iron deficiency (Table II). One had low serum ferritin (No. 12), one had low transferrin saturation (No. 2), and one had low red cell mean corpuscular volume and low transferrin saturation (No. 1). Two of these three patients had the highest erythropoietin titers in the study (Nos. 1 and 2). Also, three of the four highest oxygen consumption measurements were recorded in these patients.

Fig. 2 compares aortic oxygen saturation with hemoglobin concentrations in the 16 patients who had sufficient iron. As expected, these data show a strong negative relationship between aortic oxygen saturation and hemoglobin concentration. Two points concerning these data area of special interest. Patients with oxygen saturation above 80% had hemoglobin concentrations of 18.1 gm/dl or less, values consistent with the absence of significant hyperviscosity.6-8 Second, there is a broad range of hemoglobin concentrations for a given aortic oxygen saturation. For example, patients with aortic oxygen saturations between 75% and 79% had hemoglobin concentrations ranging from 14.8 gm/dl to 19.9 gm/dl.

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Nutnew 1, Part 1 Erythropoietin in cyanotic heart disease 13 1

DISCUSSION

This study has shown that in children with cya- notic congenital heart disease, severe hypoxemia and elevated red cell 2,3-DPG concentrations are associated with high serum erythropoietin titers. Children with aortic oxygen saturation above 75% and high-normal red cell 2,3-DPG concentrations in general had low serum erythropoietin titers. The compensatory erythrocytosis in this group of cya- notic patients with low erythropoietin titers, partic- ulary with aortic saturation was higher than 80%) was not of sufficient magnitude to cause severe hyperviscosity, which is usually associated with hemoglobin concentrations significantly higher than 20 gm/dl.B-8 These data suggest that for children less than 8 years of age, adequate physiologic compensa- tion for moderate hypoxemia can occur with modest increases in hemoglobin levels and are consistent with the observation of Berman et al.4 that systemic oxygen transport is normalized by compensatory erythrocytosis. Most patients with palliated cyanot- ic congenital heart disease are in this age group.

It is difficult to generalize our results to cyanotic patients more than 8 years of age because of the effects of age on hemoglobin concentrations. During late childhood and adolescence, hemoglobin values in acyanotic patients increase.14 In cyanotic patients a similar phenomenon may occur, since in our clinical experience it is very unusual for children less than 10 years of age to require exchange transfusions for severe polycythemia, whereas exchange transfu- sions are often necessary in older patients, even when aortic saturation is higher than 80%. In the report of Rosenthal et al.,5 14 of 22 patients who received exchange transfusions for polycythemia were older than 8 years. Five of eight patients younger than 8 years had aortic saturations less than 80%.

Other potential explanations for our findings exist that could alter the conclusions drawn from our study. If erythropoiesis is dependent on episodic stimulation of erythropoietin production, our study protocol may have underestimated the serum eryth- ropoietin titer in some patients because they were studied at rest rather than during daily activity or at other times of relatively increased hypoxemia or tissue oxygen demands. 18* 2o Our study also assumes that serum erythropoietin titers reflect total eryth- ropoietin production. Total erythropoietin produc- tion may be underestimated, for example, if an increased number of red cell precursors exist in the bone marrow and erythropoietin is sequestered on receptors of these cells. lo This situation could exist in cyanotic congenital heart disease.

23r y = -0.26 (x) + 38 r = 0.77 S.E.E. = 1.6

Fig. 2. Relationship between aortic oxygen (0,) satura- tion and hemoglobin concentration in 16 patients with normal iron stores. (See text for further discussion).

Study of erythropoietin secretion in cyanotic con- genital heart disease has previously been limited by the absence of an easily performed assay for eryth- ropoietin. Halvorsen17 showed that unoperated patients with transposition have elevated levels of erythropoietin, as determined by bioassay, at less than 1 month of age. Studies of patients with polycythemia resulting from various causes have often included a small nrcmber of patients with congenital heart disease. Erythropoietin titers in these patients have generally been >30 mU/dl. Clinical information concerning these patients has been limited.16p I9

Recently, Tyndall et al.21 studied serum erythro- poietin in a large series of patients with cyanotic and acyanotic congenital heart disease by means of a radioimmunoassay technique. Values for erythro- poietin in our two series were generally similar. These investigators also concluded that the presence of low serum erythropoietin titers in many patients with cyanotic congenital heart disease suggests that erythrocytosis can compensate for moderate hypox- emia. The results of our study add additional sup- port to this conclusion by showing increased red cell 2,3-DPG concentrations in patients with higher erythropoietin titers. In addition, our data show the severe stress of a combination of iron deficiency and severe hypoxemia, inasmuch as our two highest erythropoietin titers were in patients with this com- bination.

132 Gidding and Stockman

Results of early studies if iron supplementation in cyanotic congenital heart disease suggested that uncontrolled iron supplementation would lead to hyperviscosity.’ Results of more recent studies have shown that children less than 8 years of age have increased iron requirements, are more prone to cerebral vascular incidents when relatively anemic, and have impaired tissue oxygen delivery when iron stores are diminished.2~g~21*22 That three of our four highest oxygen consumption measurements oc- curred in patients with iron deficiency suggests that iron deficiency may increase metabolic demands.4s 23 We speculate that iron supplementation in patients with moderate hypoxemia will not lead to severe increases in hemoglobin concentration, since our patients had normal serum erythropoietin titers and sufficient iron. Thus, we do not feel that iron therapy should be withheld from moderately hypox- emia patients with clinically suspected relative ane- mia.

To summarize, we have shown that low serum erythropoietin titers are present in children with moderate hypoxemia, relatively normal tissue oxy- gen delivery, and adequate compensatory erythrocy- tosis. We have developed a regression equation relating hemoglobin concentration to aortic satura- tion in patients with sufficient iron and have shown that, in general, children less than 8 years of age with aortic oxygen saturation higher than 80% will adequately compensate for their chronic hypoxemia at levels of hemoglobin that do not produce severe hyperviscosity.

We thank Annette Gardner for performing the erythropoietin assays in the laboratory of Dr. Eugene Goldwasser, and Eric Valcourt for performing the red cell 2,3-DPG assays. We also thank Shann Bulger for her careful preparation of this manu- script.

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July 1989

American Heart Journal