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Off-pump coronary
artery bypass has emerged as a popular method of coronary revascularization to
mitigate the harmful effects of cardiopulmonary bypass. It requires surgical
and anesthetic expertise for good results. Newer minimally invasive approaches
to off pump coronary bypass provide attractive alternatives to conventional off
pump surgery, especially in achieving better cosmetic results and early
extubation. They require good perioperative pain management, with epidural and
paravertebral block being good choices. Emergency conversion of off-pump bypass
to on-pump bypass is associated with poor outcomes, so preventive measures to
avoid it should be employed.
Keywords: Off-pump coronary artery bypass, Anesthetic implications
INTRODUCTION
Off-pump coronary artery bypass (OPCAB) has become popular since 1990;
the rationale being to avoid the deleterious effects of cardiopulmonary bypass
machine. OPCAB can be done via either a full sternotomy or minimally invasive
approach. Minimally invasive OPCAB requires one-lung ventilation and the access
for multiple vessel revascularizations may be difficult. Conventional OPCAB
avoids these limitations.
Till date there is no definitive data to suggest the benefit of OPCAB
over conventional CABG with regard to long-term graft patency rates, need for
repeat revascularization, stroke and mortality [1-3]. The main reason being the
surgical expertise, however, patients with renal, or liver dysfunction, and
those with calcified/atheromatous aorta are at advantage during OPCAB. Since
this surgery involves high level of vigilance on the part of the anesthesiologist;
this article intends to focus on anesthetic implications during OPCAB.
POTENTIAL
ADVANTAGES OF OPCAB [1-3]
1.
Reduced systemic inflammatory response, renal dysfunction, short-term
neurocognitive dysfunction.
2.
Reduced need for transfusion, reoperation for bleeding.
3.
Reduced incidence of atrial fibrillation.
4.
Reduced need for inotropes.
5.
Shorter period of postoperative ventilation.
6.
Reduced intensive care unit (ICU) and hospital stay, costs.
LIMITATIONS
Intracardiac thrombus,
malignant arrhythmias, deep intra-myocardial bridging coronaries and potential
for under-grafting diseased coronary arteries.
PREOPERATIVE
ANESTHESIA ASSESSMENT
Apart from routine assessment, preoperative assessment includes a review
of the angiogram and a discussion with the surgeon regarding the surgical plan
including the order of grafting. The CPB machine, perfusionist and facilities
for defibrillation, cardiac pacing and IABP should be available. Preoperative
prophylactic intra-aortic balloon pump (IABP) insertion may be considered in
high-risk patients with severe LV dysfunction and left main disease as it has
been shown to decrease postoperative low cardiac output syndrome and risk of
death [4].
Routine monitoring for OPCAB includes 5-lead electrocardiogram (ECG) with
automated ST segment analysis, pulse oximetry, invasive blood pressure, central
venous pressure, end-tidal carbon-dioxide, urine output, transesophageal
echocardiography (TEE) and temperature. If the cardiac surgical plan includes radial artery
harvesting,
TEE is done to confirm preoperative cardiac diagnoses, monitor
preload, ventricular function, valvular regurgitation and regional wall motion
abnormalities (RWMA). During heart displacement, TEE views may be lost. The use
of stabilizers can produce artifacts of RWMAs. After revascularization, TEE may
show new RWMAs which may be associated with reperfusion injury and myocardial
stunning. Persistent and severe new RWMAs are correlated with postoperative
cardiac complications [5].
Pulmonary artery catheter can be inserted in selected patients
with severe LV dysfunction (ejection fraction <30%), pulmonary hypertension
and right-sided heart failure to provide information regarding pulmonary artery
pressure (PAP), cardiac output (CO) and mixed venous oxygen saturation. Large
waves on the pulmonary capillary wedge pressure (PCWP) tracing may indicate
myocardial ischemia or severe mitral regurgitation.
ACT is maintained between 250-300 s, with an initial heparin
dose of 2 mg/kg. During proximal coronary anastomosis, arterial pressure should
be reduced to 90 mm Hg to reduce the risk of dissection on application of aortic
side-biting clamps. The left internal mammary to left anterior descending
artery (LAD) anastomosis is done first as the LAD supplies a large myocardial
territory and requires minimal displacement of the heart. Ventilation with low
tidal volumes is required during left internal mammary artery (LIMA) harvesting
and distal anastomoses to improve exposure. This may cause atelectasis in the
postoperative period. Excessive tidal volumes can cause graft compression,
especially when bilateral mammary arteries are used.
Access to the coronary arteries on the posterior and inferior
surfaces requires the heart to be elevated by using slings, pericardial
sutures, surgical packs or a suction device like the Starfish Heart Positioner.
