The heart lung machine, also known as the cardiopulmonary bypass machine, plays a pivotal role in open-heart surgeries performed in Kenyan hospitals and specialized cardiac centers. This equipment temporarily takes over the functions of the heart and lungs during surgery, ensuring continuous circulation and oxygenation of blood. For biomedical engineers maintaining this sophisticated device, understanding its functions is essential to guarantee patient safety and surgical success. The following sections explain the core functions of the heart lung machine and their relevance in the clinical setting.

9.1 Functions of Heart Lung Machine

In the context of biomedical engineering in Kenya, particularly in major hospitals like Kenyatta National Hospital and private cardiac units, the heart lung machine’s functions must be meticulously maintained. These functions are critical for supporting life during cardiac procedures where the heart must be stopped or its function impaired. The machine’s ability to oxygenate blood, maintain circulation, regulate temperature, and remove carbon dioxide highlights its complexity and the need for precise maintenance protocols.

9.1.1 Pumping Function and Circulatory Support

The primary function of the heart lung machine is to replicate the pumping action of the heart, ensuring continuous blood circulation throughout the body during surgery. This is achieved by a mechanical pump that moves blood from the patient’s venous system to the arterial system, bypassing the heart.

Components of Pumping Function

• Venous Reservoir: Collects deoxygenated blood from the patient’s body, acting as a temporary storage before pumping.
• Mechanical Pump: Often a roller or centrifugal pump, it propels blood forward, mimicking the heart’s pumping action.
• Arterial Line: Delivers oxygenated blood back into the patient’s arterial system.
• Flow Control Mechanism: Regulates the speed and volume of blood pumped to match physiological requirements.
• Safety Alarms: Alert operators to abnormal flow rates or pressures to prevent tissue hypoxia or vascular damage.

Maintaining the pumping function requires careful calibration to ensure blood flow matches the patient’s metabolic needs. For example, during a coronary artery bypass surgery at a county hospital, the biomedical engineer must verify pump speed and volume settings to prevent complications such as low perfusion or excessive pressure that could damage blood vessels.

9.1.2 Oxygenation and Gas Exchange

Another critical function is oxygenating the blood and removing carbon dioxide, substituting for the lungs during surgery. This is accomplished through an oxygenator component within the machine.

Key Aspects of Oxygenation Function

• Membrane Oxygenator: Facilitates gas exchange by allowing oxygen to diffuse into the blood and carbon dioxide to diffuse out.
• Gas Supply System: Provides a controlled mixture of oxygen and air to the oxygenator.
• Blood-Gas Interface: The surface area and material of the membrane affect efficiency and hemocompatibility.
• Temperature Regulation: Controls blood temperature to optimize oxygen uptake and patient homeostasis.
• Monitoring Sensors: Measure oxygen saturation, partial pressures of gases, and blood pH in real time.

In practice, a biomedical engineer at a Nairobi private hospital ensures the oxygenator membrane is intact-free from clots or damage, and that gas supplies are uninterrupted. Failure in oxygenation can lead to hypoxia, jeopardizing patient outcomes during cardiac surgery.

9.1.3 Temperature Regulation and Thermal Control

The heart lung machine also regulates the temperature of the circulating blood to assist in protecting organs and tissues during surgery. Hypothermia induced by cooling reduces metabolic demand, while rewarming prepares the patient for normal physiological function post-surgery.

Functions of Thermal Control

• Heat Exchanger: Integrated within the circuit to warm or cool blood as necessary.
• Temperature Sensors: Continuously monitor blood temperature to maintain it within targeted ranges.
• Thermoregulation Protocols: Enable gradual cooling and rewarming to prevent thermal shock.
• Safety Cut-offs: Prevent blood temperatures from exceeding safe limits.
• Integration with Perfusionist Controls: Allow adjustment of temperature based on surgical stage and patient condition.

For instance, during a valve replacement procedure at a county referral hospital, the biomedical engineer collaborates with perfusionists to set temperature parameters that minimize ischemic injury to organs. Proper maintenance of heat exchangers is vital to avoid thermal regulation failures.

9.1.4 Removal of Carbon Dioxide and Blood Filtration

Besides oxygen delivery, the heart lung machine removes carbon dioxide from the blood and filters out microemboli and debris that may occur during surgery.

Components and Mechanisms for CO2 Removal and Filtration

• Oxygenator Gas Exchange Surface: Facilitates CO2 diffusion from blood to gas phase.
• Filters: Positioned in the circuit to trap air bubbles, clots, and particulate matter.
• Bubble Detectors: Sensors that detect air emboli and activate alarms or stop pumps.
• Defoaming Agents: Used within the oxygenator to reduce foam formation that can impair gas exchange.
• Circuit Design: Optimized to minimize blood trauma and ensure efficient filtration.

In clinical settings such as at a university hospital, biomedical engineers ensure that filters are replaced regularly and bubble detectors are calibrated to prevent air embolism, a potentially fatal complication. Proper CO2 removal maintains acid-base balance and respiratory homeostasis during cardiopulmonary bypass.

Chapter Summary

This chapter explored the functions of the heart lung machine, highlighting its critical role in temporarily taking over the functions of the heart and lungs during cardiac surgery to maintain circulation and oxygenation. The main parts of the machine were examined, including the pump, oxygenator, heat exchanger, and control systems, each essential for effective operation. The principle of operation was discussed, focusing on how the machine circulates and oxygenates blood outside the patient’s body while regulating temperature. Fault diagnosis techniques were presented to identify common mechanical and functional issues that can arise during use. Detailed maintenance procedures were outlined to ensure the machine remains reliable and safe, emphasizing routine checks and component servicing. Safety procedures were stressed throughout the chapter, underscoring protocols to protect both the patient and the medical team. Specifications and recommended equipment ratios for training purposes were also included to support proper handling and skill development. Overall, the chapter provided comprehensive guidance on maintaining the heart lung machine for optimal performance in the operating theatre.