Neonatal laboratory blood sampling: comparison of results from arterial catheters with those from an automated capillary device

Accounting for arterial and capillary blood gases for calculation of cerebral blood flow in preterm infants

One of the most feared neurological complications of premature birth is intraventricular hemorrhage, frequently triggered by fluctuations in cerebral blood flow (CBF). Although several techniques for CBF measurement have been developed, they are not part of clinical routine in neonatal intensive care. A promising tool for monitoring of CBF is its numerical assessment using standard clinical parameters such as mean arterial pressure, carbon dioxide partial pressure (pCO₂) and oxygen partial pressure (pO₂). A standard blood gas analysis is performed on arterial blood. In neonates, capillary blood is widely used for analysis of blood gas parameters. The purpose of this study was the assessment of differences between arterial and capillary analysis of blood gases and adjustment of the mathematical model for CBF calculation to capillary values. The statistical analysis of pCO₂ and pO₂ values collected from 254 preterm infants with a gestational age of 23-30 weeks revealed no significant differences between arterial and capillary pCO₂ and significantly lower values for capillary pO₂. The estimated mean differences between arterial and capillary pO₂ of 15.15 mmHg (2.02 kPa) resulted in a significantly higher CBF calculated for capillary pO₂ compared to CBF calculated for arterial pO₂. Two methods for correction of capillary pO₂ were proposed and compared, one based on the mean difference and another one based on a regression model.

CONCLUSION

Capillary blood gas analysis with correction for pO₂ as proposed in the present work is an acceptable alternative to arterial sampling for the assessment of CBF.

WHAT IS KNOWN

• Arterial blood analysis is the gold standard in clinical practice. However, capillary blood is widely used for estimating blood gas parameters. • There is no significant difference between the arterial and capillary pCO₂ values, but the capillary pO₂ differs significantly from the arterial one.

WHAT IS NEW

• The lower capillary pO₂ values yield significantly higher values of calculated CBF compared to CBF computed from arterial pO₂ measurements. • Two correction methods for the adjustment of capillary pO₂ to arterial pO₂ that made the difference in the calculated CBF insignificant have been proposed.

Capillary partial pressure of carbon dioxide for predicting rehospitalization in preterm infants under noninvasive respiratory support with severe bronchopulmonary dysplasia

BACKGROUND

The severity of bronchopulmonary dysplasia (BPD) is an important predictor of prognosis in preterm infants. However, the severity of BPD was determined mainly by the degree of oxygen supplementation and mode of respiratory support.

OBJECTIVES

This retrospective study aimed to examine the role of partial pressure of carbon dioxide (pCO₂ ) in predicting rehospitalization among preterm infants with severe BPD without invasive ventilation at 36 weeks' postmenstrual age (PMA).

METHODS

We assessed preterm infants aged <32 gestational weeks with severe BPD who were receiving noninvasive respiratory support at 36 weeks' PMA. Patients were compared after stratifying them according to the history of rehospitalization owing to respiratory infection before a corrected age (CA) of 1 year and pCO₂ measured by capillary blood gas analysis at 36 weeks' PMA.

RESULTS

Among 54 infants who had severe BPD with noninvasive respiratory support at 36 weeks' PMA, 16 (29.6%) experienced rehospitalization due to respiratory problems. At 36 weeks' PMA, the amount of oxygen supplementation (0.30 vs. 0.28, p = 0.021) and pCO₂ (62.1 vs. 53.6 mmHg, p = 0.006) were higher in the rehospitalization group than in the no rehospitalization group. Multivariate logistic analysis findings indicated that pCO₂  ≥ 57.4 mmHg was the only factor associated with rehospitalization (adjusted odds ratio: 8.017, 95% confidence interval 1.239-51.859).

CONCLUSION

High pCO₂ during noninvasive respiratory support at 36 weeks' PMA in severe BPD was associated with rehospitalization. Consideration of the degree of impairment in ventilatory capacity may improve the prediction of later respiratory outcomes in infants with BPD.

