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Sonoclot Analysis in Healthy Children

Pivalizza, Evan G. MBChB, FFASA*,; Pivalizza, Penelope J. MD†,; Kee, Spencer MBChB, FRCA*,; Gottschalk, Lewis I. MBChB, FFASA*,; Szmuk, Peter MD*,; Abramson, David C. MBChB, FFASA**

doi: 10.1097/00000539-200104000-00019
Pediatric Anesthesia: Research Report
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Although use of the Sonoclot device (Sienco, Inc., Morrison, CO) has been reported in isolated pediatric cases and in small reports in neonates, there are no published data for normal pediatric patients. As the device is used in situations of abnormal coagulation, such as cardiac and liver transplantation surgery, our aim was to determine normal data ranges in healthy pediatric surgical patients. Blood was withdrawn after anesthetic induction, and the Sonoclot activated clotting time, rate of clot formation, time to peak amplitude, and peak amplitude was compared among four pediatric groups (<12 mo, 13–24 mo, 25–48 mo, 49 mo–9 yr) and an adult group. The Sonoclot activated clotting time in the <12-mo and the Adult groups were shorter than the oldest group of children (P < 0.05), although all were within the anticipated normal range, and there were no significant differences in clot rate, peak amplitude, and time to peak amplitude among groups without apparent trends with increasing age. These Sonoclot variables quantify adequate global clot formation in pediatric patients and will facilitate clinical coagulation management with appropriate pediatric normal ranges, avoiding the application of extrapolated adult data to children.

Department of Anesthesiology, *Medical School, and †School of Public Health, University of Texas Health Science Center, Houston, Texas

Disposables for the Sonoclot device were supplied by Sienco, Inc., Morrison, CO.

Portions of this data were presented in abstract form at the American Society of Anesthesiologist’s Annual Meeting, San Diego, CA, October 1997, and has appeared as an abstract in Anesthesiology 1997: 87;A1037.

December 14, 2000.

Address correspondence and reprint requests to Evan G. Pivalizza, MBChB, FFASA, Department of Anesthesiology, Medical School, University of Texas Health Science Center, MSB 5.020, 6431 Fannin St., Houston, TX, 77030. Address e-mail to Evan.G.Pivalizza@ uth.tmc.edu.

IMPLICATIONS: Sonoclot variables are presented for 95 healthy pediatric surgical patients in four age groups, with small differences found in the Sonoclot (Sienco, Inc., Morrison, CO) activated clotting time between two groups and no significant differences in three other variables among groups.

The Sonoclot analyzer (Sienco, Inc., Morrison, CO) is used as a visco-elastic monitor of whole blood coagulation in a variety of perioperative settings, including cardiac and liver transplantation surgery (1,2), as well as instances of abnormal coagulation (3). The small quantity of blood required for Sonoclot analysis and the ready availability of the test as a point-of-care monitor are attractive features for application in pediatric surgical patients with potentially abnormal coagulation. However, apart from an initial study in neonates (4), there are no published normal range data for children. As differences between age-groups has been demonstrated for the Thrombelastogram® (TEG®) (Haemoscope Corp., Skokie, IL) (5), which is a measure of elasticity and shear stress of whole blood, rather than viscosity, our aim was to determine normal ranges for Sonoclot variables for healthy pediatric surgical patients.

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Methods

After IRB approval and informed, oral parental consent, healthy pediatric patients presenting for superficial or minor surgical procedures were enrolled into this prospective, observational study. Children with known coagulation abnormalities or taking medication that could affect coagulation were excluded. After inhaled anesthetic induction, 1 mL of blood was withdrawn from the IV catheter after the first 1 mL was discarded either by passive drainage or with a two-syringe technique. If excessive force was required to withdraw blood from a small vein or catheter, the patient was excluded from the study. Healthy adult patients having minor surgery, who were without coagulation defect or exposure to drugs modifying coagulation, were also enrolled after informed consent.

The Sonoclot device consists of an open-ended disposable plastic probe, mounted on an ultrasonic transducer, which is immersed in a cuvette containing 1.5 mg of celite per 0.36 mL of blood. The viscous force of the forming clot creates impedance to the vibrating probe, which is converted to an output signal (1) (Fig. 1). Sonoclot analysis for this study was performed within 2 min with 0.36 mL of blood in a standard manner by one of two investigators to ensure consistent performance. Daily quality control of the device was maintained for the duration of the study. The following variables were measured: 1) the Sonoclot activated clotting time (SonACT) (seconds), the time from probe insertion into the cuvette until onset of fibrin formation and the end of the liquid phase (the endpoint for coagulation tests such as the prothrombin and partial thromboplastin times); 2) the clot rate (the gradient of the primary slope measured as a percentage of peak amplitude per unit time), reflecting the rate of fibrin formation and the amount of available fibrinogen; 3) the peak amplitude of the signature (correlating to the maximum impedance to probe vibration, measured in clot signal units), an indicator of functional fibrinogen and platelet function; and 4) the time to peak amplitude (minutes), reflecting clot retraction away from the surface of the probe, an indicator of platelet function. The standard variables are depicted in Figure 2.

