A recent clinical negligence judgment on embolism causing stroke and the importance of logic in expert evidence (again)

A recent clinical negligence judgment on embolism causing stroke and the importance of logic in expert evidence (again)

In the case of Pickering v Cambridge University Hospitals NHS Foundation Trust [2022] EWHC 1171 (QB) [Phoebe Charmaine Pickering v Cambridge University Hospitals NHS Foundation Trust [2022] EWHC 1171 (QB) (17 May 2022) (bailii.org)], in which judgment was handed down on 17 May 2022, Mr Justice Ritchie had to determine the sole issue of whether the claimant would have avoided suffering a stroke on the basis that, but for the breach of duty of the defendant, she would have received the beneficial effects of Heparin treatment.


The trial heard only from the expert witnesses as, although witness statements from the claimant, her husband and son were served, they did not give oral evidence.

The facts

The facts were relatively straightforward. The claimant was a 52½ year old woman at the time of the events in 2015. By way of background, in 1997 she had been diagnosed with atrial fibrillation, an abnormal heart rhythm, due to left atrial dilatation. In 2004, she had declined to take a beta-blocker to slow the rhythm or warfarin to reduce the risk of emboli and instead chose just to take aspirin. In 2007 she had a pacemaker inserted. She repeatedly declined to take warfarin over the years.

On 24 September 2015, she attended A&E at Addenbrooke’s Hospital in Cambridge with an intermittently cold, white, numb right lower leg. On examination she had absent lower limb pulses. However, her symptoms resolved spontaneously. A diagnosis of a resolved ischaemic event of her right leg was made and she was discharged with safety netting advice but no heparin treatment. She was advised to restart her aspirin.

The claimant was well between on 25 and 27 September 2015.

However, on 27 September 2015, she suffered a massive stroke from left middle cerebral artery and left internal carotid artery emboli. Although she received thrombolysis treatment, the stroke worsened over two days leaving her with a profoundly life-changing injury.

The defendant’s admissions

The defendant admitted that low molecular weight heparin should have been prescribed before 25 September 2015 and it was a breach of duty to fail to do so. It was also admitted that the claimant was at high risk of suffering an embolus breaking free from a blood clot probably present in her left atrial appendage. It was likely this that had caused the lower limb ischaemia. This mechanism could also lead to a stroke. Where an embolus travels to is a matter of pure chance. The time between the admitted breach of duty and the stroke was 67 hours (2.79 days).

The stroke expert evidence

Two experts in stroke medicine provided written reports and a joint statement. In their joint statement, they agreed that the Claimant was at significant risk of further emboli after she had suffered the first blood clot in her leg, that the benefits of administering Heparin to the claimant outweighed the risks and that the stroke the claimant eventually suffered was caused by an embolism from a clot in the heart. However, they could not decide whether it was the whole clot or just part of it which travelled to the brain. On the key issue of causation, the defendant’s expert (Dr Giallombardo) deferred to the haematologists but thought that anti-coagulation would not have saved the claimant, whereas the claimant’s expert (Dr Michael) repeated his opinion that on balance the Heparin would have prevented the stroke.

Dr Giallombardo did not give oral evidence at trial. Dr Michael did and was criticised by the defendant in three respects: first, he was out of date having retired 22 years ago; secondly, he was not a stroke physician but a general neurologist; and, thirdly, he relied on a paper which did not support his position (Weitz, 1997). The Weitz paper advised that, “Heparins are safe and effective for the prevention and treatment of venous thromboembolism. They have also been used successfully in patients with unstable angina or acute thrombotic stroke” and that low molecular weight Heparin prevented thrombus growth and produced a reduction in thrombus size in 64% of patients treated. The judge accepted there was force in the defendant’s third argument because the paper did not expressly say what Dr Michael had asserted, namely a reduction in risk of embolism in patients with atrial fibrillation, and the expert accepted in evidence that he had misquoted the paper and had erred in summarising a different paper.

The haematology expert evidence

The haematologists were the key experts. Professor Mehta was for the claimant and Dr Patel was for the defendant. They each prepared written reports and together prepared a joint statement for the Court.

Professor Mehta stated that the claimant had an unstable blood clot in the left atrium. He advised that it was difficult to carry out any clinical trials relating to acute emergency situations because they were impossible to conduct. However, the lack of papers on the direct short-term effects of Heparin in acute situations did not mean that Heparin lacked effect in such situations. He dismissed as irrelevant “bridging” studies, where heparin is used as a short term therapy to “bridge” between stopping Warfarin before an operation and restarting Warfarin after an operation, because in such patients atrial clots had already been abolished, prevented or dissolved. He relied on various papers to support that Heparin was an effective and rapid anticoagulant in the emergency setting and that it commences its action within minutes and that, within 24 to 36 hours, it had been shown to reduce the risk of pulmonary embolism in patients with DVT. He stated the claimant’s case was an analogous situation. He advised that Heparin reduces blood clot formation and allows time for the body’s own natural processes to dissolve the clot.

