Catecholamine abnormalities in transient tachypnoea of the premature newborn.

The aim of the present study was to assess if there was an association between low catecholamine levels at birth in the premature infant and the development of TTN. Blood samples were collected at delivery from the umbilical artery of all preterm infants with a gestational age less than 36 weeks for determination of pH and catecholamine levels (noradrenaline and adrenaline). Amongst non-asphyxiated infants only, cord pH greater than 7.25 and/or Apgar score greater than 7 at five minutes, catecholamine levels were compared between the 10 infants who developed transient tachypnoea of the newborn (TTN) and 13 controls of a similar gestational age range (31-35 weeks) who developed no respiratory distress in the neonatal period. Infants who developed TTN were more often delivered without labour, 8 of 10 compared to 2 of 13 controls (p less than 0.01). There were no significant differences in adrenaline levels between the two groups. Noradrenaline levels, however, were significantly lower in the infants who developed TTN, being a median of 3.1 nmol/l (range 1.07-5.85 nmol/l) compared to a median of 6.4 nmol/l (range 2.38-22.83) in the controls (p less than 0.01). Infants who were delivered following labour had significantly elevated noradrenaline levels compared those delivered without labour (elective delivery) (p less than 0.05). These results suggest that low noradrenaline levels in preterm infants may explain the association in this group of TTN and "elective" delivery.     

Outcome of fetuses with lactic acidemia

Umbilical arterial lactate, pH, PO2, PCO2, and base deficit were measured at delivery of 468 live-born infants. The correlation between the 1-minute Apgar score and lactate (r = -0.34) was comparable to that between the 1-minute Apgar score and pH (r = 0.30). The multiple correlation coefficient between the 1-minute Apgar score and all five parameters combined (lactate, pH, PO2, PCO2, and base deficit) was 0.36. This was not significantly higher than the simple correlation coefficient for pH alone (0.30). Similar correlation coefficients were noted for the 5-minute Apgar score. The dividing point between normal and pathologic lactate levels was 3.70 mmol/L. A pH less than 7.20 and a lactate level greater than or equal to 3.70 mmol/L had the same sensitivity, specificity, and positive and negative predictive values for low Apgar scores (less than 7). While pH and lactate are equally correlated with fetal outcome, their combination with each other and with other blood gas parameters does not predict outcome better than either pH or lactate alone.     

The effect of blood gas and Apgar score on cord blood cardiac troponin I.

OBJECTIVES: The aims of this study were to (a) establish a reference range for cardiac troponin I (cTnI) in the cord blood of healthy infants, and (b) investigate the effect of Apgar score, cord blood gas, gestational age, and creatine kinase (CK) and creatine kinase MB (CK-MB) fraction levels on cord blood cTnI levels. METHODS: 112 perinatal hypoxic and 84 control newborns without perinatal hypoxia were enrolled in this study. Cord blood samples were collected from the babies for arterial blood gas analysis, cTnI, CK and CK-MB measurements. Gestational age, birth weight, sex, Apgar score and history of fetal distress were recorded. Hypoxic ischemic encephalopathy (HIE) group, hypoxic but without HIE group and control groups were identified according to clinical observations during the first 72 h in the newborn unit. RESULTS: HIE and perinatal hypoxic without HIE groups had a significantly higher cord blood cTnI level according to the control group (1.8 ng/mL (0-13), 0 ng/ml (0-1.1) and 0 ng/ml (0-0.3) respectively). Cord blood cTnI level did not have a correlation with birth weight and gestational age (r = -0.02, p > 0.05 and r = 0.08, p > 0.05 respectively). Cord blood cTnI level also had a negative correlation with pH, bicarbonate, base deficit, and Apgar score (r = -0.40, p < 0.001; r = -0.39 p < 0.001; r = -0.45 p < 0.001; r = -0.41, p < 0.001) respectively). Cord blood cTnI level showed a positive correlation with CK and CK-MB levels (r = 0.45, p < 0.001 and r = 0.37, p < 0.001 respectively). Receiver operator curve analysis revealed that the most sensitive factor for prediction of perinatal hypoxia is cord cTnI value [area under curve = 0.929]. The optimal cut-off value of cord cTnI was 0.35 ng/ml for hypoxia. CONCLUSION: cTnI levels in the cord blood are not affected by gestational age and birth weight. cTnI together with CK and CK-MB has been found to be elevated in hypoxic infants compared to normal infants. Therefore cTnI may be an indicator for perinatal hypoxia in neonates.     

