In addressing the subject of optimal perfusion flow rates for cardiopulmonary bypass, we must be reminded of a remark made by a pioneer in the physiology of cardiac surgery, Dr. Frank Gollan, who said \"We cannot command nature except by obeying her\" ( 1 ). During extracorporeal circulation (ECC), we try to closely approximate the physiologic state, \"but we never duplicate.\" During my travels as a perfusionist, both in the United States and abroad, I have noted that flow rates during cardiopulmonary bypass vary widely from a low flow ofl.6l/m/min in adults to a high flow of over 3.0 l/m/min in infants and children; flow rates were not written in stone and handed down to us. Flow rates are \"optimal,\" thus the variations we see are appropriate to the time, place and circumstances that prevail. Historical Considerations The subject of \"optimal perfusion flow rates\" for cardiopulmonary bypass must first be approached from a historical perspective and examined in the context of hypothermia, arterial pressure, hemodilution and pulsatility of flow. Hypothermia has played a role in medicine over a long time. Hippocrates (2), wrote that cold produces a \"moderate degree of numbness,\" and \"relieves pain,\" to various practitioners of early medicine who(3, 4) used local hypothermia by applying snow to the body for various aliments and surgical procedures. Currie (5), in 1798, reported the use of hypothermia in the treatment of a patient with a high fever by immersion in an icy cold bath. In some of the experiments he conducted, volunteers were placed in ice cold baths and cooled to an oral temperature of 32°C, the process was then reversed with warm baths. His work was the forerunner of the first method of hypothermia used in cardiac surgery. In 1950, Bigelow, et al. (6, 7, 8) published their findings on the study of hypothermia in dogs as an aid to intracardiac surgery. He hypothesized that if the body's metabolism, especially the cardiac and cerebral metabolism, could be reduced by lowering the entire body temperature, circulation could be arrested for a time without permanent damaging effects. It was discovered that if the metabolism is reduced by Address correspondences to: Bennett A. Mitchell, 2865 Kingsbridge Terrace, Bronx, NY 10463 Volume 22, Number 4, Spring 1991 10%, then the oxygen demands of the organs would be also reduced by the same proportion. Therefore, it will take more time than in the normothermic state to become hypoxic following circulatory arrest. These classic experiments proved that the body temperature could be lowered and reversed without incurring an oxygen debt. This work was followed by Boerema (9), who in 1951 added a new dimension to the fledgling art of intracardiac surgery. In 1952, Lewis and Tauffic (1 0) performed the first operation using hypothermia on a child with an atrial septal defect, employing cin;:ulatory arrest for a period of five minutes. This procedure was followed by Swan (11) who performed his first 35 cases without mortality. Hypothermia Hypothermia causes a variety of physiologic, hemodynamic, and rheologic changes in human beings. Changes in anesthetic requirements occur due to analgesic effects on the neuromuscular system, pain perception, consciousness and coordination (12, 13). Variations also occur in metabolic rates in patients undergoing cardiopulmonary bypass (CPB). Hypothermia induces three basic changes in gas transport: 1. The solubility of gases in liquids is inversely related to temperature so substantially more oxygen and carbon dioxide will be carried in physical solution in the blood under hypothermic conditions. 2. As the blood temperature is lowered, the oxyhemoglobin dissociation curve shifts to the left so that for a given partial pressure of oxygen in the tissues, less of the gas is unloaded from the hemoglobin. 3. Decreasing temperatures increase the carrying power of blood for carbon dioxide through an increased activity of the blood buffers. The animal cell depends upon the utilization of energy for its viability. This energy is supplied by a complex breakdown of several organic molecules that requires a constant supply of oxygen and results in the formation of carbon dioxide and water. Together with smaller metabolites of other end products, they are immediately excreted from the cell. This process depends upon a supply of oxygen to enhance the removal of waste products. The lack of oxygen does not stop the production of energy, however a transition to anaerobic metabolism takes The Journal of Extra-Corporeal Technology 165 place, which allows for an accumulation of metabolites in the breakdown process to occur. Anaerobic conditions are limited because of an accumulation of these metabolites in the breakdown process, which are mainly acidic in origin and will result in a state of metabolic acidosis. In brief, then: hypothermia retards the metabolic process in cells and protects them from anoxic death. The degree of retardation is dependent in large extent to the degree of hypothermia employed. The metabolic rates of organs vary and hence their anoxic survival time, which corresponds with the duration and depth of temperature.

Carbon dioxide, waste product or elixir?

Optimal perfusion flow rates for cardiopulmonary bypass

The purpose of this chapter is to identify the factors that play determinants roles in affecting the blood flow during cardiopulmonary bypass (CPB). Under normal physiological conditions, blood is contained within the circulatory system which is closed circuit. The system is influenced by many factors will be presented as the initial step of this discussion.

Carbon dioxide, waste product or elixir?

Citations

Optimal perfusion flow rates for cardiopulmonary bypass

Physiology of blood flow during cardiopulmonary perfusion

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