Heart-Lung Machine

Overview

A heart-lung machine performs cardiopulmonary bypass: it drains venous blood from the patient, adds oxygen, removes carbon dioxide, controls blood temperature, and pumps the blood back into the arterial system. With the circulation externalised, surgeons can stop the heart with cardioplegia solution and operate on a still, bloodless field. The machine was first used successfully by John Gibbon in 1953; a modern console supports bypass runs of several hours during valve replacement, coronary grafting, and aortic surgery.

The machine itself is the permanent console, pumps, monitors, gas supply, and water circuit, while the blood-contacting path is a sterile single-use Tubing System (Disposable Circuit) mounted on it before each case. A perfusionist operates the console throughout the procedure.

Blood path

Venous blood drains by gravity or vacuum assist from the right atrium through the Cannula Set into the Venous Reservoir, a hard-shell tank of 3–4 litres that buffers volume changes, defoams, and filters cardiotomy suction returns. From the reservoir the main arterial pump pushes blood through the membrane oxygenator, the Arterial Line Filter (a 38 µm screen that traps any remaining emboli), and back through the aortic cannula. Total prime volume of the circuit is 1.2–1.8 litres, usually filled with crystalloid so the patient is hemodiluted when bypass begins.

Pumps

The console carries two pump types. The Roller Pump Module modules are positive-displacement peristaltic pumps: a Pump Rotor Arm sweeps two Occluding Rollers around a length of tubing held in the Pump Raceway, pushing a fixed volume per revolution. The Occlusion Adjuster is set so the rollers almost, but not quite, fully flatten the tubing, under-occlusion loses output, over-occlusion destroys red cells. Roller pumps serve the cardiotomy suckers, the ventricular vent, and cardioplegia delivery, where their precise, pressure-independent output is wanted.

Systemic flow normally comes from the Centrifugal Pump Drive. Its disposable Disposable Pump Head contains a vaned or smooth-cone impeller spun to 4,000 rpm through a Magnetic Coupler, so no shaft penetrates the blood chamber. Output depends on both speed and downstream resistance, which makes the pump inherently pressure-limited: it cannot burst a clamped line the way a roller pump can. Actual delivered flow is therefore measured directly with the ultrasonic Ultrasonic Flow Probe. Every pump position retains a Emergency Hand Crank so perfusion can continue by hand if all power is lost.

Gas exchange and temperature

The disposable oxygenator hangs on the Oxygenator Holder and receives sweep gas from the Gas Blender, which proportions oxygen and air to a set FiO2; flow through each Gas Flowmeter determines CO2 removal, much as minute ventilation does in the lung. A Vaporizer Mount lets volatile anesthetic be added to the sweep gas, and the Oxygen Analyzer confirms delivered oxygen fraction.

Blood temperature is managed through the Heat Exchanger Circuit: water held in the Water Reservoir is heated or chilled between 2 °C and 41 °C and circulated through the oxygenator's integral heat exchanger via Insulated Water Hoses. Cooling the patient to 28–32 °C reduces metabolic oxygen demand and adds a safety margin; the Water Temperature Controller then rewarms at a limited gradient to avoid gaseous emboli forming as solubility falls.

Safety systems

The Safety Sensor Suite suite interlocks the pumps. An ultrasonic Ultrasonic Bubble Detector on the arterial line stops the pump and fires the Electric Tube Clamp within milliseconds if air above roughly 0.3 mL is detected, since arterial air embolism is the most feared bypass accident. The capacitive Reservoir Level Sensor on the venous reservoir slows or stops the arterial pump before the reservoir can empty and entrain air. Four Pressure Sensor channels watch arterial line and cardioplegia pressures, and Temperature Probes track arterial, venous, and water temperatures.

Console and controls

The Console Base is a wheeled steel Console Chassis with rails accepting interchangeable pump modules and two Accessory Masts for holders and monitors. The Control and Monitoring System gives each pump its own Microcontroller-based controller and LCD Panel, with a central System Display aggregating flows, pressures, temperatures, and bypass timers. All electronics run from an isolated medical-grade Power Supply, and the Backup Power System battery bank carries the full pump load for at least an hour, long enough to finish a case or come safely off bypass during a mains failure.