The relationship between Apgar score, umbilical artery pH and operative delivery for fetal distress in 2778 infants born at term

A total of 2778 infants born at term were studied to determine the relationship between Apgar scores after 1 min, umbilical artery pH values, mode of delivery, a diagnosis of fetal distress leading to operative delivery, and sex. Eighty-three percent of the population had normal Apgar scores (greater than or equal to 8) and normal pH values (greater than 7.15) in which 10% were operatively delivered for fetal distress (ODFD). Sixty-one percent of the children with low Apgar scores (less than or equal to 7) had normal pH values, and 74% of the infants with acidosis (pH less than or equal to 7.15) had normal Apgar score. Twenty-four percent of the infants with a low Apgar score and/or acidosis were ODFD (sensitivity). Ninety percent of the infants who had Apgar scores and pH values were not ODFD (specificity). The predictive value (a low Apgar score and/or acidosis) of ODFD was 33%, and the negative predictive value (normal Apgar score and a normal pH) of ODFD was 85%. A significantly higher incidence of ODFD and acidosis was found in boys.     

Apgar scores and umbilical arterial pH in preterm newborn infants

One- and five-minute Apgar scores and umbilical cord arterial pH values were compared in preterm newborn infants of various gestational ages. The more premature the infant, the more likely the Apgar score was low in the presence of a pH greater than or equal to 7.25. Conversely, the closer to term, the more frequently an infant with a pH of less than 7.25 had an Apgar score of greater than or equal to 7. Therefore, in preterm infants, there is little congruity between the Apgar score and umbilical cord pH. Based on these findings, it is not appropriate to label preterm newborn infants as asphyxiated based on a low Apgar score.     

Umbilical cord unbound free fatty acid concentration and low apgar score

Increased levels of unbound Free Fatty acid (FFAu) have been found in adults undergoing coronary angioplasty as a result of acute hypoxia-ischemia. We hypohesized that infants suffering from a 1-minute Apgar score of less than 5 will demonstrate elevated FFAu levels in the cord blood. One hundred ninety-nine infants between 25 and 41 weeks gestational age were enrolled in the study. Infants with an Apgar score of less than 5 at 1 minute served as the study group. Blood samples were collected from the umbilical cord and serum FFAu levels were measured with the fluorescent probe acrylodan-derivatized intestinal fatty acid binding protein. The low Apgar score group (n=32, birthweight 3153+/-780 g, gestational age 37.9+/-3.1 weeks) and normal Apgar score group (n=167, birthweight 3067+/-847 g, gestational age 37.5+/-3.5 weeks) were significantly different with respect to Apgar score at 1 minute (3.0+/-1.2 versus 8.4+/-1.1), Apgar score at 5 minutes (6.9+/-versus 8.9+/-0.5), cord pH (7.16+/-0.12 versus 7.28+/-0.07), and in the frequency of meconium passage (40.6% versus 14.9%). Cord FFAu levels were 4.4+/-1.7 versus 3.2+/-1.2 nM (p<0.001), respectively. Cord FFAu correlated inversely with Apgar score at 1 minute (r=-0.31, p<0.05) and with cord pH (r=-0.12, p<0.05), but not with birthweight or gestational age. In infants with low 1-minute Apgar scores, cord free fatty acid levels were significantly elevated compared with those from controls.     

Routine umbilical cord blood gas determinations?

Between 1986 and 1988, 1924 term nulliparous patients with spontaneous onset of labor were studied to assess the importance of obtaining umbilical cord blood gas levels on all deliveries. The umbilical cord arterial and venous pH values (expressed as mean +/- 2 SD) were 7.24 +/- 0.14 (n = 1694) and 7.32 +/- 0.12 (n = 1820), respectively. The incidence of newborn depression (1- or 5-minute Apgar score less than 7) was 14.1%; of these depressed newborns, the incidence of normal umbilical cord arterial pH values (greater than or equal to -2 SD) was 77.8%. Of the vigorous newborns, there was a 2.1% incidence of umbilical cord arterial blood acidemia. Umbilical cord arterial blood acidemia in vigorous newborns was not highly predictive of specific morbidity in the immediate newborn period. Regression analysis demonstrated the umbilical cord arterial pH to correlate best with the Apgar scores when compared with all other arterial or venous blood gas measurements. We reached the following conclusions: (1) that obtaining cord arterial pH values in vigorous newborns should be considered since the values will provide objective documentation or normal fetal acid base balance in 98% of infants. (2) Only a cord arterial pH determination is recommended since it reflects fetal or newborn status more accurately than all other measurements. Additional measurements increase the likelihood of abnormal results and do not contribute to neonatal management. (3) An umbilical cord blood pH value is extremely useful in ruling out the diagnosis of birth asphyxia in the depressed newborn.     

