Cardiopulmonary Bypass: Patient Monitoring, Blood Pressure Measurement, and Myocardial Preservation

Introduction

• CPB is a cornerstone of modern cardiothoracic surgery, enabling surgeons to safely perform high-risk operations in the mediastinum while temporarily taking over the functions of the heart and lungs.
• Optimal outcomes during CPB support depend on meticulous patient monitoring, careful blood pressure management, and robust myocardial preservation strategies.¹
• Together, these factors maintain patient perfusion and safety while allowing surgeons to complete procedures that would otherwise compromise cardiorespiratory function, such as those requiring diastolic heart arrest.

Patient Monitoring

• The American Association of Thoracic Surgeons’ 2017 American Society of ExtraCorporeal Technology (AmSECT) Standards and Guidelines for Perfusion Practice outlines monitoring standards for patients maintained on CPB.²
• These include patient arterial blood pressure monitoring, CPB circuit monitoring (arterial line pressure and blood flow), cardioplegia delivery (flow and pressure), patient and device temperatures, blood gas analysis at regular intervals, and situations requiring cerebral oximetry.^1,2

Arterial Pressure, Arterial Line Pressure, and Blood Flow

• Arterial blood pressure and arterial line pressure should be continuously measured during CPB. A radial arterial catheter is most often used to measure arterial blood pressure; however, the most suitable anatomic location for arterial monitoring may be modified by the proposed surgical plan.²
• Arterial line pressure is measured via a pressure transducer connected to the arterial return limb of the CPB circuit. Similarly, blood flow is traditionally measured using flow probes, which are connected directly to the arterial outflow tubing of the CPB circuit.
• The AmSECT guidelines recommend that arterial line pressure and blood flow be continuously measured while the patient is on CPB. Further, it is critical that arterial return flow be measured at a point in the CPB circuit that accurately reflects blood flow returned to the patient and the flow monitor must therefore be distal to any intra-circuit shunts in the CPB machine.²

Cardioplegia Delivery and Pressures

• Cardioplegia, a hyperkalemic solution used in cardiac surgery to arrest and cool the heart, is delivered to the heart either antegrade through the aortic root and coronary arteries or retrograde via the coronary sinus.
• Dose, delivery method, and dosing interval of cardioplegia must be carefully monitored during CPB.
• The AmSECT guidelines recommend that the cardioplegia dose and the pressure of the cardioplegia delivery system be continuously monitored during cardioplegia delivery.
• In cases where cardioplegia is delivered in a retrograde fashion, coronary sinus pressure should be measured continuously via a transduction lumen built into the retrograde cardioplegia cannula.^2,3

Patient and Device Temperatures

• While maintained on CPB, temperature is monitored at several different sites simultaneously.^2,3
• Specifically, core patient and blood temperatures are monitored in tandem with temperatures returning to (venous limb) and emanating from (arterial limb) the CPB circuit itself.^2,3
• Core temperature is typically measured via bladder temperature, nasopharyngeal or oropharyngeal temperature, or the thermistor on the distal tip of a pulmonary arterial catheter. Regional temperature variation is frequent and may be exaggerated by longer bypass times and lower temperature nadirs.
• Brainstem temperature is best approximated by nasopharyngeal monitoring, while bladder temperature is often the “target” used to determine adequate rewarming before eventual separation from the bypass circuit, as a better representation of the body’s visceral core temperature.
• Due to re-equilibration of temperatures in the high- and low-flow body compartments, it is not uncommon for core body temperature to fall after bypass separation, even when normothermia is ensured at the time of separation.

Blood Gas Analysis

• The AmSECT guidelines recommend that blood gas analyses be monitored serially in patients on CPB.^2,4
• In-line blood gas monitoring is recommended when available, allowing for real-time assessment and rapid interventions should derangements occur, and serial standard blood gas panels at regular intervals are considered requisite. Serial monitoring of anticoagulation adequacy is also mandatory, with activated clotting time measurements at similar intervals.^2,4

