Diffusional arteriovenous shunting in the heart.

Previous indicator dilution experiments in isolated blood-perfused dog hearts suggested that there was intramyocardial diffusional shunting of water relative to a flow-limited solute, antipyrine. Two sets of studies have been done to assess the importance of this shunting, since it implies the possibility of a diffusional bypass for oxygen and other substances, which may be important in ischemia. Nonconsumed tracers were used to show the phenomenon. In the first set, bolus injections of 133Xe dissolved in saline were made into the coronary inflow and the tracer content of the organ recorded by an external gamma detector. The initial Xe washout was disproportionately rapid at low flows, and the late phase was also relatively retarded. In the second set, boluses of cool saline containing indocyanine green were injected into the coronary arterial inflow while coronary sinus outflow dilution curves were recorded via a thermistor and a dye densitometer over a wide range of flows. The thermal curves showed emergence of heat preceding the dye; the degree of precession was much greater at low flows, and, unlike the dye curves, the thermal dilution curves showed dramatic differences in shape at different flows. A model for diffusional countercurrent exchange shows similar changes in residue curves and outflow dilution curves. The conclusion is that there is diffusional shunting of small lipid-soluble molecules whose diffusion coefficients in tissue are high. While the shunting of heat is great, the shunting of soluble gases will not be large and that of normal substrates will be negligible.

Ventilation-perfusion relationship during high-frequency ventilation.

The efficiency of oxygenation and the uniformity of the distribution of regional ventilation (Vr) to regional perfusion (Qr) along the vertical and horizontal axes was compared in anesthetized dogs between conventional mechanical ventilation (CMV) and high-frequency ventilation (HFV) at 5.8, 15.0, and 29.8 Hz. Both CMV and HFV were adjusted to result in similar arterial CO2 tensions. The distribution of Vr/Qr during HFV at 5.8 Hz tended to be more uniform than during HFV at 15.0 or 29.8 Hz or during CMV. Consistent with this observation, arterial O2 tension (PaO2) tended to be higher during HFV at 5.8 Hz (means +/- SD, 90 +/- 9 Torr) than during HFV at 15.0 Hz (83 +/- 9 Torr) or 29.8 Hz (78 +/- 10 Torr); PaO2 was significantly higher during HFV at 5.8 Hz than during CMV (83 +/- 7 Torr).

Gas mixing in the airways of dog lungs during high-frequency ventilation.

Washout of insoluble inert test gases of different diffusivity (He and SF6 or He and Ar) from dog lungs was studied during high-frequency ventilation (HFV). Test gas equilibrium and subsequent washout were performed with HFV, succeeding measurements being performed at different stroke volumes (1.5-2.5 ml/kg body wt), oscillation frequencies (10-30 Hz), and with different lung volumes (32-74 ml X kg-1). Test gas concentrations were continuously measured by a mass spectrometer. The time course of washout could be described as the sum of two exponentials. There were no consistent differences in the time courses of washout between He and SF6 or between He and Ar. It is concluded that gas mixing in the airways during HFV is not significantly limited by diffusion, and this is suggested to apply during HFV to steady-state transport of respiratory gases (e.g., O2 and CO2) as well as to the transient state of inert gas washout.

Long-term high-frequency ventilation in dogs.

The lungs of 11 anesthetized dogs (with or without preexisting lung disease) were ventilated for 36 h by high-frequency, small-volume ventilation (HFV) to determine the effects of HFV on pulmonary gas exchange, mechanical properties of the respiratory system, and function of the cardiovascular system. For comparison, the lungs of 5 other anesthetized dogs without preexisting lung disease were ventilated with conventional mechanical ventilation (CMV) for a similar period. All dogs without preexisting lung disease whose lungs were ventilated with either HFV or CMV maintained satisfactory respiratory and cardiovascular functions for the duration of the study. After 36 h of HFV, no evidence for changes in the mechanical behavior of the lungs was detected. Small amounts of transudates were observed in the pleural space of all dogs ventilated with HFV; no dog ventilated with CMV had pleural effusions.

Gas transport during high-frequency ventilation.

