Extracorporeal blood pumping during heart-lung bypass.

This article describes three different types of extracorporeal blood pumps and the physiology of blood flow and analyzes the circulatory changes introduced by the pumping during heart-lung bypass. Associated clinical problems are briefly discussed.

Vascular resistance and transit time of sickle red blood cells.

Changes in vascular resistance and mean transit time caused by the introduction either normal or sickled red blood cells (SSRBC) into the circulation were measured in the coronary and mesenteric vascular beds. The cat mesentery was perfused at constant flow through the superior mesenteric artery with a Ringer solution containing albumin. A rise in input pressure was observed after introducing RBC's. (10% hematocrit) at a constant rate, into the perfusate. The pressure rise caused by SSRBC's relative to that caused by normal RBC's represents the relative change in vascular resistance caused by the former cells as the perfusion flow was the same for both infusions. Similar measurements were performed in Ringer's perfused rabbit hearts in vitro. SSRBC's produced a 2 +/- 0.12 (M +/- SE) times relative increase in resistance in the mesentery and a 4 +/- 0.5 times increase in the coronary vasculature. Mean transit time, t was measured by monitoring the light transmission change caused by the injection of a small amount of RBC's into the arterial side of a Ringer's perfused rabbit heart. For normal RBC's t was 3.67 +/- 0.08 s and it rose to 6.1 +/- 0.6 s when SSRBC's were used. The simultaneous measurement of both mean transit time and pressure rise may help to identify the cell changes that cause circulatory impairment is SS cell anemia patients and it may also provide a convenient index to relate with the severity of the crisis.

Secondary driving forces affecting transcapillary osmotic flows in perfused heart.

We evaluated secondary hydrostatic and osmotic pressure gradients caused by an osmotic flow across the wall of rabbit heart capillaries in vitro. Tissue fluid loss after addition of 80 mM sucrose to the Ringer perfusate depressed interstitial fluid pressure (IFP) by 13.2 +/- 1.1 cmH2O, as measured with a fine intramyocardial needle. This, plus a calculated rise in capillary pressure, caused a difference of 4.3 cmH2O in transcapillary pressures which represents 1.4% of the simultaneous and opposite osmotic pressure difference. The effect of transient-induced experimental changes in interstitial effect of transient-induced experimental changes in interstitial concentration of NaCl (main resident solute) during an osmotic transient and osmotic balance experiments using opposing transcapillary flows (Jv) and a sigma sucrose = 2.9 sigma NaCl. Long-term perfusion with solutions having abnormal concentrations of NaCl did not influence Jv, whereas inclusions of sucrose as resident solute depressed Jv by 10%. Results indicate that secondary hydrostatic and osmotic forces are small relative to the main osmotic driving force. Compliance measurements and calculations suggest that cells contribute three-fourths organ water loss during an osmotic transient, thus buffering volume, pressure, and concentration changes in the interstitial spaces.

Transcapillary osmotic flows in the in vitro perfused heart.

Hearts were removed from rabbits anesthetized with pentobarbitol sodium, heparinized, and then perfused through the aorta with Ringer solution. Addition of sucrose to the perfusate caused an osmotic transcapillary flow (Jv). Measurements of Jv with two independent methods, one using the rate of organ weight change and the other the variation of effluent concentration of an impermeant dye (Blue dextran), agreed very closely, giving an initial Jv per unit concentration and wet heart weight of 0.306 (microliters/s) (mmol/l).g (corrected for viscosity, 25 degrees C). Because dye dilution was free of vascular volume interference, its agreement with weight measurements suggests that vascular volume changes were relatively small. Measurements using 51Cr-labeled erythrocytes supported the above conclusion. The effect of temperature and concentration on Jv was ascribed to viscosity changes. Organ condition remained stable for several hours, based on maximum ventricular pressure (107 +/- 6.4 cmH2O) and dP/dt (1,145 +/- 98 cmH2O/s) values close to those in blood-perfused rabbit hearts. Repeat weight response to osmotic testing showed approximately 5% variation during an experiment.

Permeability and model testing of heart capillaries by osmotic and optical methods.

Rabbits were anesthetized with Nembutal; their hearts were removed and perfused with Ringers solution. Osmotic weight transients were then produced by test solute infusion into the perfusate. The rate constant for organ weight change was used to predict test solute venous concentration (CV) and the distribution volume (V) during an osmotic transient by use of the Johnson and Wilson (6) model for capillary tissue exchange. By use of Cr EDTA, strongly purple in solution, we compared the above predictions with a value of CV obtained directly by optical measurement of outflow venous concentration and values of V calculated from our measurements of organ weight and the known sucrose distribution volume. The close agreement between both sets of values with a permeability coefficient of 5 x 10(-5) cm/s and a volume of distribution of 3.2 ml/10 g heart lead us to conclude that the model used closly represents the conditions of the isolated perfused heart, and that both the osmotic transient and the extraction measurements provide good estimates of organ capillary permeability.