A PROSPECTIVE STUDY OF THROMBOELASTOGRAPHY (TEG) AND FILTER LIFE DURING CONTINUOUS VENO-VENOUS HEMOFILTRATION

Abstract

Anticoagulants are commonly used to prolong circuit life during continuous hemofiltration. However, a clear correlation between routinely performed blood coagulability tests and circuit life has not been demonstrated. This lack of correlation may derive from the limited ability of such tests to describe the likelihood of in vivo clotting. We hypothesized that thromboelastography (TEG), which derives its variables from a closer reproduction of in vivo coagulation, would significantly correlate with filter life. Accordingly, we conducted a prospective pilot study of the correlation between filter life and TEG-derived variables in 21 hemofilters used in 6 critically ill patients admitted to a tertiary intensive care unit. It involved the performance of TEG during steady state anticoagulation, measurement of circuit life, and of routine coagulation variables. The results showed that the mean circuit life was 20.7 ± 4.0 h despite an average aPTT of 67.7 ± 12.8 s and a mean heparin dose of 472.5 ± 96.2 IU / h. The mean INR was 1.4 ± 1 and the mean platelet count was 118 ± 16 × 10³ / mm³. Although several TEG variables correlated with heparin dose (p < 0.03), no correlation was found between any of the routine coagulation variables or any of the TEG variables and circuit life. In conclusion, no significant correlation between TEG derived variables or routinely measured coagulation variables and circuit life could be demonstrated. These findings suggest that such tests are not useful indicators of circuit anticoagulation adequacy and that factors other than blood coagulability may play a role in circuit failure.

INTRODUCTION

Anticoagulation is commonly used in continuous renal replacement therapies (CRRT) to maintain circuit patency and function [1-4] The adequacy of such anticoagulation is typically monitored by means of routine measurements of blood coagulability (INR, aPTT, ACT and platelet count) [[4]]. Despite this clinical approach, there is little evidence of a correlation between the intensity of conventional heparin-based anticoagulation and circuit life. Also, there is no evidence of a correlation between the values of routinely monitored tests of coagulability and filter life.

Thromboelastography (TEG) is a global measure of clot formation [[5]]. While standard clotting indices such as ACT, aPTT, INR and platelet count measure discrete components of the hemostatic system, TEG provides data on the dynamic interplay of these factors [5-6]. Its major clinical applications have been in th emonitoring of coagulation during liver transplantation [[6]] and cardiopulmonary bypass (CPB) [7-9]. Such usefulness during CPB suggests that TEG may be helpful in predicting the duration of function and adequacy of anticoagulation in other extracorporeal circuits such as CRRT. TEG-derived variables, therefore, may correlate with circuit life and prove clinically useful in predicting which filters are likely to clot prematurely. Accordingly, we conducted a prospective pilot study of the correlation between TEG-derived variables, routinely measured coagulation tests, and circuit life in a cohort of critically ill patients requiring CRRT.

MATERIALS AND METHODS

We studied twenty-one filters in six critically ill patients receiving CRRT. In all patients, CRRT was provided in the form of continuous veno-venous hemofiltration (CVVH). The BMM10-1 blood monitor (Gambro, Lund, Sweden) was used to maintain blood flow at 200 mL/min through the filter using an AN69 hemofilter (Hospal, Lyon, France) and pump-controlled ultrafiltration (UF) at 33 ml/min. Fluid replacement was delivered in the pre-dilution mode. CVVH was otherwise performed in accordance with a previously described protocol [10-11]. Circuit anticoagulation was achieved using a standardized approach with dilute heparin (10,000 IU / L saline) with dosage selected according to clinical judgement or by means of continuous prostacyclin infusion with dose selection on the basis of clinical judgement.

Cessation of circuit life was defined as the inability to further operate the circuit due to the presence of clot in the filter or in the air-trap or elsewhere in the circuit.

The apparatus for performing TEG is made of two mechanical parts: a heated (37°C) cuvette, which is oscillated and a pin suspended freely from a torsion wire. Freshly drawn blood (0.35 mL) is placed in the cuvette. While the sample remains in the liquid phase, the motion of the cuvette does not affect the pin. Once the clot starts to form however, the fibrin strands “couple” the motion of the cuvette to that of the pin, thus transmitting the shear modulus and elasticity of the clot through the pin. Such shear effect is then amplified to yield the TEG trace. This trace is simultaneously recorded on heat sensitive paper (moving at a rate of 2 mm/min). Various TEG parameters (r, K, ANG, MA, Clot lysis at 30 minutes and 60 minutes were measured and studied. TEG blood sampling was performed randomly on 20 consecutive CVVH circuits during steady state anticoagulation (4–8 hours into filter function).