Fetal heart rate variability: an approach to automated assessment.

Three hundred seventy-five hours of fetal heart rate (FHR) data derived from the direct fetal electrocardiogram (ECG) were studied. This data had been stored on magnetic tape from 83 intrapartum patients. By means of a computerized technique, the FHR variability was assessed quantitatively. The degree of variability was then related to: (1) state of labor, (2) fetal scalp pH values, and (3) the 1-minute Apgar score. FHR variability was computed from differences between consecutive R-R intervals measured from the R wave of each fetal ECG. A trend of increasing variability was seen with advancing labor, defined by either time prior to delivery or cervical dilatation, but values were not statistically significant. Significantly less FHR variability was encountered when fetal scalp pH values below 7.20 were compared to higher values. FHR variability assessed during the 20 minutes immediately preceding delivery was significantly lower in infants with 1-minute Apgar scores less than 7. Machine assessment of FHR variability thus could be correlated with fetal condition as determined by scalp pH and neonatal outcome determined by Apgar score.     

Increased arterial catecholamines in pre-eclampsia

Arterial and venous plasma catecholamines were measured in 13 pre-eclamptic and 13 normotensive pregnant women. In the pre-eclamptic group, arterial concentrations were higher for adrenaline (p less than 0,001), noradrenaline (p less than 0.05) and dopamine (p less than 0.01) than in the normotensive group, whereas in venous plasma only adrenaline (p less than 0.01) and dopamine levels were higher (p less than 0.05). Arterial adrenaline concentrations in the pre-eclamptic group were, on average, three times as high as normotensive arterial adrenaline. The arterial-venous (a-v) differences were higher for adrenaline (p less than 0.001) and dopamine (p less than 0.05) in the pre-eclamptic than in the normotensive group. In the pre-eclamptic group, arterial adrenaline was correlated with mean arterial blood pressure (r = 0.89, p less than 0.001) and with increased heart rate (r = 0.78, p less than 0.01). According to these results, both sympathetic nervous and sympathetic adrenal activities are increased in patients with pre-eclampsia.     

Monitoring arterial oxygen saturation in the neonate

Term neonates (N = 35) with an one minute Apgar score of greater than or equal to 8 and mean umbilical artery pH values within normal were monitored by pulse oximetry. SaO2 monitoring started one minute after delivery. The initial SaO2 ranged from 40 to 75%. Neonates with a SaO2 above 80% five minutes after delivery remained untreated, neonates with a SaO2 below 80% received mask CPAP (figure 1). The initial difference in SaO2 between the groups was statistically significant (p less than 0.05). Mean umbilical artery pH and one, five and ten minutes Apgar score values were statistically not significant between the groups (p greater than 0.05). CPAP had been terminated as soon as SaO2 had reached 90%. This had been the lowest value monitored in spontaneously breathing neonates one day after delivery. Our findings indicate that neonates may sustain prolonged periods of decreased SaO2 which had not been detected by umbilical artery pH nor by the Apgar score. SaO2 monitoring by pulse oximetry served as a valuable method in the immediate newborn evaluation.     

Screening with Doppler velocimetry in labor

Doppler flow velocimetry was performed on 273 nonselected patients in labor. All patients were at least 2 cm dilated and 80% effaced (58% greater than 4 cm) and were delivered of infants within 24 hours. Fetal heart rate tracing performed at the same time was read by another observer and compared with Doppler flow velocity measurements (systolic/diastolic ratio, pulsatility index). A significant correlation was noted (r = 0.234, p less than 0.001) between the two modalities. Both fetal heart rate tracings and umbilical flow velocity measurements were correlated with fetal outcome. Significant associations were noted between the umbilical systolic/diastolic ratio and fetal distress. 5-minute Apgar scores, the need for intubation independent of gestational age, and umbilical arterial cord pH and base excess. It appears that the information obtained from flow velocimetry had additional benefits compared with that of fetal heart rate tracings alone.     