Concerns about hemodynamic status when the heart is first positioned should be
communicated to the surgeon because once the coronary artery is opened; the
surgical options become limited until completion of the anastomosis. Access
also requires the Trendelenberg position, which beneficially increases venous
return. Tissue stabilizers like the Octopus device (Figure 1) use suction to fix the target coronary artery to
facilitate distal anastomosis. Blood flow can be controlled using a suture
applied around the coronary artery proximal to the graft site. A blower using
humidified carbon dioxide is used to prevent blood obscuring the arteriotomy
site. Once opened, a shunt can be inserted into the artery, reducing blood in
the surgical field and allowing perfusion of the distal myocardium.
Minimally invasive OPCAB
Minimally invasive OPCAB
involves a thoracotomy, partial sternotomy or subxiphoid incision in contrast
to a full sternotomy. It can be performed via direct, endoscopic or robotic
access. Avoidance of full sternotomy which is required in conventional OPCAB is
beneficial in patients with a prior sternotomy or bypass grafts. The
disadvantages of minimally invasive approaches are less adequate exposure to
achieve multivessel revascularization and increased duration of surgery. The
advantages include potential for improved cosmesis and wound healing along with
reduced morbidity, pain, cost, transfusion and length of stay in the ICU [12].
Patient selection is crucial in ensuring success of these techniques due to
limited access to surgical site, requirement of one-lung ventilation, potential
for incomplete revascularization and postoperative pain management. Provision
for external defibrillation is a must in minimally invasive cardiac surgery.
Endoscopic CABG will require insufflation of carbon-dioxide along with one-lung
ventilation. Continuous monitoring of insufflation pressure, airway pressure
and central venous pressures is required. One-lung ventilation has potential to
cause hypoxia and may require use of permissive hypercarbia. This may
necessitate use of inotropes and cause heart irritability, both of which are
undesirable in performing revascularization on a beating heart.
PAIN MANAGEMENT
Adequate control of pain is
crucial for fast-tracking extubation of cardiac surgery patients. Thoracotomy
is considered to be more painful than median sternotomy. Regional anesthetic
techniques are known to reduce opioid consumption, reduce stress response,
improve coronary blood flow and pulmonary function, and facilitate early
ambulation [7-12] Commonly used regional techniques include epidural block,
paravertebral block, intrathecal opioids, intercostal nerve block and
intrapleural block. Catheters for regional analgesia should be inserted the day
before surgery or at least an hour prior to heparinization. Surgery should be
delayed for 24 h in the event of a traumatic tap. Standard guidelines should be
followed to ensure safe removal of these catheters post-surgery if the patient
is receiving heparin.
Complications of epidural
blockade are hypotension, intravascular injection and epidural hematoma.
Thoracic paravertebral block is comparable to thoracic epidural anesthesia in
procedures involving unilateral thoracotomy incision [13]. It can decrease the
risk of epidural hematoma while avoiding the hemodynamic effects of the
sympathectomy seen with neuraxial blockade [14-16]. Complications of paravertebral
block are vascular puncture and pleural puncture with pneumothorax. Despite
proven benefits and the low risk of epidural hematoma, these techniques are yet
to gain widespread acceptance. NSAIDs and paracetamol are important adjuncts in
multimodal analgesia.
Complications and conversion to
on-pump complications of OPCAB are severe hypotension unresponsive to
inotropes, malignant arrhythmias, new regional wall motion abnormalities or
complete cardiovascular collapse. They require emergency conversion to on-pump
CABG. Conversion to CPB has been shown to increase morbidity and mortality with
operative mortality ranging from 8.5 to 18%. Congestive heart failure, redo
surgery, low ejection fraction, recent myocardial infarction, mitral
regurgitation, and chronic obstructive pulmonary disease have been reported as
independent predictors of emergency conversion to CPB.
SUMMARY
Cardiopulmonary bypass is a safe
technology that has led to innovations in cardiac surgery with better outcomes.
Knowledge of complications associated with it is important to avoid
catastrophes. Good communication between the entire surgical team is vital. OPCAB,
with good surgical and anesthetic experience can provide similar results to
on-pump CABG, along with certain specific advantages.
1.
Cheng DC, Bainbridge D, Martin JE, Novick RJ (2005)
Does off-pump coronary artery bypass reduce mortality, morbidity and resource
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Outcomes Research. Circulation 111: 2858.
4.
Yang F, Wang J, Hou D, Xing J, Liu F, et al. (2016)
Preoperative intra-aortic balloon pump improves the clinical outcomes of
off-pump coronary artery bypass grafting in left ventricular dysfunction
patients. Sci Rep 6: 27645.
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