Pulmonary Atelectasis Clinical Trial
Official title:
Ventilation During Cardiopulmonary Bypass (CPB) for Cardiac Surgery - Effect on Pulmonary Atelectasis Post-operatively, a Pilot Study
During most types of heart surgery cardiopulmonary bypass (CPB) is used. CPB enables blood to
be directed away from the heart and the lungs and pumped through the body while the heart is
not beating. Surgery on the heart easier when it is not beating and bloodless area. Generally
during CPB the lungs do not need to be ventilated, as no blood is flowing through the lungs
and the body received oxygen from a machine (oxygenator) attached to the CPB pump.
In this study we are investigating the difference in lung collapse after heart surgery in
patient who did have their lungs ventilated during CPB, compared to patients who did not have
their lungs ventilated during CPB. We will use lung ultrasound scans to determine the degree
of lung collapse at various time periods before and after the heart surgery. We will also
investigate if ventilation during CPB will affect: a.) the rate of lung infection or
pneumonia after the operation b.) the time it takes for a patient to have the breathing tube
removed in the intensive care unit after the operation c.) the time for a patient to be
discharged home from the hospital d.) the concentration of oxygen in the blood after the
operation.
Objective and Hypothesis:
To determine if continued mechanical ventilation (low tidal volume ventilation) during
cardiopulmonary bypass (CPB) in adult patients undergoing elective cardiac surgery will
reduce the degree of pulmonary atelectasis post-operatively, when compared to controls not
ventilated during CPB.
The degree of pulmonary atelectasis will be assessed and quantified using non-invasive lung
ultrasound (LUS) imaging.
We will conduct a preliminary observational study (pilot study) to assess the feasibility of
this study. The pilot will be conducted on 40 patients, all of whom will be non-ventilated
during CPB. The specific feasibility questions we would like to determine are: 1) the degree
of atelectasis that can be demonstrated by LUS on patients that are not ventilated while on
CPB, 2) the time needed to complete a LUS in the operating room (OR) and in the intensive
care unit (ICU) and whether this will be efficient and possible given the busy operating room
schedule, 3) to help determine the optimal time to scan patients to show the maximal degree
of atelectasis, and 4) to help estimate the number of patients we will need to show a
difference in atelectasis in the definitive study.
Primary hypothesis: low tidal volume ventilation during CPB for adult cardiac surgery reduces
the amount of pulmonary atelectasis compared to controls not ventilated during CPB.
Background:
Respiratory dysfunction after cardiac surgery is relatively common, with an incidence
anywhere from 2- 25% quoted in the literature. Pulmonary complications are a significant
factor in morbidity and mortality of post-surgical patients, and therefore a burden on
healthcare resources.(1)
The etiology of post CPB pulmonary dysfunction is multi-factorial, including CPB-induced
systemic inflammatory response, hypothermia, surgery, anesthesia, ventilation and
transfusion. (2-3) Modifications in the above-mentioned factors, amongst others, may lead to
better lung function postoperatively. Strategies include reducing CPB time, or minimizing the
extracorporeal surface area with the use of miniaturized circuits. The partial restoration of
pulmonary artery perfusion during CPB possibly contributes to prevent pulmonary ischemia and
lung dysfunction. The use of ultrafiltration to scavenge pro-inflammatory factors may be
protective for lung function. (4) Similarly, the use of corticosteroids and leukocyte
depletion filters may be protective. (5-6) Reducing the use of the cardiotomy suction device,
as well as the contact-time between free blood and pericardium, may prove beneficial. The
evidence for outcome benefit for most of these interventions is, however, weak. (3-7)
In standard care, lungs are unventilated and deflated and blood circulation is shunted away
from the pulmonary vasculature during CPB. Restoration of normal perfusion results in the
release of oxygen free radicals, causing lipid peroxidation with pulmonary endothelial damage
(ischemia-reperfusion injury). (8-9)
Continued ventilation during CPB has been experimentally shown to preserve pulmonary arterial
endothelial function. (10) A significant consequence of reperfusion injury is dysfunction of
the pulmonary vascular endothelium with diminished nitric oxide (NO) production, causing
secondary vasoconstriction and increased vascular permeability. This leads to pulmonary
hypertension, pulmonary edema and hypoxia. (10)
Non-ventilation may result in ischemic lung injury. During bypass there is no blood flow
through the pulmonary arteries. The only vascular and hence oxygen supply to the lungs is via
the bronchial circulation. (11) In an experimental porcine bypass model it has been shown
that bronchial artery flow is significantly decreased during CPB. (12-13) It is hypothesized
that the repetitive inflation and deflation of lungs at physiological intra-alveolar pressure
is necessary for normal bronchial arterial flow secondary to the cyclical compression and
relaxation of the vessels. In that case, cessation of ventilation during bypass would reduce
bronchial flow and predispose to ischemic lung injury. (13)
John et al (11) showed in their randomized study that ventilation during CPB with tidal
volumes of 5 ml/Kg resulted in significant smaller extra-vascular lung water (measured using
the Pulse Contour Cardiac Output (PiCCO) system) and a shorter time-to-extubation. However,
the evidence for clear clinical benefits of maintaining ventilation during CPB is
inconsistent. Problematic in these studies are the heterogeneous outcome endpoints and small
sample sizes. (11;14)
This study aims to investigate if continued low volume ventilation during CPB would decrease
the amount of pulmonary atelectasis present post-operatively in cardiac surgery patients.
De-recruitment of lung volumes occurs during the period of non-ventilation on CPB.
Atelectasis may lead to pulmonary complications post-operatively, if not addressed in the
perioperative period with active lung recruitment and physiotherapy. Low-volume ventilation
can be applied easily and safely in most patients undergoing CPB without hampering the
surgical progress.
Lung ultrasound (LUS) will be used to grade the pulmonary atelectasis present at various
points in time (both pre-operatively and during the post-operative course). LUS is a
non-invasive, easy-to-use, point-of-care investigatory tool. LUS is well validated and used
in the operating room and the intensive care unit to diagnosis pulmonary pathology. It has
been shown to be more accurate than plain film chest X-ray in determining pulmonary
consolidation, as well as other pulmonary pathology. (13) LUS has also shown tight
correlation in previous studies with pulmonary CT finding. In 2012 the first International
evidence-based recommendations for point-of-care LUS were published. (16) It has become a
useful and widely used modality in clinical practice. (17-18)
Several ultrasound studies based on the examination of the left lower lobe during
transesophageal echocardiography have confirmed the feasibility of assessing lung recruitment
(19-20). Another study has shown that resolution of ventilator-associated pneumonia with
antimicrobial therapy can be accurately assessed by LUS: a tight correlation was found
between an increase in computed tomography (CT) lung aeration after 7-day antibiotic
administration and an ultrasound re-aeration score based on observed changes in ultrasound
patterns (21). Therefore, it is reasonable to hypothesize that a change in lung aeration
resulting from any therapy (for example, ventilation during CPB) will be detected by
corresponding changes in LUS patterns.
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