Flushing and locking of intravenous catheters are thought to be essential in the prevention of occlusion. The clinical sign of an occlusion is catheter malfunction and flushing is strongly recommended to ensure a well-functioning catheter. Therefore fluid dynamics, flushing techniques, and sufficient flushing volumes are important matters in adequate flushing in all catheter types. If a catheter is not in use, it is locked. For years, it has been thought that the catheter has to be filled with an anticoagulant to prevent catheter occlusion. Heparin has played a key role in locking venous catheters. However, the high number of risks associated with heparin forces us to look for alternatives. A long time ago, 0.9% sodium chloride was already introduced as locking solution in peripheral cannulas. More recently, a 0.9% sodium chloride lock has also been investigated in other types of catheters. Thrombolytic agents have also been studied as a locking solution because their antithrombotic effect was suggested as superior to heparin. Other catheter lock solutions focus on the anti-infective properties of the locks such as antibiotics and chelating agents. Still, the most effective locking solution will depend on the catheter type and the patient’s condition.
Flushing and locking have been strongly associated with the prevention of catheter occlusion. The causes of catheter occlusion might be thrombotic, related to drug or parenteral nutrition (PN) precipitates or mechanical. Thrombotic obstruction is caused by an intraluminal clot or a catheter tip thrombus. Precipitates might be formed by drug mixtures with an extreme pH, calcium phosphate crystals, or lipid deposits. Examples of mechanical obstruction are sleeve formation resulting in partial or total embedding of the catheter tip, a catheter tip abutting the vein wall, a pinch off, a kinked or twisted catheter or tubing, tight sutures, or an incorrect Huber needle placement [
Visible adhesions to the catheter wall.
Build-up of deposits of fibrin and/or infusion fluids and/or drug precipitates.
The aim of this paper is to clarify issues related to flushing and locking and to describe the available evidence relating to the benefits of interventions in relation to occlusion. All types of intravenous (IV) catheters are considered apart from apheresis and haemodialysis catheters and catheters in neonates due to the specific context of these devices.
In this context of rinsing the catheter, flushing of an IV catheter is defined as a manual injection of 0.9% sodium chloride or so called normal saline (NS) in order to clean the catheter. Locking is defined as the injection of a limited volume of a liquid following the catheter flush, for the period of time when the catheter is not used, to prevent intraluminal clot formation and/or catheter colonization. Traditionally, an anticoagulant, such as diluted heparin, is used. Generally, flushing and locking are described ambiguously in guidelines and in the scientific literature which leads to confusion and misunderstanding. Moreover, flushing and locking are terms that are mutually exchanged [
Important aspects related to flushing are syringe diameter and injection flow dynamics. Traditionally, syringes with at least a diameter of 10 mL are recommended for long-term central venous catheters. However, this issue arises only when force applied meets resistance. Flushing with a small syringe diameter or with high force applied to the plunger in cases of resistance increases the risk of catheter damage [
The dynamic of the injection flow plays a pivotal role in adequate flushing. Vigier and colleagues showed in a qualitative
An adequate flush volume is needed to be able to remove debris and fibrin deposits in the catheter and port reservoir. Recommendations state the following: “use at least twice the volume of the catheter and add-on devices” [
Flushing the catheter is the most important factor in preventing malfunction by maintaining catheter patency. The fact that fibrin and other deposits are impeded in attaching to the intraluminal catheter wall is paramount. Therefore a major recommendation is to flush before and after administration of medication, also known as the SAS acronym. The order of IV injections is as follows: a normal saline flush (S), followed by the administration (A) of drugs or fluids, followed by a normal saline flush (S). The use of the similar sequence is even more important for blood sampling procedures due to the viscous nature of blood: SBS, a normal saline flush (S), followed by the blood sampling (B), followed by a normal saline flush (S). If the procedure ends with a heparin (H) lock the acronym is SASH and SBSH. The first NS flush provides a clean intraluminal surface which precludes attachment of drug deposits or fibrin. The flush at the end of the IV administration or blood sampling procedure prevents accumulation by intraluminal drug deposits or fibrin and a clean surface impedes attachment from microorganisms to the inner wall. A 10 mL flushing volume after blood sampling is appropriate because fibrin contact with the catheter wall is limited to some minutes. In contrast, after a blood transfusion a flush of 20 mL is required because fibrin might deposit to the catheter wall during a prolonged time. Similarly, accidental blood reflux into the catheter and infusion line, for example, when a infusion bag is empty, requires a manual flush of at least 10 mL of NS.