The following TEG variables were used for the purpose of the study:

r (reaction time)::	=	This is the time elapsed from the point when the sample is placed in the cuvette until TEG tracing amplitude reaches 2 mm. It represents the rate of initial fibrin formation, which is dependent on plasma clotting factors and circulating inhibitor activity. Prolongation of the r time suggests coagulation factor deficiencies, anticoagulation or severe hypofibrinogenemia. In contrast, a small r value may represent a hypercoagulable state. The normal range is 6 to 8 minutes (15 to 30 mm).
K (clot formation time)::	=	This variable is measured from r time to when the TEG amplitude tracing reaches 20 mm. It is the time for a fixed degree of viscoelasticity to be achieved by the forming clot resulting from fibrin build up and cross-linkage. This is effected by the activity of clotting factors, platelets and fibrinogen. The normal range is 3 to 6 minutes (6 to 12 mm).
Alpha angle (ANG)::	=	This is the angle formed by the slope of the TEG tracing from the r to the K value. It denotes the speed of clot formation. Decreased values may indicate hypofibrinogenemia or thrombocytopenia. The normal range is 50 to 60°.
MA (maximum amplitude)::	=	This is the greatest amplitude on the TEG trace and is a reflection of the absolute strength of the fibrin clot. It is a direct function of the maximum dynamic properties of fibrin and platelets. Both quantitative and qualitative platelet defects affect the value of the MA. The normal range is 50 to 60 mm.
A60::	=	This is the amplitude of the TEG tracing 60 minutes after MA is achieved, It measures clot lysis or retraction. The normal range is MA-5 mm. CLI (Clot Lysis Index): It is derived from the formula [A60 / MA × 100] %. It is a measure of the amplitude as a function of time and reflects loss of clot integrity through lysis. The normal range is > 85 %.
Clot Lys 30 and 60::	=	This is a measure of clot breakdown at 30 and 60 minutes. This variable is also a measurement of thrombolytic activity.

Routine tests of coagulability were also performed including (INR, aPTT, and platelet count). The duration of filter life was recorded.

Statistical Analysis. Summary descriptive statistics of all measured variables were derived and analysis was performed with a proprietary statistical software package (StatView™ Abacus Concepts Inc, Berkeley, CA). Correlations were tested for using Spearman's correlation test due to the non-parametric nature of data. A p value < 0.05 was considered statistically significant.

RESULTS

A total of 21 circuits were studied in six consecutive critically ill patients. The distribution of circuit life in these patients is presented in Figure 1. The degree of anticoagulation applied to maintain filter patency was variable and dictated by clinical concerns regarding the risk of bleeding. The type and dose of anticoagulants used are summarized in Table 1. Mean hemofilter lifespan was 20.7 ±,4.0 hours. The mean aPTT was 67.7 ± 12.8 seconds achieved with a mean heparin dose of 472.5 ± 96.2 IU / h. However the aPTT value had no correlation with circuit life (p = 0.463). The mean INR was 1.4 ± 0.1 and also had no correlation with circuit life. Patients were anemic with a mean hemoglobin of 96 ± 3 g/L and relatively thrombocytopenic, with a mean platelet count of 118 ± 15.8 10³ / mm³. In fact, of the 21 circuits studied, 11 were used in patients with a platelet count < 100 000 / mm³. The platelet count, however, did not correlate with circuit life (p = 0.249). On the other hand, when circuits with a clinically acceptable life span (arbitrarily taken to be equal to or more than 15 hours) were compared to those that failed prematurely, there was a difference in platelet counts with the count being lower for circuits with an acceptable life span (p = 0.046).

Figure 1. Histogram showing the distribution of circuit life span.