Severe hypocalcemia and seizures after normalization of pCO2 in a patient with severe bronchopulmonary dysplasia and permissive hypercapnia

Objectives

To describe the association between the rapid normalization of pCO2 after intubation in a patient with severe bronchopulmonary dysplasia managed with permissive hypercapnia, with the risk of developing hypocalcemia and seizures, and to make health care providers aware of this risk in similar cases.

Case presentation

An extreme premature infant, born at 25 weeks of gestational age (GA), developed a severe bronchopulmonary dysplasia (BPD) and, after several extubation failures could be managed with non-invasive ventilation and permissive hypercapnia, with capillary pCO2 of up to 80 mmHg and pH >7.20. At 46 postmenstrual age (PMA) he was intubated because of severe hypercapnia and compensating metabolic alkalosis. About 20 h after intubation, after normalization of pH and pCO2, he developed hypocalcemia and seizures, that remitted after iCa normalization. A comparison between arterial and capillary blood gases showed a significantly greater correlation between pH and iCa in arterial than in capillary samples.

Conclusions

Our findings emphasize the importance of avoiding the abrupt reduction of pCO2 and the close monitoring of acute metabolic changes after its correction in chronic patients with permissive hypercapnia, as well as the potential superiority of arterial samples over capillaries to improve the precision of this control.

Agreement between Arterial and Capillary pH, pCO2, and Lactate in Patients in the Emergency Department

Background

Blood gas analysis (BGA) is a frequent painful procedure in emergency departments. The primary objective of the study was a quantitative analysis to assess the mean difference and 95% confidence interval of the difference between capillary and arterial BGA for pH, pCO₂, and lactate. Secondary objective was to measure the sensitivity and specificity of capillary samples to detect altered pH, hypercarbia, and lactic acidosis. Adults admitted to the ED were screened for inclusion. We studied the agreement between the two methods for pH, pCO₂, and lactate with Bland-Altman bias plot analysis and receiver operating characteristic curves.

Results

One hundred ninety-seven paired analyses were included. Mean difference for pH between arterial and capillary BGA was 0.0095, and 95% limits of agreement (LOA) were −0.048 to 0.067. For pCO₂, mean difference was −0.3 mmHg, and 95% LOA were −8.5 to 7.9 mmHg. Lactate mean difference was −0.93 mmol/L, and 95% LOA were −2.7 to 0.8 mmol/L. At a threshold of 7.34, capillary pH had 98% sensitivity and 97% specificity to detect acidemia; at 45.9 mmHg, capillary pCO₂ had 89% sensitivity and 96% specificity to detect hypercarbia. At a threshold of 3.5 mmol/L, capillary lactate had 66% sensitivity to detect lactic acidosis.

Conclusion

Capillary BGA cannot replace arterial BGA despite high concordance between the two methods for pH and pCO₂ and moderate concordance for lactate. Capillary measures had good accuracy when used as a screening tool to detect altered pH and hypercarbia but insufficient sensitivity and specificity when screening for lactic acidosis.

Acute physiological effects of packed red blood cell transfusion in preterm infants with different degrees of anaemia

OBJECTIVE

The safe lower limit of haematocrit or haemoglobin that should trigger a red blood cell (RBC) transfusion has not been defined. The objective of this study was to examine the physiological effects of anaemia and compare the acute responses to transfusion in preterm infants who were transfused at higher or lower haematocrit thresholds.

METHODS

The authors studied 41 preterm infants with birth weights 500-1300 g, who were enrolled in a clinical trial comparing high ('liberal') and low ('restrictive') haematocrit thresholds for transfusion. Measurements were performed before and after a packed RBC transfusion of 15 ml/kg, which was administered because the infant's haematocrit had fallen below the threshold defined by study protocol. Haemoglobin, haematocrit, RBC count, reticulocyte count, lactic acid and erythropoietin were measured before and after transfusion using standard methods. Cardiac output was measured by echocardiography. Oxygen consumption was determined using indirect calorimetry. Systemic oxygen transport and fractional oxygen extraction were calculated.

RESULTS

Systemic oxygen transport rose in both groups following transfusion. Lactic acid was lower after transfusion in both groups. Oxygen consumption did not change significantly in either group. Cardiac output and fractional oxygen extraction fell after transfusion in the low haematocrit group only.