Flushing recommendations that are based on research and insights are summarized in Table
Flushing and locking recommendations.
Flushing recommendations | |
|
|
Technique | Use a pulsatile flow when flushing |
Use a flush with 10 × 1 mL boluses with a time interval of 0.4 s between 2 boluses | |
Use SAS and SBS order for the administration of mediation/fluids and blood sampling procedures | |
|
|
Volume | Use a 10 mL flush for all IV catheters (except for peripheral cannulas, use 5 mL) |
Use a 20 mL flush after infusion of viscous products like blood components, parenteral nutrition, and contrast media | |
|
|
Regimen | Flush with NS before and after administration of drugs of fluids (SAS) |
Flush with NS before and after blood sampling (SBS) | |
|
|
Locking recommendations | |
|
|
Technique | Use the positive pressure technique when disconnecting a syringe |
Close clamps and let them closed when not in use | |
|
|
Volume | 1.0 mL for peripheral cannulas |
1.5 mL for midlines, PICCs, nontunnelled CVCs, and small bore tunnelled catheters (≤1 mm ID) | |
2.5 mL for large bore tunnelled catheters (>1 mm ID) and TIVADs (reservoir volume up to 0.6 mL, Huber needle volume not included) | |
|
|
Regimen | q8h–q24h for short-term catheters |
Weekly in long-term catheters | |
q6w–q8w in TIVADs |
The goal of an adequate catheter lock is prevention of premature termination of catheter function by maintaining patency when the catheter is not in use. The optimal lock solution prevents clot formation in the catheter and at the catheter tip, and also prevents microorganism adhesion and biofilm formation.
As far back as in 1987, Shearer suggested using the positive pressure technique to prevent backflow of blood into the catheter. This technique was defined as withdrawing the syringe from the injection site while still exerting pressure on the syringe plunger when injecting the last 0.5 mL [
Although the idea of preventing blood influx at the catheter tip by the positive pressure technique is reasonable, some issues arise. This technique prevents only blood influx at the moment of locking of the catheter. Once the syringe is removed, other effects might influence the internal volume such as the clamp that might be opened and closed or external catheter parts that might be pinched. This phenomenon causes a push out of locking solution and once the pressure of the pinching/clamping is lifted, the same volume that has been pushed out will create a backflow of blood at the catheter tip by negative pressure. From
The locking volume must be sufficient to fill the entire catheter. Therefore the volume of add-ons might be added to the priming volume of the catheter. Internal catheter volumes are relatively small: approximately 0.03 mL for a peripheral catheter, 0.4 mL for a 4 Fr midline, 0.6 mL for a 4 Fr single lumen nontunnelled central venous catheter (CVC), 0.7 mL for a 4 Fr PICC, 0.7 mL, and 1.5 mL for a small and large bore tunnelled catheter (75 cm), respectively, and 1.3 mL for a TIVAD (large reservoir volume of 0.5 mL), Huber needle with extension set included.
For trimmed catheters with a circular diameter the catheter volume might be calculated easily per cm. The mathematic formula of a cylinder is
Internal volume of single lumen venous catheters in mL.