Table 1. Values of standard anticoagulation parameters, type and dose of anticoagulants used

Patient/Circuit	I.N.R.	aPTT (seconds)	Platelet (× 10^3/mm^3)	Haematocrit (%)	Heparin (IU/h)
1	1.3	83	76	27	700
2	1.1	45	226	29	700
3	1.3	60	86	27	700
4	1.3	66	160	26	700
5	1.2	76	60	28	700
6	1.1	68	107	28	700
7	1	46	230	23	600
8	1.3	38	40	28	nil
9	2.2	58	48	31	nil
10	39	204	27	nil	nil
11	1.7	58	141	26	nil
12	1.4	40	48	29	250
13	2.2	105	71	28	Prostacyclin
14	2	58	64	29	Prostacyclin
15	1.1	35	47	27	700
16	1	44	53	30	1500
17	1.1	43	92	27	800
18	1.2	47	130	32	800
19	1.1	44	106	29	1000
20	1.1	76	277	31	300
21	2.4	300	153	40	nil

None of the TEG derived parameters (TEG index, r, K, r + K, MA, alpha angle and clot lysis) were significantly correlated with circuit life. On the other hand, the dose of heparin was significantly correlated with several of the TEG parameters (alpha angle: p = 0.013, K: p = 0.06, r: 0.03, MA: 0.002) (Table 2) as a reflection of its anticoagulant activity and the detection of such anticoagulant activity by the test.

Table 2. Correlation between heparin dose and TEG or coagulation variables

Correlation	p value
Heparin dose & Circuit Life	0.02
Clot Lysis Time @ 30 min & Circuit Life	0.05
Clot Lysis Time @ 30 min & Heparin Dose	0.001 ± inverse)
Clot Lysis Time @ 30 min & aPTT	0.35
Clot Lysis Time @ 60 min & INR	0.019
Clot Lysis Time @ 60 min & Platelet Count	0.016 ± invese)
Clot Lysis Time @ 60 min & Heparin Dose	0.022 ± inverse
ANG & Heparin Dose	0.013
K & Heparin Dose	0.006
r & Heparin Dose	0.03
MA & Platelet Count	0.002
Heparin Dose & INR	0.0001 ± inverse)

DISCUSSION

Anticoagulation during CVVH can be achieved through the use of heparins, heparinoids, prostacyclin, citrate and nafamostat mesilate [12-13]. In the clinical environment, the adequacy of such anticoagulation is usually monitored by means of routinely available tests of blood coagulability. Such tests typically include aPTT, INR and platelet count. Clinical experience, however, has shown that these tests are poor predictors of circuit life [14-15]. In particular, there has never been a demonstration that their prolongation results in an increased duration of circuit function. Furthermore, when heparin is used, little data support a correlation between the dose of heparin and the duration of circuit function. The lack of correlation between routinely performed tests of blood coagulability and circuit life has a number of potential explanations. One of them is that these tests are inaccurate descriptors of in vivo coagulability. If this explanation accounted for their inability to predict circuit life, a test or tests that more closely approximated in vivo coagulation may prove superior in predicting and correlating with the duration of circuit function during CRRT. One such test is thromboelastography (TEG). It has the theoretical advantage of being a global and dynamic measure of clot formation [[6]]. In fact, while standard clotting indices such as ACT, aPTT, INR and platelet count measure single aspects of the clotting cascade, TEG provides data on the way different components of the cascade interact to produce clot formation and subsequent clot lysis [[17]]. Furthermore, TEG has proven useful in other fields of medicine involving extracorporeal circulations such as cardiopulmonary bypass (CPB) [[18]]. Such application during CPB suggests that TEG may also be useful in assessing adequacy of anticoagulation during CRRT. TEG-derived variables, therefore, may correlate with circuit life and prove clinically useful in predicting which filters are likely to clot prematurely. Such information would then allow early intervention to prevent unexpected circuit clotting and loss of patient blood due to inability to return it electively. To test whether TEG would provide a clinically relevant advantage over other routinely performed coagulation tests, we conducted a prospective pilot study of the correlation between TEG-derived variables, routinely measured coagulation tests and filter life in a cohort of critically ill patients requiring CRRT.

We studied twenty-one filters in six critically ill patients receiving CVVH. Circuit anticoagulation was achieved using a standardized approach and cessation of circuit life was also defined in a standard way so that confounding variables would not affect our findings. Blood was randomly sampled from the 21 circuits during steady state anticoagulation to determine the standard indices as well as the TEG parameters. This form of sampling sought to ensure that findings would be representative of the level of anticoagulation for a given circuit. Despite these precautions, commonly used coagulation tests aPTT, platelet count and INR as well as TEG-derived parameters (TEG Index, r, k, MA, ANG and clot lysis at 30 and 60 minutes) could not be correlated with circuit life. The only significant finding was obtained comparing platelet counts for circuits with an arbitrarily defined clinically acceptable lifespan (> 15 hours) to circuits with a shorter (< 15 hours) duration of function. The filters with a longer duration of function were operated during periods of significantly lower platelet counts.