Hypoxanthine: a marker for asphyxia

It has been hypothesized that hypoxanthine concentrations in the blood of newborn infants are a marker of asphyxia. To test this hypothesis, we measured serum hypoxanthine levels in relationship to perinatal and neonatal asphyxia, and compared arterial hypoxanthine levels with arterial pH and base deficit. We also compared hypoxanthine levels of survivors with those of asphyxiated non-survivors. Forty-two newborns were classified as asphyxiated by either of two methods: 1) Infants from whom umbilical cord hypoxanthine levels were taken were classified as asphyxiated if they had an Apgar score of 6 or less at 1 or 5 minutes, fetal heart rate below 100 beats per minute, or meconium-stained amniotic fluid; and 2) infants from whom peripheral arterial hypoxanthine samples were taken were classified by clinical assessment, whereby one author, blinded to the infants' hypoxanthine levels, prospectively assessed each patient's condition for evidence of asphyxia. Hypoxanthine levels correlated with increased base deficit (P less than .001; r = 0.8) and with decreased pH (P less than .001; r = -0.5). By both of our asphyxia classification methods, hypoxanthine levels were significantly higher (P less than .002) in the asphyxiated groups. We also noted a higher hypoxanthine level in asphyxiated non-survivors as compared with all survivors (P less than .02). We propose that serum hypoxanthine levels may help define asphyxia. Because hypoxanthine, when metabolized by xanthine oxidase, generates oxygen radicals that are highly destructive to tissue, hypoxanthine levels may have important therapeutic implications for asphyxiated patients.     

Elevated plasma Met-enkephalin levels in the human newborn are a poor indicator of perinatal stress.

OBJECTIVE: This study was designed to investigate whether plasma Met-enkephalin peptides could serve as markers of physiologic stress in the neonate. STUDY DESIGN: Infants (n = 115) between 1.2 and 4.7 kg and 28 and 42 weeks of gestation were studied at birth. Seventy-four infants were delivered by the vaginal route, 31 by cesarean section after labor, and 10 by cesarean section before labor. Correlations were sought between plasma enkephalin peptides and epinephrine, norepinephrine, and arterial blood gases with linear regression analysis. Various clinical data were also analyzed. RESULTS: Plasma Met-enkephalin levels were significantly greater in infants exposed to labor (440 +/- 36 vs 260 +/- 30 pg/ml, p less than 0.05). The large-molecular-weight forms of enkephalin peptides were also greater in these infants (approximately 50 +/- 4 vs 23 +/- 2 ng/ml). There was no correlation between plasma enkephalin peptides and catecholamines, arterial pH, or Apgar scores. There was a significant but weak correlation between plasma Met-enkephalin levels and birth weight (r = 0.34, p = 0.03) and PaO2 (r = -0.28, p less than 0.05). CONCLUSION: The lack of correlation between Met-enkephalin plasma levels and umbilical plasma catecholamine concentrations, acid-base status, or Apgar scores suggests that circulating Met-enkephalin is a poor indicator of stress in the newborn.     

Meconium aspiration and fetal acidosis

Meconium in labor is associated with increased perinatal morbidity and mortality. To identify the infants at risk, 53 women with moderate-to-thick meconium were followed in labor after obtaining baseline fetal scalp blood pH levels. Although 28 of the newborns (53%) exhibited an arterial cord pH of less than 7.25 at delivery, there were no significant predictive variables found in the electronic fetal monitoring score, Apgar score, or mode of delivery. Nine of the infants with a pH value of less than 7.25 had meconium below the vocal cords at delivery, but none in the infants with pH levels greater than or equal to 7.25. The P50 value for cumulative acidosis is 55 minutes, indicating a more rapid deterioration than an average-for-gestational-age fetus without meconium. Therefore, the presence of thick meconium implies that fetal stress must be avoided during labor, and early intervention is warranted when there is deviation from normal labor progress or fetal heart rate pattern.     