CONCLUSIONS

These study's results demonstrate no acute physiological benefit of transfusion in the high haematocrit group. The fall in cardiac output with transfusion in the low haematocrit group shows that these infants had increased their cardiac output to maintain adequate tissue oxygen delivery in response to anaemia and, therefore, may have benefitted from transfusion.

Does lactate level in the first 12 hours of life predict mortality in extremely premature infants?

AIMS

To determine if high lactate levels within the first 12 h of life independently or in combination with Clinical Risk Index for Babies (CRIB) II can predict mortality in extremely premature babies.

STUDY DESIGN

A retrospective review of medical charts of babies born between 2001 and 2003 with birthweight <1000 g or gestation <28 weeks was performed. Blood gases and highest umbilical lactate levels in first 12 h of life were noted. Area under the curve (AUC) was calculated for lactate, CRIB and CRIB II as a predictor of mortality. The AUC for lactate and CRIB II were combined using discriminant analysis.

RESULTS

Two hundred nineteen infants were included in the study, 41 (18.7%) of whom died. The AUC for lactate was 0.67 (P < 0.001), while AUCs for CRIB and CRIB II score were 0.81 (P < 0.001) and 0.82 (P < 0.001), respectively. The AUC for the combined measure of lactate and CRIB II was 0.82, similar to CRIB II.

CONCLUSIONS

Lactate predicts mortality in premature infants, but was found to be inferior to CRIB and CRIB II. Adding lactate level to CRIB II score does not improve its predictive ability.

PCO2 and room air saturation values in premature infants at risk for bronchopulmonary dysplasia

OBJECTIVE

To determine the capillary partial pressure of carbon dioxide (PCO(2)) and room air transcutaneous hemoglobin saturation (RA SAT) at 36 weeks' postmenstrual age (PMA) in infants born with weight between 501 and 1250 g.

STUDY DESIGN

Multicenter, prospective investigation with primary data collection within 72 h of 36 weeks PMA or discharge, whichever first. PCO(2) and RA SAT determinations were done at rest on infants not requiring mechanical ventilation or nasal continuous positive airway pressure (NCPAP).

RESULT

A total of 220 infants were enrolled (mean gestational age 27.7 weeks, mean birthweight 951 g). In infants with traditionally defined chronic lung disease (CLD) compared to those without CLD, the mean PCO(2) was significantly higher (54 versus 45 mm Hg) and the median RA SAT significantly lower (<80 versus 97%). In infants with the new classification of bronchopulmonary dysplasia (BPD), there was a significant linear trend toward increasing PCO(2) with increasing severity of BPD (45, 47, 54 and 62 mm Hg in No, Mild, Moderate and Severe BPD). There was a significant linear trend toward decreasing RA SAT with increasing severity of BPD (97, 95 <80, <80% in No, Mild, Moderate and Severe BPD).

CONCLUSION

Defining CLD as BPD based upon a RA SAT test is a more discriminate, objective method to categorize lung injury. PCO(2) is an objective measure of lung function that inversely correlates with RA SAT. These determinations done together at 36 weeks PMA may provide more precise and accurate estimates of lung injury that might allow for better understanding of pulmonary therapies and clearer comparison of BPD rates and severities among NICUs.

Guide to capillary heelstick blood sampling in infants

Capillary blood sampling is an essential method of blood collection performed by nurses of all skill levels to obtain samples for routine laboratory tests in neonates. Accuracy of results depends on proper heelstick and sample collection technique. Recent advances including development of devices designed specifically for heelstick capillary blood sampling and research into expanded safe heel capillary sampling sites are discussed. A step-by-step guide to capillary blood sampling is outlined along with evidence-based practice incorporating neonatal-appropriate disinfection and nonpharmacological analgesia that contribute to improved infant safety and comfort during and after the procedure.