Catheter length (cm) | Internal diameter (mm) | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1 | 1.1 | 1.2 | 1.3 | 1.4 | 1.5 | 1.6 | |
10 | 0.02 | 0.03 | 0.04 | 0.05 | 0.06 | 0.08 | 0.09 | 0.11 | 0.13 | 0.15 | 0.18 | 0.20 |
15 | 0.03 | 0.04 | 0.06 | 0.08 | 0.10 | 0.12 | 0.13 | 0.17 | 0.20 | 0.23 | 0.26 | 0.30 |
20 | 0.04 | 0.06 | 0.08 | 0.10 | 0.13 | 0.16 | 0.17 | 0.23 | 0.27 | 0.31 | 0.35 | 0.40 |
25 | 0.05 | 0.07 | 0.10 | 0.13 | 0.16 | 0.20 | 0.22 | 0.28 | 0.33 | 0.38 | 0.44 | 0.50 |
30 | 0.06 | 0.08 | 0.12 | 0.15 | 0.19 | 0.24 | 0.26 | 0.34 | 0.40 | 0.46 | 0.53 | 0.60 |
35 | 0.07 | 0.10 | 0.13 | 0.18 | 0.22 | 0.27 | 0.30 | 0.40 | 0.46 | 0.54 | 0.62 | 0.70 |
40 | 0.08 | 0.11 | 0.15 | 0.20 | 0.25 | 0.31 | 0.35 | 0.45 | 0.53 | 0.62 | 0.71 | 0.80 |
45 | 0.09 | 0.13 | 0.17 | 0.23 | 0.29 | 0.35 | 0.39 | 0.51 | 0.60 | 0.69 | 0.79 | 0.90 |
50 | 0.10 | 0.14 | 0.19 | 0.25 | 0.32 | 0.39 | 0.43 | 0.57 | 0.66 | 0.77 | 0.88 | 1.00 |
55 | 0.11 | 0.16 | 0.21 | 0.28 | 0.35 | 0.43 | 0.47 | 0.62 | 0.73 | 0.85 | 0.97 | 1.11 |
60 | 0.12 | 0.17 | 0.23 | 0.30 | 0.38 | 0.47 | 0.52 | 0.68 | 0.80 | 0.92 | 1.06 | 1.21 |
65 | 0.13 | 0.18 | 0.25 | 0.33 | 0.41 | 0.51 | 0.56 | 0.73 | 0.86 | 1.00 | 1.15 | 1.31 |
70 | 0.14 | 0.20 | 0.27 | 0.35 | 0.45 | 0.55 | 0.60 | 0.79 | 0.93 | 1.08 | 1.24 | 1.41 |
75 | 0.15 | 0.21 | 0.29 | 0.38 | 0.48 | 0.59 | 0.65 | 0.85 | 0.99 | 1.15 | 1.32 | 1.51 |
80 | 0.16 | 0.23 | 0.31 | 0.40 | 0.51 | 0.63 | 0.69 | 0.90 | 1.06 | 1.23 | 1.41 | 1.61 |
85 | 0.17 | 0.24 | 0.33 | 0.43 | 0.54 | 0.67 | 0.73 | 0.96 | 1.13 | 1.31 | 1.50 | 1.71 |
90 | 0.18 | 0.25 | 0.35 | 0.45 | 0.57 | 0.71 | 0.78 | 1.02 | 1.19 | 1.38 | 1.59 | 1.81 |
Examples of corresponding internal and outer diameters in different types of single lumen catheters.
Internal diameter in mm | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1 | 1.1 | 1.2 | 1.3 | 1.4 | 1.5 | 1.6 | |
Outer diameter in French | ||||||||||||
Ports, PUR catheters, BBraun | 4.5 | 5 | 6.5 | 8.5 | ||||||||
Ports, Chronoflex CRBard | 6 | 8.5 | ||||||||||
Ports, Silicone catheters, BBraun | 6.5 | 8.5 | 10 | |||||||||
PICC, PowerPICC CRBard | 4 | |||||||||||
Tunneled catheter, Hickman CRBard | 2.7 | 4.2 | 6.6 | 9.6 |
The aim of a lock is to fill the catheter entirely. However the risk of “leakage” of the lock over time has been described and therefore it is suggested that catheters should be overfilled by approximately 15–20%. However, this extra volume can only be recommended if the locking solution does not cause adverse effects when systemically injected [
To avoid confusion and given the available variation in catheter length and diameter, uniform volumes for different catheter types are suggested. Table
Calculation of recommended locking volumes if lock does not cause adverse effects when systemically injected.