Growth hormone, somatomedin C, and nonsuppressible insulin-like activity levels compared in premature, small, average birth weight, and large infants

We have investigated the relationship between growth hormone, somatomedin C, nonsuppressible insulin-like activity, weight, gestational age, and 1-minute Apgar score in newborn infants. The 153 infants were categorized as small for gestational age (n = 19), average for gestational age (n = 59), large for gestational age (n = 60), and premature (gestational age at birth, 36 weeks or less (n = 15). Our study showed that (1) growth hormone levels were elevated in premature infants and correlated with Apgar scores and birth weights; (2) somatomedin C and nonsuppressible insulin-like activity levels were significantly lower in premature than in term infants; and (3) the birth weight of all infants studied had a significant overall effect on both somatomedin C and nonsuppressible insulin-like activity levels, suggesting that these factors may be involved in fetal growth. However, because in small for gestational age infants somatomedin C and nonsuppressible insulin-like activity were similar to levels in average for gestational age infants, it is suggested that other factors may inhibit fetal growth.     

Comparison of epidural and general anaesthesia for elective caesarean delivery according to the effects of apgar scores and acid-base status

The objective of this study was to determine the effects of lumbar epidural anaesthesia on the Apgar score and acid-base status of the newborn. Umbilical artery blood gases were obtained in 85 singleton, term, uncomplicated pregnancies delivered by elective Caesarean section. The umbilical artery blood pH, PaCO2, PaO2 and HCO3 values and Apgar scores (1 and 5 minutes) were compared between lumbar epidural and general anaesthesia groups. General anaesthesia was used in 45 (52.9%) women and lumbar epidural anaesthesia in 40 (47.1%). Only 2 of the newborns exposed to epidural anaesthesia had umbilical artery blood pH values 7.19 or less. The mean umbilical artery blood pH was found to be significantly lower in the newborns exposed to lumbar epidural anaesthesia (p = 0.011). None of the newborns in the 2 groups were severely depressed (Apgar scores less than 4). The mean umbilical artery blood PaCO2, PaO2 and HCO3 values did not show any significant difference between the groups. In conclusion, lumbar epidural anaesthesia is associated with lower umbilical artery blood pH values, occasionally with severe fetal acidaemia.     

The reliability of fetal pulse oximetry: the effect of fetal oxygen saturation below 30% on perinatal outcome

OBJECTIVES: The objective of this prospective study was to determine the reliability of fetal pulse oximetry (FPO) in cases of abnormal fetal heart rate (FHR) pattern and fetal oxygen saturation (FSpO(2)) below 30%. STUDY DESIGN: Three hundred and one deliveries complicated by abnormal FHR pattern were monitored with both CTG and FPO. Cases of FSpO(2) values below 30% (n=52) formed the study group and those above 30% (n=249) were the control group. pH and base excess (BE) in the umbilical cord artery, 5-min Apgar score, and pH and BE in the newborn were measured and compared. Statistical analysis included the Epi info 2002 program, Chi-squared test, and linear regression. RESULTS: FSpO(2) values were consequently below 30% in 52 and above 30% in 249 cases. When FSpO(2) values were below 30%, umbilical pH was significantly lower than in controls (P<0.01), while other parameters were not significantly different. If the time interval between the measurement of the first low (<30%) FSpO(2) value and delivery did not exceed 32.4+/-2.5 min, umbilical pH and Apgar scores did not show fetal acidosis. CONCLUSION: Our findings suggest that FPO is a reliable method of estimating fetal condition and may help reduce the number of cesarean sections performed due to abnormal FHR tracings.     

Findings of subpartal monitoring, status of the newborn infant and evaluation of cardiotocographic findings]

The parameters of the intrapartal fetal CTG and the actual pH are related to each other and to the Apgar score of the newborn. The interpretation was carried out by means of groups of 300 to 600 cases. The actual fetal pH has the most compact relation to the Apgar score of the newborn (C equals 0,31). But also this contingence is unsatisfactory. At fetal acidosis most of the Apgar scores are pathological, in that manner the pH is the most important test for indication of operative delivery. The rare CTG pattern as dip II and variable decelerations combined with tachycardia of bradycardia, the bradycardia with 80 bpm or less, are likewise indications of urgent delivery.     