Cerebral Oximetry

• Cerebral oximetry uses the principles of near-infrared optical spectroscopy, in which a sensor placed on a patient’s forehead emits light of specific wavelengths and measures the light reflected back to the sensor to measure the pooled tissue hemoglobin saturation.⁴
• Several platforms measure cerebral oximetry and may differ in expected normal values due to variability in tissue sampling. A fall of greater than 25% of baseline values may herald neurological events secondary to decreased cerebral oxygenation.⁴

Blood Pressure Measurement

• The AmSECT guidelines recommend that arterial blood pressure be continuously measured while patients are on CPB; however, it is nearly universal to monitor arterial blood pressure continuously via transduction from an indwelling arterial line, as non-pulsatile CPB flow precludes non-invasive oscillometric blood pressure monitoring. Some centers implement transcranial Doppler to guide MAP targets; however, this is not included in the AmSECT guidelines and outside the scope of this review.
• Target MAP ranges for patients maintained on CPB during cardiac surgery are typically individualized, with targets adjusted for comorbidities such as chronic hypertension and cerebrovascular disease, in which autoregulation curves may shift to the right.⁵
• General patients: most guidelines recommend targeting a MAP of 60-70 mmHg for patients maintained on CPB, though there is no universal consensus.⁵
• High-risk patients: as noted above, higher MAP targets, such as 70-80 mmHg, may be considered for patients with comorbidities, such as chronic hypertension and cerebrovascular disease, as the lower limit of the autoregulation curve may be shifted to the right.
• While several trials have failed to show that higher MAP targets (typically >70 mmHg) consistently reduce the incidence of acute kidney injury, stroke, or mortality compared to standard targets, some studies have demonstrated a reduction in the incidence of postoperative delirium in select patients.^6,7
• Moreover, aggressive treatment of intraoperative hypotension as guided by arterial blood pressure transduction should be pursued, as increased time spent below the lower limit of the cerebral autoregulation curve is associated with increased risk of stroke, renal dysfunction, and other complications.

Myocardial Preservation

• The goals of myocardial protection are threefold:^8,9
  • Facilitate surgery in a still, bloodless field, therefore allowing for safe, optimal operating conditions
  • Avoid iatrogenic injury induced by myocardial ischemia during aortic cross-clamping
  • Prevent ischemia-reperfusion injury upon release of the aortic cross-clamp

Cardioplegia

• Regardless of its type, composition, or mode of delivery, cardioplegia induces rapid, reversible cardiac arrest by depolarizing myocardial cell membranes, thereby halting electrical activity.8 Traditionally, cardioplegia is composed of a hyperkalemic solution, which is delivered either antegrade through the aortic root and coronary arteries or retrograde via the coronary sinus.
• Some centers have instituted “polarized” cardioplegia solutions containing lidocaine and adenosine, which have been theorized to reduce metabolic stress and ionic imbalances compared with traditional cardioplegia solutions.9
• Two primary types of cardioplegia include:^8,9
  • Blood cardioplegia, in which the patient’s own blood is mixed with additives (such as potassium and magnesium). This formulation has been shown to confer superior myocardial protection and lower rates of perioperative myocardial infarction. It typically needs to be re-dosed by the surgeon at various intervals during surgery.
  • Crystalloid cardioplegia, containing electrolytes and buffering agents in a crystalloid solution without admixture with patient blood. It is generally considered to confer less myocardial protection than blood-based cardioplegia.

Adjunctive Strategies

• Other adjuncts to improve myocardial protection during CPB include, but are not limited to, topical cooling of the heart with ice, temperature management (such as systemic mild hypothermia and alpha-stat blood gas management), optimizing respiratory parameters, and remote ischemic preconditioning.⁸

Preconditioning Strategies

• While several studies have investigated preconditioning mechanisms to attenuate myocardial reperfusion injury following diastolic arrest during CPB,¹⁰ clinical experience has not yet yielded an encouraging experimental model for routine use.
• Remote ischemic preconditioning (RIPC), typically performed by applying a limb tourniquet and then reperfusion before diastolic arrest, has been a major area of study. Proposed mechanisms of myocardial protection include the induction of protective secondary mediators produced by ischemic muscle, and a variety of chemical mediators have been investigated.¹⁰
• A variety of other strategies to attenuate myocardial injury after reperfusion have been studied, including volatile anesthetics and opiates. However, the evidence is mixed, and no clear strategy has become dominant in clinical practice.