During high-frequency small-volume ventilation (HFV), the transport rate of gas from the mouth to a lung region is a function of two conductances (conductance is the transfer rate of a gas divided by its partial pressure difference): regional longitudinal gas conductance along the airways (Grlongi) and gas conductance between lung regions (Ginter). Grlongi per unit regional lung (gas) volume [Grlongi/(Vr beta g)] was determined during HFV in 11 anesthetized paralyzed dogs lying supine. The distribution of Grlongi/(Vr beta g) was nearly uniform during HFV when stroke volumes were less than approximately two-thirds of the Fowler dead-space volume. By contrast, the distribution of Grlongi/(Vr beta g) was nonuniform when the stroke volume exceeded approximately two-thirds of the Fowler dead-space volume and the oscillation frequency was 5 Hz. Gas conductance along the airways per unit lung gas volume [average Glongi/(V beta g)], for the entire lung, increased with stroke volume at all frequencies, but for a given product of oscillation frequency and stroke volume, the average Glongi/(V beta g) was greater when stroke volume was large and oscillation frequency was low. The average Glongi/(V beta g) increased with frequency up to a maximal value; the frequency at which the maximum occurred depended on the kinematic viscosity of the inspired gas mixture.

Intrapulmonary gas transport and perfusion during high-frequency oscillation.

The effects of high-frequency oscillation (HFO) on 1) regional pulmonary 133Xe clearance after equilibration, 2) regional distribution and subsequent clearance of 133Xe after right atrial bolus injection, and 3) pulmonary gas exchange were examined in anesthetized supine dogs. After equilibration 133Xe cleared similarly from all lung regions with HFO at 16 and 30 Hz and a stroke volume of 2.6 ml/kg. Pulmonary gas exchange was adequate. 133Xe, injected as a bolus into the right atrium, was preferentially distributed to dependent lung regions during both HFO and apnea, indicating vertical gradients in pulmonary perfusion. During the subsequent pulmonary clearance of 133Xe, regional 133Xe concentrations (CrXe) increased initially in nondependent regions. By contrast, CrXe decreased immediately in the dependent lung region; after CrXe's became similar in dependent and nondependent regions, all lung regions started to clear at similar rates. The initial increases of CrXe in nondependent regions were attributed to interregional mixing, which may contribute to the uniformity in regional pulmonary 133Xe clearance after equilibration.

Inspiratory flow and intrapulmonary gas distribution.

The effect of flow of inspired gas on intrapulmonary gas distribution was examined by analysis of regional pulmonary 133Xe clearances and of total pulmonary 133Xe clearance measured at the mouth after equilibration of the lungs with 133Xe. Five awake healthy volunteers (24 to 40 yr of age) and another 5 healthy, anesthetized-paralyzed volunteers (26 to 28 yr of age) were studied while they were in the right lateral decubitus position. The awake subjects were studied at 3 inspiratory flows (0.4, 0.7, and 1.0 L/s) and the anesthetized-paralyzed subjects at 4 inspiratory flows (0.2, 0.5, 1.1, and 1.6 L/s). Interregional differences in 133Xe clearances along the vertical axis were significantly less during anesthesia-paralysis and mechanical ventilation than during spontaneous breathing in the awake state. No differences in the regional or total pulmonary 133Xe clearances were detected at these different flows in either of the two states, i.e., the difference between the awake and anesthetized-paralyzed states persisted.

Chest wall motion and distribution of inspired gas in anesthetized supine dogs.

Changes in the anterior-posterior (AP) and lateral diameters of the rib cage and abdomen were assessed by magnetometry in seven anesthetized supine dogs during spontaneous respiration (SR) and mechanical ventilation after muscle paralysis (MV). Regional distribution of inspired gas was measured for both modes of ventilation by determining regional 133Xe clearances. Marked differences in chest wall motion were observed between SR and MV: during MV, the changes in lateral rib cage diameter from FRC to end inspiration were larger, and the changes in both abdominal diameters smaller than during SR. AP rib cage diameter changes were similar for both modes of ventilation. Inward motion of the lateral rib cage during initial inspiration was observed in four dogs during SR; it disappeared consistently with MV. Regional 133Xe clearances were not significantly different: there was no cephalocaudal gradient, and the vertical gradient in regional ventilation was similar with MV and SR. We conclude that significant changes in chest wall motion and shape are not necessarily associated with detectable differences in the distribution of regional ventilation.

Ventilation-perfusion relationship in young healthy awake and anesthetized-paralyzed man.