Therefore, TEG, like other routine tests of anticoagulation adequacy failed to provide useful information regarding filter life. Only the presence of a low platelet count (<100×10³/mm³) may be seen as a weak marker of greater filter longevity. Our findings are in keeping with existing literature [[19]]. They highlight the very limited usefulness of monitoring of routine coagulation tests as a measure of adequacy of anticoagulation for the prevention of filter clotting. They also support the view that TEG provides no additional clinically useful information. The findings of our study, in conjunction with previous literature support the view that monitoring of anticoagulation adequacy by either aPTT or INR or TEG can not provide the clinician with an accurate measure of the adequacy of circuit anticoagulation and is, therefore, not indicated for such purposes. Such monitoring, however, may be useful to ensure the safety of a given anticoagulation regimen.

There are several possible explanations for this failure of anticoagulation monitoring to provide clinically useful information regarding expected CRRT circuit life. One possible explanation is that such tests are all inaccurate representations of the complex extracorporeal circuit-hemostatic pathways interaction [[19]]. Another explanation may derive from the fact that the above tests only partly assess the fibrinolytic system [[20]]. It is likely that this system is operative at variable levels in these critically ill patients and that its activation or dysfunction participate in the pathogenesis of circuit failure [[20]]. Platelet counts provide no information on platelet function. In critically ill patients there are variable levels of platelet activation or dysfunction, which may also affect filter life. Other proteins involved in the regulation of coagulation such as activated protein C are also deficient in critical illness [[20]] making the laboratory assessment of in vivo coagulability even more difficult. Furthermore, many patients with multi-organ failure have deficiencies of antithrombin III, the co-factor for heparin [[21]]. In these patients, therefore, the dose of heparin is unlikely to provide a guide to the likelihood of filter clotting.

Another explanation may be due to the fact that circuit lifespan is dependent on more than just the interplay of the membrane and tubing with the coagulation and fibrinolytic systems. Hydraulic, rheological and other physicochemical factors [[22]] within the hemofilter fibres could be equally important. Also vascular access dysfunction with fluctuations in the actual blood flow delivered to the filter [[23]], inducing stasis within the circuit or filter may be extremely important in the pathogenesis of circuit clotting. Such mechanical factors may overwhelm even the most effective circuit anticoagulation. The interface between air and blood in the venous bubble trap of the circuit may be extremely thrombogenic and also overcome the effect of heparin on the coagulation cascade. Close to 20% of our circuits fail because of clot in the venous chamber.

This pilot study has several limitations. Firstly, the sample size was small. However, [[21]] consecutive filters were studied giving us the opportunity to detect even a moderate correlation between the tests performed and circuit life. Secondly, there was no control group to demonstrate whether, in the absence of any monitoring of coagulation, circuit life would be affected. Thirdly, more sophisticated tests of coagulation or fibrinolysis could not be performed to study the link between in vitro and in vivo clotting. However, the goal of our investigation was to establish whether routine tests or TEG had any predictive ability or clinical usefulness in the monitoring of adequacy of anticoagulation during CRRT. Finally, we could not make any statements about the usefulness of monitoring of coagulation tests in predicting the risk of bleeding because of the small sample of patients and because this was not the intent of the study. None of the patients studied experienced clinically detectable bleeding.

In conclusion, this study showed that standard coagulation indices do not correlate with circuit life span. Heparin dose per se also does not correlate with circuit life. The additional use of TEG confers no advantage to the clinician seeking to monitor the adequacy of anticoagulation during CRRT. Filter clotting remains an unpredictable event and the adequacy of prophylactic anticoagulation cannot be established by readily available tests of coagulation. Accordingly, measurement of aPTT, INR and TEG derived variables, although potentially useful in determining the safety of a given regimen of anticoagulation, does not appear useful in determining its efficacy as antithrombotic prophylaxis during CRRT.

ACKNOWLEDGMENT

We wish to express our sincere thanks and gratitude to all the nurses of the Department of Intensive Care, Austin and Repatriation Medical Centre, Heidelberg, Melbourne, Victoria, Australia, without whose help this project would not have been possible.