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Tight K

Tight K is a study investigating the strategy of controlling potassium levels after cardiac surgery.

Usual care is to maintain potassium levels above 4.5 mEq/L following cardiac surgery. This study will compare usual care against maintaining potassium levels above 3.6 mEq/L.

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Tight K is a pragmatic, multicentre, non-inferiority randomised clinical trial based in the UK. The study will investigate whether a strategy of maintaining potassium levels at ≥3.6 mEq/L is non-inferior to maintaining at ≥4.5 mEq/L on the occurrence of new onset atrial fibrillation after cardiac surgery.

1684 patients who are undergoing isolated CABG surgery will be recruited into the study.

Who we are

The study is managed by the Clinical Trials Unit at the London School of Hygiene & Tropical Medicine, and funded by the British Heart Foundation.

The sponsor of the study is Barts Health NHS Trust. 


All our study documentation can be accessed here if you want to find out more about how to participate in this research.


A list of our publications can be accessed here.

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About Tight K 2 columns
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Tight K is the first appropriately powered non-inferiority multicentre randomised trial of a strategy of 'relaxed' potassium supplementation versus a strategy of 'tight' potassium supplementation, which is defined as standard of care. The routine maintenance of serum [K+] ≥4.5 mEq/L is of unproven efficacy, may be unpleasant or hazardous for patients, and is costly. The findings will have important consequences for patients and clinicians, regardless of whether or not potassium supplementation is found to be non-inferior for the prevention of AF after cardiac surgery. A survey of practice patterns in Europe and North America suggests that there is genuine equipoise, with 67% of caregivers practising in Europe reporting that their institution has a protocol for maintaining high-normal serum potassium levels after cardiac surgery. So one in three do not.

The Tight K Trial will set out to test the hypothesis that AF after cardiac surgery will be no more common after CABG surgery when serum potassium levels are maintained ≥3.6 mEq/L as when they are maintained ≥4.5 mEq/L.

Inclusion criteria

  1. Scheduled to have isolated CABG surgery
  2. Patient in sinus rhythm

Exclusion criteria

  1. Age less than 18 years
  2. Previous history of Atrial Fibrillation, Atrial Flutter and/or Atrial Tachyarrhythmia
  3. Pre-operative high-degree atrioventricular (AV) block (defined as Mobitz type 2 second degree AV block or complete heart block)
  4. Pre-operative serum [K+] greater than 5.5 mEq/L
  5. Current or previous use of medication for the purposes of cardiac rhythm management
  6. Dialysis-dependent end-stage renal failure
  7. Concurrent patient involvement in another clinical study assessing cardiac rhythm post-operative interventions
  8. Unable to provide informed consent

If you are a patient interested in taking part in the study, you can contact the research team at your local hospital to discuss this further.

Who we are
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Ben O’Brien

Prof Diana Elbourne


Professor of Health Care Evaluation

Richard Evans

Senior Manager of the Clinical Trials Unit
Main contact

Kiran Bal

Assistant Trial Manager
For queries about setup

Laura Van Dyck

Data Manager
Queries about data
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Tight K trial protocol

Tight K trial protocol

Tight K patient information sheet

Tight K trial patient information sheet

Tight K trial consent form

Tight K trial consent form

Tight K trial Holter monitor training

Tight K trial Holter monitor training

The video below describes the process for applying the Holter monitors to Tight K trial participants.

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FAQs Tight K 2 columns
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Do we need to do additional potassium level monitoring?

No. Sites do not need to change the frequency of monitoring patient’s potassium levels. On ICU potassium may be measured every 4 hours, but on the wards it is common at many sites to monitor only once a day, and this is also fine. 

Where do I send Holter monitors to for analysis?

Holter monitors are to be sent to the Core Lab at Wythenshawe Hospital in Manchester. Please ensure that you use the pre-paid labels provided, using one label per box. Please do not attach multiple boxes together. 

When should we give potassium if the participant is randomised to the TIGHT arm?

Potassium is only to be given when serum levels are less than 4.5.

When should we give potassium if the participant is randomised to the RELAXED arm?

Potassium is only to be given when serum levels are less than 3.6. The only exception is if the participant has a period of AFACS, as defined in the Tight K protocol, after which treatment can return to local standard of care. 

Does a high serum potassium level prior to surgery automatically exclude a participant from the trial? 

An isolated high pre-operative potassium level does not necessarily exclude a participant. If there is a suspicion that the result may be inaccurate, either because it is an older measurement or an isolated high reading, it is recommended to repeat the measurement. If the repeated measurement is lower than 5.5 the patient would be eligible to be enrolled. 

What about ectopics, as patients seem to do well with potassium?

Similar to AF, there is currently no evidence that high potassium levels help with ectopics. Therefore please try to ensure that potassium isn’t given outside the protocol for ectopics as this would be considered a protocol violation.