Prediction of seizures in asphyxiated neonates: correlation with continuous video-electroencephalographic monitoring

BACKGROUND

After perinatal asphyxia, predicting which infants will develop significant hypoxic-ischemic encephalopathy and neonatal seizures remains a difficult task. High-risk markers (Apgar score, acidosis, nucleated red blood cells, and resuscitation) have been used to predict neonatal seizures with varying success. The "3 strikes" of Apgar score of <5 at 5 minutes, pH <7.00, and need for intubation have been cited as having a positive predictive value of 80%. We examined whether the predictive values of these markers would be increased if early continuous electroencephalographic monitoring allowed us to accurately identify all neonatal seizures and to grade the encephalopathy.

METHOD

We recruited term infants with perinatal asphyxia. Continuous video electroencephalography was commenced soon after birth and continued for 24 to 72 hours. The abilities of high-risk markers to predict electroencephalographic seizures, background electroencephalographic activity, and Sarnat grade were examined.

RESULTS

Forty-nine infants were suitable for analysis. Electrographic seizures occurred in 11 of the 49 infants. Encephalopathy was scored by using Sarnat grade (6, severe; 18, moderate; 25, mild) and electroencephalographic findings (4 inactive, 4 major abnormalities, 16 moderate abnormalities, and 25 normal/mildly abnormal). Apgar score of <5 at 5 minutes, pH <7.0, and the need for intubation had positive predictive values for neonatal seizures of 18%, 16%, and 21%, respectively. Combining these markers gave a positive predictive value of 25% and a negative predictive value of 77%. Substituting base deficit or lactate for pH in the 3-strikes model did not improve its predictive value. Apgar score of <5 at 5 minutes, nucleated red blood cells, and a base deficit less than -15 mEq/L showed some association with Sarnat grade. Only 5-minute Apgar score was significantly associated with both Sarnat grade and electroencephalographic grade.

CONCLUSION

After perinatal asphyxia, neither the condition at birth nor the degree of metabolic acidosis reliably predict neonatal seizures.

A Scottish study of heel-prick blood sampling in newborn babies

OBJECTIVE

to conduct a randomised-controlled trial to investigate the effectiveness of two heel-prick devices (Tenderfoot and Genie Lancet) used in the newborn-baby screening test.

DESIGN

a randomised-controlled trial.

PARTICIPANTS AND SETTING

the homes of 340 healthy term newborn babies discharged from the maternity unit of Stirling Royal Infirmary, Scotland. Data were collected between April and November 2003.

INTERVENTIONS

babies were randomly allocated to be tested with either the Tenderfoot or Genie Lancet heel-prick device.

MEASUREMENTS

primary study outcomes include (1) quality of the blood sample; (2) time taken to collect the sample; (3) number of heel pricks required to take the sample; (4) whether squeezing of heel was required; (5) pain expressed by the baby; and (6) presence of bruising. A potential intervening variable was the experience of the midwife.

FINDINGS

on all outcomes, the Tenderfoot device was more effective than the Genie Lancet. Experienced midwives were more efficient in sample collection.

KEY CONCLUSIONS

this study shows that the Tenderfoot device saves significant time for midwifery staff, improves baby care and reduces the need for more than one heel prick at each test, making it superior to the Genie Lancet device.

IMPLICATIONS FOR PRACTICE

there is a case to be made for midwives to be issued with the Tenderfoot device for neonatal screening.

Neonate capillary blood gas reference values

OBJECTIVES

Because biological data are instrument-dependent and because technology has evolved over the last two decades, the published capillary blood reference values for blood gases, lactate, ionized calcium (iCa) and glucose may not reflect the present day situation. Hence, we report such values for healthy term neonates at 48 +/- 12 h of life.

DESIGN AND METHODS

The Institution Ethics Review Board for Research on Human Subjects has accepted the protocol. Extra blood sample was obtained at the time heel-pricks were performed in the frame of the Quebec genetic screening program. One hundred twenty-six term neonates (39.6 +/- 1.2 weeks of gestation) were included in the study. pH, pO2, pCO2, lactate, ionized calcium and glucose were simultaneously measured with selective electrodes on the ABL 735 blood gas analyzer (Radiometer).

RESULTS

All variables exhibited a Gaussian distribution. Since there was no gender effect, all data were pooled.