Catheter type | Total lock volume in mL | Minimum catheter volume in mL |
Extra volumea |
---|---|---|---|
Peripheral catheters | 1.0 | 0.04 (0.03 + 0.006) | 0.9 |
Midline | 1.5 | 0.5 (0.4 + 0.1) | 1.0 |
PICC | 1.5 | 0.7 (0.6 + 0.1) | 0.8 |
Nontunnelled CVC | 1.5 | 0.7 (0.6 + 0.1) | 0.8 |
Small bore tunnelled catheter (≤1 mm ID) | 1.5 | 0.8 (0.7 + 0.1) | 0.7 |
Large bore tunnelled catheter (>1 mm ID) | 2.5 | 1.6 (1.3 + 0.3) | 0.9 |
TIVADs (reservoir volume up to 0.6 mL) | 2.5 | 1.6 (1.3 + 0.3) | 0.9 |
For most low concentration locking solutions (e.g., a 100 U/mL heparin) the lock solution does not need to be aspirated. When the lock is renewed, the new locking solution may be instilled without aspiration or flushing with NS. Some locking solutions, which might be causing adverse events when injected into the blood circulation, must be first aspirated before renewal for example, a 5000 U/mL heparin lock. Most guidelines recommend a nonspecified “regular” flush regimen. The optimal time between two locking procedures when the catheter is not in use, is not well studied. Commonly a time period between 8 and 24 hours is suggested, although in PICCs and long-term CVCs periods of 1 week or more are also used.
For TIVADs, when accessing the port for the intermittent flushing procedure, it is recommended to flush first with a 10 mL NS, before a heparin lock. If the Huber needle is not correctly located in the reservoir, the paravenous administration of NS, in contrast to heparin, is not harmful. There is also a tendency to prolong the interval between intermittent accesses for TIVAD maintenance from monthly to every 6 to 8 weeks [
Locking recommendations that are based on research and insights are summarized in Table
A heparin lock was discussed back in the 1970s when IV peripheral cannulas were locked as alternative to a continuous heparin infusion to keep the cannula patent [
Discontinuation of heparin as locking solution seems to be attractive because it eliminates the risks associated with heparin while it prompt savings in nursing time, supplies, and costs for the patient and/or the institution and/or the society. Therefore the hypothesis that there is no statistical difference for locking a catheter with heparin or NS has been investigated many times in different types of catheters. A literature review was conducted to investigate level I-II evidence [
RCTs and meta-analyses comparing NS and heparin as locking solution.
Authors, year | Evidence regarding patency with the use of NS versus heparin | Concentration, volume of heparin | Volume of NS | Frequency | Remarks |
---|---|---|---|---|---|
Peripheral cannulas | |||||
|
|||||
Goode et al. |
No statistically significant difference | 2.5, 3.3, 10, 16.5, 50, 100, 132 U/mL |
NR | q8h–q24h | Small number of studies, variation in methodological quality |
|
|||||
Peterson and Kirchhoff |
No statistically significant difference | 1–5 mL, 10 to 100 U/mL | 1–5 mL | q8h, q12h, q24h | Small number of studies, few pediatric studies, variation in methodological quality |
|
|||||
Randolph et al. |
No statistically significant difference | 10 U/mL |
NR | q6h, q8h, q12h | Small number of studies |
Lower patency rate in NS group | 100 U/mL |