Fetal tobacco syndrome and perinatal outcome

OBJECTIVE: The aim of this study was to evaluate perinatal outcome in newborns of mothers who are smokers. METHODS: The study included 87 pregnant women with a single pregnancy in the cephalic position, 64 of them nonsmokers (group 1), 13 who smoked 5-20 cigarettes per day (group 2) and 10 who smoked more than 20 cigarettes per day (group 3). Maternal demographic variables and laboratory hemoglobin concentration, hematocrit and erythrocyte count in the last trimester were recorded. Perinatal outcome included type of delivery (vaginal or cesarean section), birth weight, occurrence of meconium in the amniotic fluid, 5-min Apgar score, umbilical arterial blood pH postpartum, sex of the newborn, need for treatment at a neonatal intensive care unit (NICU) and clinically and neurosonographically verified postpartum neurologic complications in the newborn. RESULTS: A statistically significant correlation (p < 0.01) was found with the mean gestational age at delivery in all three groups of women, especially in those smoking >20 cigarettes per day, who had a higher incidence of premature deliveries. Maternal laboratory findings also differed significantly among the three groups of women, i.e. erythrocyte count (p < 0.01), hemoglobin concentration (p < 0.01) and hematocrit (p < 0.001). The rate of delivery by cesarean section was significantly higher in the groups of smokers, irrespective of the number of cigarettes per day (groups 2 and 3). Birth weight was lower by about 250 and 350 g (p < 0.001) in groups 2 and 3, respectively. Five-minute Apgar score and umbilical arterial blood pH were lower in group 3 as compared with groups 1 and 2 (p < 0.01). NICU treatment was required in more than 50% of infants born to group 3 mothers, in whom 70% of perinatal neurologic complications such as subependymal hemorrhage, periventricular hemorrhage, porencephalic cysts, intracranial hemorrhage and swallowing disturbance of the newborn were recorded (p < 0.001). The infants born to group 3 mothers had a longer and more difficult period of adaptation, thus often requiring an NICU stay. CONCLUSION: Our study confirmed that pregnancy burdened with smoking, especially in the case of >20 cigarettes a day, is associated with a high risk due to the development of maternal anemia and fetal hypoxia and polyglobulia, which in turn result in a significantly poorer perinatal outcome in infants born to smoking mothers and compromised subsequent development of the child, as evidenced by the morphological substrates on the brain resulting from the fetal mechanism of defense against hypoxia. Clinically, there was no other (etiologic) reason for (chronic) fetal hypoxia; thus, the clinical substrate of fetal tobacco syndrome could be presumed to have developed consequentially to chronic smoking during pregnancy, as a preuterine factor of fetal hypoxia. Other gestational or gestation-related diseases (e.g., gestosis, diabetes) that may potentially cause nutritional and respiratory insufficiency of the placenta were ruled out.     

The relationship of maternal erythrocyte oxygen transport parameters to intrauterine growth retardation

The relation of fetal growth and maternal oxygen transport as assessed by red blood cell 2,3-diphosphoglycerate, hemoglobin oxygen affinity, hemoglobin, pH, and PCO2 was evaluated in 21 pregnant women. The study was performed in the third trimester and each subject evaluated had sonographic evidence of fetal growth retardation without other obvious abnormalities. Decreased maternal 2,3-diphosphoglycerate/hemoglobin molar ratio and hemoglobin oxygen affinity were related linearly to the birth weight normalized for the expected sea level values of gestational age expressed as a birth weight (gestational age-normalized) Z score. The correlation coefficients and p values were r = 0.71, p less than 0.001 and r = 0.67, p less than 0.001, respectively. The ponderal index-normalized Z score correlated with the 2,3-diphosphoglycerate/hemoglobin molar ratio (r = 0.46, p less than 0.04), but the relation was not as strong as the birth weight-normalized Z score. The crown-heel length/head circumference ratio did not correlate with the 2,3-diphosphoglycerate/hemoglobin molar ratio (r = 0.29, NS). The birth weight (gestational age)-normalized Z score did not correlate with hemoglobin, PCO2, or pH. In the regulation of hemoglobin oxygen affinity, calculations indicated that the 2,3-diphosphoglycerate/hemoglobin molar ratio played a highly significant role (p less than 0.001), pH was minimally significant (p less than 0.025), but PCO2 had little or no significant effects in this study. It appears that fetal growth is related to the maternal red blood cell oxygen transport parameters 2,3-diphosphoglycerate/hemoglobin molar ratio and hemoglobin oxygen affinity. Moreover, the 2,3-diphosphoglycerate/hemoglobin molar ratio is the principal regulator of hemoglobin oxygen affinity.