G. L. Riedel

Autoregulation of canine gastric mucosal blood flow

Although autoregulation of total blood flow has been demonstrated in the stomach, autoregulation of gastric mucosal blood flow has not been investigated due to the limitations of previously available mucosal blood flow measuring techniques. We recently evaluated laser-Doppler velocimetry for use in the stomach and found it to yield continuous, superficial measurements of either mucosal or muscularis blood flow. In the present study, simultaneous measurements of total, mucosal, and muscularis blood flows and arteriovenous oxygen difference were made during step decreases in perfusion pressure in chambered segments of resting canine stomach. As perfusion pressure was decreased from 180 to 35 mmHg, oxygen consumption remained relatively constant at 1.5 ml/min X 100 g and only became blood flow-dependent when perfusion pressure was set below 90 mmHg and total blood flow fell below 35 ml/min X 100 g. Oxygen consumption was maintained partly by increased oxygen extraction; however, there was also a progressive increase in the tendency for total blood flow autoregulation after each drop in perfusion pressure. Mucosal blood flow was also well autoregulated over the physiologic range of perfusion pressure and remained relatively constant during the plateau portion of the oxygen consumption curve. Our results indicate that in the resting stomach, oxygen consumption is maintained by changes in oxygen extraction in conjunction with autoregulation of mucosal and total blood flow.

Local control of canine gastric mucosal blood flow.

The hemodynamic responses to venous pressure elevation and the effect of metabolic stimulation on reactive hyperemia and autoregulation were assessed in chambered segments of the canine gastric corpus perfused by a pressurized reservoir. Arteriovenous oxygen difference, mucosal pH, and total, mucosal, and muscularis blood flow were monitored continuously. Pentagastrin increased acid secretion, oxygen consumption, and both total and mucosal blood flow. Before and after pentagastrin, the magnitude of reactive hyperemia was correlated with the occlusion duration. During stepwise reductions in perfusion pressure, oxygen consumption was relatively constant and blood flow-independent over most of the pressure range. Mucosal blood flow was well maintained except at the lowest perfusion pressure. Pentagastrin did not enhance autoregulation in the mucosa or muscularis, but did enhance the autoregulation of total blood flow. A myogenic vasoconstriction occurred during 20-mmHg venous pressure elevations. Our results indicate that both metabolic and myogenic mechanisms regulate the gastric mucosal circulation.

Evaluation of Light-Emitting Diodes for Whole Blood Oximetry

The difference between the optical extinction coefficients of oxy-and deoxyhemoglobin is a primary concern in the design of blood oxygen instrumentation. In this communication, we report the apparent values for the molar extinction coefficients of hemoglobin under conditions of interest to instrument designers, namely, with whole blood and with unfiltered LED illumination. Extinction coefficient values are reported for four commercially available LEDs with typical spectral half-intensity bandwidths and peak emissions ranging from 660 to 940 nm.

Effects of hemodilution on gastric and intestinal oxygenation

To determine the effects of hemodilution on gastric and intestinal oxygenation, isolated segments of canine stomach and small bowel were perfused by a pressurized reservoir with blood at hematocrits of 40 and 20%. Arteriovenous O2 difference, blood flow, and arterial and venous pressures were monitored continuously as perfusion pressure was reduced in 30-mmHg steps from 180 to 30 mmHg. O2 consumption was calculated as the product of the steady-state arteriovenous O2 difference and blood flow at each perfusion pressure. Gastric and intestinal O2 uptake were relatively well maintained over most of the pressure range when the hematocrit was set at 40%. After hemodilution, gastric O2 uptake decreased significantly only at 90 and 60 mmHg, but intestinal O2 uptake was significantly reduced except at 30 mmHg. When gastric and intestinal O2 uptake were plotted as a function of blood flow, the O2 uptake vs. blood flow relationship were shifted down and to the right by hemodilution. Hemodilution also linearized the O2 uptake vs. blood flow relationship in the intestine. However, when O2 uptake was plotted as function of O2 delivery, the gastric O2 uptake vs. delivery curves at the two hematocrits were superimposed on each other, but the O2 uptake vs. delivery curves for the intestine diverged except at low rates of O2 delivery. We conclude that by reducing the O2-carrying capacity of the blood, hemodilution adversely affects gastric and intestinal oxygenation. Our results also indicate that hemodilution lowers gastric O2 uptake by reducing O2 delivery; however, hemodilution lowers intestinal O2 uptake not only by reducing O2 delivery but also by impairing O2 extraction.