NR | q6h, q8h | Small number of studies | |
|
|||||
Gyr et al. 1995 [ |
Lower patency rate in NS group | 10 U/mL |
NR | q1h–q8h | Pediatric population |
|
|||||
LeDuc 1997 [ |
No statistically significant difference | 3 mL 10 U/mL | 3 mL | 0.5 h–24 h | Pediatric population in emergency department setting |
|
|||||
Niesen et al. 2003 [ |
No statistically significant difference | 1 mL 10 U/mL | 1 mL | q12h | Pregnant woman in emergency department setting, limited statistical power |
|
|||||
Mok et al. 2007 [ |
No statistically significant difference | (1) 1 mL 1 U/mL |
1 mL | q6h, q8h | Pediatric population |
|
|||||
White et al. 2011 [ |
No statistically significant difference | 1 mL 10 U/mL | 3 mL | q8h | Pediatric population, small sample size |
|
|||||
Bertolino et al. 2012 [ |
Lower patency rate in NS group | 3 mL 100 U/mL | 3 mL | q12h | Large medical population |
|
|||||
Midlines | |||||
|
|||||
No evidence available | |||||
|
|||||
Nontunneled short-term CVCs | |||||
|
|||||
Rabe et al. 2002 [ |
Lower patency rate in NS group | (1) 0.5 mL 5000 U/mL | (2) 0.5 mL | q48h | (3) Third arm was Vit C 200 mg/mL, 10 mL |
Schallom et al. 2012 [ |
No statistically significant difference | 3 mL 10 U/mL | 10 mL | q8h | ICU and medical ward, limited statistical power, ICU and medical ward |
|
|||||
PICCs | |||||
|
|||||
Bowers et al. 2008 [ |
No statistically significant difference | 5 mL 100 U/mL | 10 mL | q12–24h | A positive displacement connector was used in the 3 groups, small study |
|
|||||
Lyons and Phalen 2014 [ |
No statistically significant difference | (1) 5 mL 10 U/mL |
(3) 10 mL | q12h | A neutral connector was used in the 3 groups, home care setting |
|
|||||
Tunneled catheters | |||||
|
|||||
Smith et al. 1991 [ |
No statistically significant difference | 5 mL 10 U/mL | 9 mL | q12h |
Small sample size, paediatrics, onco-hematology patients |
|
|||||
TIVADs | |||||
|
|||||
Goossens et al. 2013 [ |
No statistically significant difference | 3 mL 100 U/mL | 10 mL | Heparin at discharge or q8w | Onco-hematology patients |
In peripheral cannulas, evidence was found for the discontinuation of the use of heparin locks in two meta-analyses in the early nineties. In these meta-analyses, studies with different heparin concentrations, ranging from 2.5 U/mL to 100 U/mL, are included [
Although the use of neutral and positive displacement connectors implies no heparin lock requirement, two RCTs with PICCs used the locking solution as dependent variable for occlusion rather than the connector. In the first study a positive displacement system was combined with the use of a 10 mL NS lock versus 5 mL of heparin (100 U/mL) [
In tunnelled catheters, one RCT with a small sample size found no difference in nonpatency between a twice daily flush with 5 mL heparin (10 U/mL) versus a weekly flush of 9 mL NS [
We can conclude that the use of a heparin lock at a concentration of 10 U/mL does not have any added value over the use of a NS lock in peripheral cannulas. The available scientific evidence regarding the efficacy of NS versus heparin (100 U/mL) locking in all types of catheters is weak due to the limited available methodological rigorous studies.
