Discussion
CPB is arguably one of the greatest developments to allow for the
expansion and advancement of the field of cardiac
surgery1,12. Over the decades, preexisting
technologies have improved with the goal of reducing CPB-related
complications. Some of these technologies include centrifugal pumps,
improved oxygenators, and heparin-bound circuit
tubing2. These advancements aim to reduce air emboli
and reduce thrombus formation within the CPB circuit, and thus eliminate
thromboembolic complications. Despite such advancements, MRI-adjudicated
studies have revealed shockingly high rates of new, albeit mostly
clinically insignificant, brain lesions following on-pump cardiac
surgery13,14. Additionally, CPB runs are also linked
to elevation of pro-inflammatory cascades, which can result in the
development of systemic vasoplegia and end-organ
malperfusion15–19. Though necessary for the field of
cardiac surgery, CPB technology remains an area for improvement.
Because of the known complications and increased mortality risk with
prolonged CPB usage9–11, it is universally accepted
that pump runs be kept to a minimum, and that cardiac procedures be
planned accordingly to be conducted in such a way to eliminate
unnecessary CPB and cross-clamp times. Nissinen and Madhavan have
suggested that the optimal CPB time to be under 180-240 minutes in order
to minimize risk of severe complications and/or
mortality7,11. In the analysis by Nissinen and
colleagues, increasing CPB time (per 30 minutes OR 1.47, 95% CI
1.27-1.71 was associated with increased odds of 30-day mortality,
independent of case complexity or patient comorbidities. In their
series, only 30 patients had a CPB time longer than 300 minutes, and
30-day mortality was 56.7% in this cohort. In our cohort of 293
patients, operative mortality was 22.5%, significantly lower than
previously described. In an evolving era of cardiac surgery,
increasingly complex procedures are being performed, along with complex
redo operations, which not surprisingly require lengthier pump runs.
Although prior studies have correlated poorer outcomes with increasing
CPB times, the current limitations of our existing technology are not
well understood.
As stated, increased CPB times have been linked to worse survival,
renal, failure, and increased intensive care unit
utilization4,7,9,20, which likely are associated with
increased healthcare costs. There are limited studies that have
specifically investigated outcomes of patients in which very-long CPB
perfusion times were required. One such study evaluated patients with
aortic cross-clamp times exceeding 300 minutes with an average CPB time
of 420 minutes, and demonstrated a 30-day mortality of
12.4%21. In patients surviving at least 30 days, the
1-year survival was 92% in their series, again supporting the notion
that longer term survival is favorable if early mortality is avoided in
this patient cohort.
As expected, this population was prone to a high degree of complications
in our analysis, namely prolonged mechanical ventilation, renal failure,
and need for reoperation and blood transfusion. Furthermore, one fifth
of patients died within the perioperative window. However, at one year,
more than 60% of patients had survived with roughly one third of
patients requiring readmission for any cause. Such findings suggest that
although these cases with excessive perfusion times, whether planned or
unplanned, are faced with a very high risk of operative mortality and
morbidity, mid-term results are acceptable with the majority of patients
being alive. Therefore, cardiac operations requiring excessive CPB times
should not be considered futile.
The granular causes of very-long CPB times can be multifactorial and
were not captured in our data registry. These causes can be categorized
as multi-component procedures that require lengthy operative times even
if performed efficiently, or cases that should have limited CPB times
but where intraoperative complications or issues led to a very prolonged
operative course. Nearly 50% of the cases in our series were
multi-component cases consisting of coronary revascularization combined
with a valve procedure, or a multi-valve operation. The most common
procedure was an aortic root replacement. By comparison, the mean CPB
times reported for combined CABG with aortic valve replacement or
isolated aortic root replacement are 123-203
minutes22,23 and 122-237
minutes24–26, respectively. This suggests that even
in these typically lengthier cases, the CPB times typically do not
exceed 300 minutes. Factors such as having to revise distal anastomoses
in CABG, replacing a valve in which a suboptimal repair was performed,
or reapplying a cross-clamp to address a paravalvular leak in valve
replacement surgery can add time to the operation. Intraoperative
complications, although rare, can add substantial operative time,
including such events as atrioventricular groove disruption during
mitral valve replacement, root disruption during aortic valve surgery,
or poor coronary flow after a root replacement requiring coronary
revascularization.
The longer CPB times may also reflect reperfusion time after an
unsuccessful attempt at weaning the patient from CPB at the conclusion
of the case. Most surgeons, after an initial CPB wean, will empty and
reperfuse the heart for a longer period of time if the ventricular
function is suboptimal and the patient cannot be weaned successfully and
safely from CPB. This can be the result of poor baseline ventricular
function, suboptimal myocardial protection, or temporary insults such as
air embolism. Protamine reactions can also occur requiring reinstitution
of CPB with added time.
Another important factor in achieving reasonable success in these more
complex scenarios is early institution of temporary mechanical
circulatory support. This is reflected in the 20% of patients in whom
we utilized an intraoperative IABP. In cases where the patient cannot be
weaned from CPB, we typically will attempt weaning with IABP support,
and then escalate to ECMO if the patient is still unable to be weaned.
In addition, we prefer to utilize IABP support in marginal cases where
the patient can be weaned from CPB but is requiring high levels of
inotropic support in the setting of depressed ventricular function. The
use of mechanical support depends heavily on the surgeon’s judgement.
However, an increasing body of evidence underscores the importance of
these devices in reducing native cardiac stress and distension,
improving contractility and myocardial recovery, and increasing
end-organ perfusion, all of which contribute to the overall mortality
and morbidity of the patient27–31.