Dr Patel asserted that the evidence for the effectiveness of Heparin was lacking. Although he accepted Heparin was >90% effective in preventing DVT and PE within 2 to 3 days of starting treatment, he nevertheless asserted that Heparin had a failure rate in cases involving mechanical heart valves, in that it did not prevent stroke when Warfarin was stopped, and likewise for atrial fibrillation cases when Heparin was used to “bridge” for elective surgery. Dr Patel advised that it was, therefore, wrong to extrapolate Heparin’s success in dealing with venous thrombosis to its ability to deal with left atrial thrombosis.

The judge analysed the literature in very considerable detail [paragraphs 96-131 of the judgment]. The judge also analysed the haematology experts’ oral evidence and their credibility as experts in some detail. He concluded that Professor Mehta had a tendency under cross-examination to descend into lectures whereas Dr Patel had a tendency to be rigid and to produce rather extreme opinions. The judge was also struck by Dr Patel’s reliance on the bridging papers “to prove something that I consider they just do not prove” [140]. The judge noted that Dr Patel accepted that the effectiveness of Heparin for DVT and PE was better than 90% and that such effect was shown in the first 24 to 48 hours, but that Dr Patel simply refused in his live evidence to detail why and how Heparin’s great success in abolishing the risk of emboli from blood clots in DVT and PE should be occurring so quickly and why it was irrelevant to atrial clots.

The judgment

The judge concluded that there were three processes that explained why Heparin was so quickly effective against DVT and PE. The first was that Heparin prevents new clots forming and prevents the “mother clot” growing more (propagating). The second was that Heparin very quickly allows the body’s natural process to start dissolving the blood clots. The third was that the body’s natural defences organise and/or solidify and/or stabilise and/or “wall off” the blood clot.

The judge found that, “Had Heparin been administered to the claimant on 25 September, as it should have been, the Heparin would have started working within 1 to 3 hours and the effect would have been to start the body’s natural processes of dissolving the friable or unstable clot and working towards stabilising it and/or organising it and/or “walling it off” and/or adhering it to the atrial wall” and that “Taking into account the TEE [trans-esophageal echocardiogram] studies carried out by Collins et al the effect of the Heparin on the claimant’s atrial clot, in the 67 hours (2.79 days), would have been that the clot would have reduced in size considerably and probably by over 50% in size”. The judge considered that “At the same time as the thrombus was being dissolved and reduced in size by the body’s natural mechanisms, encouraged and assisted by Heparin’s effects on the blood, I find that the structure of the claimant’s LAA [left atrial appendage] thrombus would have been becoming more organised, less friable, more stable and more attached to the atrial wall”.

In reaching his conclusion, the judge also took into account that:

The effects of Heparin on blood clots in DVT and PE cases after the first 48 hours were very beneficial in preventing emboli breaking away from the blood clots in the venous system [157].
The agreed evidence was that Heparin prevents the formation of new clots in the left atrium and the left atrial appendage and prevents propagation of the mother clot. In finding that Heparin would have done so in the claimant over the 67 hours, the judge concluded that this would therefore probably have prevented new clot formation on the surface of the claimant’s mother clot from embolising and causing the stroke [158].
He preferred the evidence of Professor Mehta that Heparin stabilises or organises or makes atrial clots less friable and/or more adherent over that of Dr Patel that Heparin does not enable the body to reduce the embolic danger created by a blood clot and it merely allows the body to dissolve the blood clot [160].
Dr Patel advised that Heparin reaches peak effect in 3-4 hours.
The judge concluded for the claimant [162] that Heparin would have prevented new clot formation, prevented “mother clot” propagation of the existing clot and would have enabled the claimant’s body not only to reduce the size of the mother clot in the claimant’s left atrial appendage but also to make it less friable, more stable and more organised, so that on the balance of probability no embolus would have been “fired off” on 27 September 2015 and the claimant would not have suffered a stroke.


The judgment is a detailed and thorough analysis of the expert evidence, the experts’ reliance on literature (or its absence), how that reliance affected their opinion on the key issues and also their credibility. The judge preferred the claimant’s haematology expert’s evidence, concluding that the defendant’s haematology expert’s “fixed thinking and questionable logic on these matters … undermines the credibility of his evidence” [139]. Ultimately, it is apparent that Dr Patel’s assertions and conclusions did not stand up to close scrutiny and logical analysis, an exercise the judge himself had carried out in great detail, and so the claimant’s expert’s evidence was preferred. This is yet another example of how important it is for an expert’s opinion to withstand logical analysis. As Green J., as he then was, stated in his since oft repeated judgment in C v North Cumbria University Hospitals NHS Trust [2014] EWHC 61, regarding the assessment of expert evidence [at 25(vii)]: “By far and away the most important consideration is the logic of the expert opinion tendered”; in recent years, logic has repeatedly been shown to be a paramount factor in assessing the credibility of an expert’s opinion, and experts would be advised to heed the point.