Randomized trial of liberal versus restrictive guidelines for red blood cell transfusion in preterm infants

OBJECTIVE

Although many centers have introduced more restrictive transfusion policies for preterm infants in recent years, the benefits and adverse consequences of allowing lower hematocrit levels have not been systematically evaluated. The objective of this study was to determine if restrictive guidelines for red blood cell (RBC) transfusions for preterm infants can reduce the number of transfusions without adverse consequences.

DESIGN, SETTING, AND PATIENTS

We enrolled 100 hospitalized preterm infants with birth weights of 500 to 1300 g into a randomized clinical trial comparing 2 levels of hematocrit threshold for RBC transfusion.

INTERVENTION

The infants were assigned randomly to either the liberal- or the restrictive-transfusion group. For each group, transfusions were given only when the hematocrit level fell below the assigned value. In each group, the transfusion threshold levels decreased with improving clinical status.

MAIN OUTCOME MEASURES

We recorded the number of transfusions, the number of donor exposures, and various clinical and physiologic outcomes.

RESULTS

Infants in the liberal-transfusion group received more RBC transfusions (5.2 +/- 4.5 [mean +/- SD] vs 3.3 +/- 2.9 in the restrictive-transfusion group). However, the number of donors to whom the infants were exposed was not significantly different (2.8 +/- 2.5 vs 2.2 +/- 2.0). There was no difference between the groups in the percentage of infants who avoided transfusions altogether (12% in the liberal-transfusion group versus 10% in the restrictive-transfusion group). Infants in the restrictive-transfusion group were more likely to have intraparenchymal brain hemorrhage or periventricular leukomalacia, and they had more frequent episodes of apnea, including both mild and severe episodes.

CONCLUSIONS

Although both transfusion programs were well tolerated, our finding of more frequent major adverse neurologic events in the restrictive RBC-transfusion group suggests that the practice of restrictive transfusions may be harmful to preterm infants.

Comparison of capillary blood sampling using an automated incision device with and without warming the heel

OBJECTIVE

To determine if heel warming prior to heelstick increases the volume of blood collected compared to no warming.

STUDY DESIGN

Randomized controlled trial involving 100 preterm and term infants requiring capillary blood sampling (CBS), randomized to receive warming (Control) (n=50) or nonwarming (Experimental) (n=50).

RESULTS

This sample of infants showed no benefit of warming the heel prior to CBS. Volume of blood, collection time, crying time, and repeat procedures were not different between groups. More infants in the Control group received squeezing during the procedure compared to the Experimental group.

CONCLUSION

Heel warming prior to CBS may be an unnecessary technique in preterm and term infants that expends nursing time and hospital financial cost.

Hematocrit correlates well with circulating red blood cell volume in very low birth weight infants

Although circulating red blood cell (RBC) volume is a better measure of total body oxygen delivering capacity than hematocrit (HCT), circulating RBC volume is more difficult to measure. Thus, the HCT is often used in RBC transfusion decisions. However, several previous studies of low birth weight infants have reported that the correlation between HCT and circulating RBC volume is poor. Using a robust nonradioactive method based on in vivo dilution of biotinylated RBC enumerated by flow cytometry, the present study reexamined the correlation between HCT and circulating RBC volume in very low birth weight infants. Venous and capillary HCT levels were compared with circulating RBC volume measured using the biotin method. Twenty-six stable very low birth weight infants with birth weights less than 1300 g were studied on 43 occasions between 7 and 79 d of life. Venous HCT values correlated highly with circulating RBC volume (r = 0.907; p < 0.0001). However, the mean 95% confidence limits for prediction of circulating RBC volume from venous HCT (the average error of prediction) was +/-13.4 mL/kg. The correlation between HCT and circulating RBC volume is strong in older stable very low birth weight infants. However, clinically important uncertainty exists in estimating circulating RBC volume and the associated RBC transfusion needs of an individual infant based on venous HCT. Because direct measurement of circulating RBC volume is not yet practical, the HCT (or the blood Hb concentration) remains the best available indirect indicator.