Operation theatre environments in Kenya rely heavily on advanced biomedical equipment to support complex surgical procedures. Among these, the heart lung machine plays a vital role in cardiothoracic surgeries by temporarily taking over the functions of the heart and lungs, ensuring patient stability during operations. Proper understanding of the heart lung machine’s functions is crucial for biomedical engineers tasked with maintenance, as any malfunction can have critical consequences. This chapter provides an in-depth exploration of the functions of the heart lung machine within the Kenyan healthcare setting.

9.1 Functions of Heart Lung Machine

The heart lung machine, also known as the cardiopulmonary bypass (CPB) machine, is an essential device in open-heart surgeries and other cardiac procedures requiring cessation of the heart’s activity. It maintains systemic circulation and oxygenation, allowing surgeons to operate on a still and bloodless heart. Biomedical engineers supporting institutions like Kenyatta National Hospital or county hospitals must comprehend the machine’s core functions to ensure reliability and patient safety during operations.

9.1.1 Mechanical Circulation and Blood Pumping

Mechanical circulation is the primary function of the heart lung machine, where it replaces the pumping action of the heart. This function is vital during surgeries that require the heart to be stopped, such as valve replacements or coronary artery bypass grafting.

Components and Operation

• Blood Pump: The machine uses a roller or centrifugal pump to propel blood through the extracorporeal circuit. The pump must maintain a steady flow rate tailored to the patient’s body surface area and metabolic needs.
• Flow Control: Precise control mechanisms regulate the blood flow, preventing hemolysis or excessive pressure that can damage blood cells or vascular structures.
• Pressure Monitoring: Continuous monitoring of arterial and venous pressures ensures the machine mimics physiological conditions, preventing complications such as embolism or ischemia.
• Circuit Integrity: Tubing and connectors must be free from leaks and kinks to maintain uninterrupted circulation.
• Emergency Bypass: Systems are in place to quickly switch to manual operation in case of pump failure, preserving patient safety.

Example: At a county referral hospital, a malfunctioning roller pump led to reduced blood flow during bypass, highlighting the need for regular calibration and maintenance by biomedical engineers.

9.1.2 Oxygenation of Blood

The heart lung machine oxygenates venous blood, replacing the lungs’ function during surgery. This process is critical for maintaining tissue oxygen supply when pulmonary ventilation is suspended.

Oxygenator Functionality

• Gas Exchange Membrane: The oxygenator contains a semi-permeable membrane allowing oxygen to diffuse into the blood and carbon dioxide to diffuse out.
• Sweep Gas Control: The machine regulates the flow of oxygen and air mixture (sweep gas) to optimize oxygenation and carbon dioxide removal.
• Temperature Regulation: Oxygenators often incorporate heat exchangers to warm or cool blood, assisting in patient temperature management during surgery.
• Blood Compatibility: The oxygenator’s materials are biocompatible to minimize activation of coagulation or immune responses.
• Monitoring Gas Levels: Sensors verify oxygen saturation and carbon dioxide levels in the blood to ensure adequate respiratory function.

In Nairobi’s private hospitals, the use of modern hollow-fiber membrane oxygenators has improved patient outcomes by reducing inflammatory responses during bypass.

9.1.3 Removal of Carbon Dioxide and Waste Gases

While oxygenating blood, the heart lung machine simultaneously removes carbon dioxide and other metabolic waste gases, preventing acidosis and maintaining acid-base balance.

Mechanisms of Gas Removal

• Diffusion Gradient: Carbon dioxide diffuses from the blood across the oxygenator membrane into the sweep gas, following a concentration gradient.
• Sweep Gas Flow Rate: Adjusting the sweep gas flow controls the rate of carbon dioxide removal, critical for maintaining normal blood pH.
• pH Monitoring: Integrated sensors monitor blood pH to detect and correct imbalances promptly.
• Ventilation Simulation: The machine simulates lung ventilation, ensuring that gas exchange meets metabolic demands.
• Waste Gas Exhaust: The system safely vents removed gases away from the operating room environment.

At a teaching hospital in Mombasa, precise control of sweep gas flow during pediatric cardiac surgery was essential to avoid CO2 build-up, underscoring the importance of machine calibration.

9.1.4 Temperature Control of Blood

Temperature regulation is another critical function of the heart lung machine, as hypothermia or hyperthermia can significantly affect patient outcomes during surgery.

Temperature Management Features

• Heat Exchanger: Integrated heat exchangers warm or cool blood before it returns to the patient, allowing controlled hypothermia or normothermia.
• Temperature Sensors: Continuous monitoring of blood temperature ensures target temperatures are maintained throughout the procedure.
• Therapeutic Hypothermia: Cooling the blood reduces metabolic rate and protects organs during prolonged surgeries.
• Rewarming Phase: Gradual warming prevents complications such as arrhythmias and coagulopathy as surgery concludes.
• Alarm Systems: Alerts notify the surgical team of temperature deviations, enabling immediate corrective action.

In a Nairobi hospital, effective temperature management via the heart lung machine contributed to reduced postoperative complications in cardiac surgery patients.

Chapter Summary

This chapter examined the heart lung machine, beginning with its essential functions in supporting circulatory and respiratory processes during cardiac surgery. It detailed the key parts of the machine, explaining their roles in ensuring effective blood oxygenation and circulation. The principle of operation was explored, highlighting how the machine mimics the heart and lungs to maintain patient stability. Fault diagnosis techniques were discussed to identify common mechanical or functional issues that could compromise performance. Maintenance procedures were outlined to ensure the machine remains reliable, including routine checks and component servicing. Finally, safety procedures were emphasized to protect both patients and operators during use, underscoring the importance of strict adherence to protocols in the operating theatre environment.