Figure 1

Figure 1

Figure 2

Figure 2

Subjects were divided into four age groups: <12 mo, 13–24 mo, 25–48 mo, and 49 mo–9 yr, and were compared with each other and the healthy Adult group. Based on extrapolation from pediatric TEG® variables (5) and our preliminary analysis, power analysis to detect a possible 20% difference among groups in measured Sonoclot variables with an α of 0.05 and a β of 0.1 predicted a minimal sample size of 18–22 in each group for the respective variables. Sonoclot variables were compared among groups by using analysis of variance and subsequent t-test if significance was found. Significance was assumed with P < 0.05.

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Results

Ninety-five children were enrolled (25, 24, 22, and 24 in each age group) with 22 patients in the Adult group. The average age in the infant group was 7.1 ± 3.5 mo, and no neonates were included, with the youngest patient being 9 wk old. Sonoclot variables are shown in Table 1. There were minor statistically significant differences in the SonACT between the <12-mo group and Adult group compared with the oldest pediatric group (P < 0.05). However, nearly all values were within the anticipated manufacturer’s normal range of 115 ± 30 s. There were no significant differences in clot rate (P ≥ 0.5), peak amplitude (P ≥ 0.5), and time to peak amplitude (P ≥ 0.3).

Table 1

Table 1

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Discussion

This study presents the first published Sonoclot data for healthy pediatric surgical patients. Statistically significant prolongation of the SonACT (±13% from baseline) was observed in the oldest pediatric group compared with the youngest pediatric and adult groups. There were no significant differences in the clot rate, peak amplitude, or the time to peak amplitude without obvious trends related to age. The statistical differences in the SonACT do not imply any clinically significant delay in thrombin generation or subsequent fibrin formation, especially in the context of the rest of the variables showing normal rates of fibrinogen to fibrin conversion, and normal platelet function. The SonACT findings are compatible with previous evidence of slight prolongation of bleeding times in children compared with adults (6). These findings will be relevant to physicians who previously had to use adult-derived normal values to make clinical decisions in pediatric patients. Decisions based on this normal pediatric data, with SonACT differences up to 13 seconds, may prevent inappropriate intervention if comparisons are made to shorter adult-derived SonACTs.

The Sonoclot device, a visco-elastic measure of whole blood coagulation is used in cardiac and liver transplant surgery and has been described in other perioperative populations (1,2,7,8). In the pediatric population, there is a report in which the device was used to evaluate plasma exchange in a teenager with meningococcal septicemia (3). Blifeld et al. (4) found a shorter lag time (similar to the SonACT) and shoulder-to-peak interval and a steeper primary slope in 53 neonatal cord blood samples compared with adults, with poorer indices in premature infants (29 weeks gestation). The shoulder-peak interval that was used in this study is not routinely measured now, and confounding variables included sampling from heparin-flushed umbilical catheters in the preterm group and recalcification after citrate storage. The recalcification process may influence results of viscoelastic devices, as a TEG® study of recalcification showed significant differences related to the specific time of citrate storage (9). SonACT and clot rates have also been published for 10 neonates undergoing extracorporeal membrane oxygen: in addition to the potential coagulation abnormalities in this population, recalcification was also used after citrate storage (10). Recalcification was not used in the current pediatric study, and meticulous technique avoided potential acceleration of coagulation as a result of thromboplastin release.

The adequacy of clot formation as measured by the Sonoclot in this study compares favorably with previous findings in neonatal cord blood (10). In addition, a study with the TEG®, a different monitor of whole blood coagulation which measures clot strength and shear stress, also confirmed functional maturity of coagulation in children <two years old with a nonsignificant trend toward hypercoagulable variables in a <6-month age group compared with adults (5). These findings and the present data confirming normal whole blood coagulation in children are useful for clinicians, as isolated laboratory measures of coagulation and coagulation factors reveal consistent age-related changes with continuing maturity of the hematological system for infants (6) and older children (11). The only other study to investigate age-related effects with the Sonoclot found an increased clot rate with increasing age in adult orthopedic patients without pediatric data (12).

In summary, we have provided normal data ranges for four measured Sonoclot variables in healthy children, demonstrating nonage-related statistically significant (although clinically minor) changes in SonACT between two age groups without differences in three other variables. Although the small magnitude of the changes imply less clinical significance given the preserved global coagulation, these changes are relevant for clinicians using the device in situations of abnormal coagulation because clinical management will be facilitated by comparison to pediatric data rather than extrapolation from slightly different adult-derived variables.

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References

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© 2001 International Anesthesia Research Society