The use of the positive pressure technique might avoid blood influx at the catheter tip when disconnecting a syringe. This procedure is strongly associated with the knowledge and skills of the healthcare worker. To overcome this problem, supporting technologies such as valves incorporated in the catheter tip (e.g., Groshong, C.R. Bard) or at the catheter hub (e.g., PASV Technology, Navelyst Medical) have been developed. The integrated valves in PICCs, tunnelled, and port catheters are designed to avoid blood influx because the opening pressure of the valve is higher than the pressures found in the venous circulation. The valve opens only during positive pressure (injection) or negative pressure (aspiration). Needleless connectors with neutral or positive displacement have also been developed to prevent blood influx at the catheter tip. The need for a heparin lock is eliminated with the use of these valves and connectors. Therefore heparin as locking solution is no longer recommended by the manufacturers of these technologies. Few RCTs with a focus on catheter patency and locking with heparin versus NS with the help of these technologies (valves and connectors) are available. Two RCTs compared valved catheters versus nonvalved catheters. A first study from Hoffer and colleagues found a statistically significant lower occlusion rate in valved PICCs locked with NS versus nonvalved PICCs locked with heparin (10 mL, 10 U/mL) [
Only one RCT investigated a weekly NS lock with a positive displacement connector versus a twice weekly heparin lock with a standard cap in tunnelled catheters in the paediatric onco-hematology population. A lower patency rate was found with a NS lock and positive displacement connector than a heparin lock 200 U/mL (volume not reported) and a standard cap. No difference in total catheter dwell time was found [
Finally, three systematic reviews which included all types of catheters, with or without needleless connectors, valved or nonvalved CVCs are available. Mitchell and colleagues found weak evidence that locking with a heparin solution versus NS reduces the occlusion rate. Due to methodological concerns, no strong conclusions could be drawn [
It is obvious that the available studies included different patient populations, different catheter types with different locking regimens. Moreover different malfunction definitions are used and although all of these studies had a strong methodological design a lot of them ended up with small sample sizes. All these issues might explain why mixed results are found. There is an urgent need for further well-designed studies using uniform terminology and outcome measures to investigate potential differences in malfunction rates between heparin and NS as locking solution for venous catheters. Till then, the choice to abandon heparin as locking solution is more one of weighing up advantages and disadvantages.
Lepidurin is an anticoagulant which acts through direct thrombin inhibition. Only one small study investigated this locking solution versus heparin in IV catheters. A lepidurin (100
Urokinase is a thrombolytic agent and therefore effective in the treatment of thrombotic occlusion. This fibrinolytic drug may also be used in a more prophylactic way. Moreover the use of periodic fibrinolytic therapy was also suggested in the prevention of catheter-related infectious complications [
Due to the number of manipulations over time, long-term venous catheters are prone to breaches in aseptic technique during the manipulation of the catheters. The intraluminal source of infection is associated with more prolonged dwell times [
A meta-analysis of trials in oncology showed weak scientific proof for effectiveness of antibiotic-based lock solutions compared to heparin in preventing CRBSI. However, in the included studies, the investigated antibiotic locks were heterogeneous (vancomycin, amikacin, and ciprofloxacin) and the outcome measurement used was nonspecific (sepsis and noncatheter related sepsis) [
Nonantibiotic locks or antiseptics kill bacteria through physical effects rather than specific biochemical pathways and may not induce microbial resistance [
Ethanol also has the potential to remove established biofilm (bacteria). A systematic review suggested that a prophylactic ethanol lock decreases the rates of infection and unplanned catheter removal and that ethanol lock treatment appears efficacious in combination with systemic antibiotics. However the review was based mainly on retrospective studies [
Taurolidine, a derivative of the amino acid taurine, is an antimicrobial agent showing a broad spectrum of antimicrobial activity against both bacteria and fungi [
Some antimicrobial and antisepticlocks are not always considered as traditional “locks.” They do not fulfill all conditions of the earlier definition that a lock is instilled for the period of time when the catheter is not in use. Antimicrobial and antiseptic locks might dwell for a limited time and a common locking solution, such as heparin, might be utilised in between.
Maintaining patency has always been considered essential for all types of venous catheters. Flushing with NS is important and probably the most crucial factor in the prevention of malfunction. However, evidence on flushing techniques, volumes, and regimens is lacking. Moreover, also the available scientific basis for catheter locking with heparin is weak. Hence, clinical studies with a strong methodological design and a focus on flushing and locking in relation to malfunction are urgently needed. Uniform malfunction definitions, terminology, and measurements should be used.
Meanwhile, more standardised flushing and locking volumes should be used. Flushing volumes should be at least 10 mL in order to rinse the catheter sufficiently. Locking volumes should be minimal and based on the catheter volume. A maximum of 1 mL lock volume surplus is suitable to safely fill the catheter and any add-ons. For peripheral cannulas, a high flushing and locking volume of the catheter is not needed due to the small internal volume of the catheter.
The prevention of CRBSI due to biofilm formation is an increasingly important issue. For long-term CVCs and especially in susceptible patients an antimicrobial or antisepticlock must be considered.
The author declares that there is no conflict of interests regarding the publication of this paper.