Distributions of ventilation and perfusion relative to Va/Q were determined in seven young healthy volunteers (24-33 yr) while they were either in the supine or right lateral decubitus position. The subjects were studied first awake and then while anesthetized-paralyzed and breathing 30% oxygen and again while breathing 100% oxygen. In the awake state, no statistically significant differences were observed in the distribution of ventilation and perfusion relative to Va/Q between the supine and right lateral decubitus positions or on changing the inspired oxygen concentrations. After induction of anesthesia-paralysis, Va/Q mismatching increased significantly but only small right-to-left intrapulmonary shunts developed. Ventilating the lungs with 100% oxygen further increased the dispersion of blood flow distribution during anesthesia-paralysis; lung units with low Va/Q or right-to-left intrapulmonary shunts (or both) developed. With induction of anesthesia-paralysis and intubation of the trachea, the anatomic dead space was decreased and the alveolar dead space increased.

Regional intrapulmonary gas distribution in awake and anesthetized-paralyzed prone man.

The intrapulmonary distribution of inspired gas (ventilation/unit lung volume, VI), functional residual capacity (FRC), closing capacity (CC), and the slope of phase III were determined in five awake and five anesthetized-paralyzed volunteers who were in the prone position with the abdomen unsupported. After induction of anesthesia-paralysis, FRC was less in four of five subjects and CC was consistently less. At FRC there was no difference in the vertical gradient of regional lung volumes between the awake and anesthetized-paralyzed prone subjects. Also, there was no difference in VI between the two states. The normalized slope of phase III decreased consistently with induction of anesthesia-paralysis, but the vertical distribution of a 133Xe bolus inhaled from residual volume was not different between the two states. The data of the study are compatible with 1) a pattern of expansion of the respiratory system during anesthesia-paralysis and mechanical ventilation different than that during spontaneous breathing and 2) a more uniform intraregional distribution of inspired gas and/or a different sequence of emptying during anesthesia-paralysis.

Nitrous oxide exposure in the operating room.

One-hundred and eight-five pairs of gas samples were collected from inspired gas (10 cm behind the head at nose level) and end-tidal gas of persons administering anesthesia in 3 operating rooms during daily routine anesthesia. Mean operating-room N2O concentrations from 22 to 144 ppm (volume/volume [V/V]) were measured by gas chromatography, and large moment-to-moment variations (temporal gradients) were seen in individual operating rooms. Mean end-tidal N2O concentrations from 51 to 114 ppm (V/V) were observed. There were low correlations between inspired and end-tidal N2O concentrations (r values as low as r = 0.35). This poor relationship is presumably due to spatial and temporal gradients of N2O in the operating rooms. We conclude that the temporal and spatial gradients in N2O concentrations within active operating rooms are sufficiently large to invalidate estimation of exposure of anesthetic personnel to N2O from "spot" or "grab" samples collected in the breathing area.

Closing capacity in awake and anesthetized-paralyzed man.

Functional residual capacity (FRC), closing capacity (CC), and (FRC--CC) were determined in 61 supine patients using the 133Xe bolus test. In 28 of the 61 patients measurements were made both while the patients were awake and during anesthesia-paralysis. Both FRC and CC decreased significantly after induction of anesthesia-paralysis. The magnitude of the reduction in CC, but not of FRC, was dependent on the relationship between FRC and CC in the awake state. Patients whose FRC was larger than their CC while awake (group I) showed less decrease in CC than FRC, i.e., (FRC--CC) decreased. By contrast, those patients whose CC was larger than their FRC while awake (group II) showed a greater decrease in CC than in FRC, i.e., (FRC--CC) became less negative. The reduction in CC after induction of anesthesia-paralysis may result from an increased elastic recoil of the lung. The larger reduction in CC in group II patients may have been due to a larger increase in elastic recoil, possibly due to the development of atelactasis.

COMPUTER GRAPHICS IN SIMULATION OF CARDIOVASCULAR TRANSPORT PHENOMENA.

Simulation is a necessary tool if we are to understand better the complexities involved in cardiovascular transport. While some of the phenomena modeled can be described analytically, perusal of the equations alone often doesn't result in full appreciation of the model system. It therefore becomes pertinent to utilize computer graphics in order to enhance simulation of physiologic transport processes. Graphic representation not only facilitates interaction between the investigator and the simulation, it provides a juxtaposition of the model to the real system, as well as a simplification of relationships between various features of the model.Increased mathematical sophistication required in the investigation of cardiovascular transport phenomena often makes traditional graphic representation cumbersome. Therefore several different types of graphics have been utilized, including 2-, 3-, and 4-dimensional displays. The methods and algorithms for these displays have been generalized to make them easy to use over a broad spectrum of applications. In some cases we have generated motion pictures of sequential model solutions which have increased and accelerated model comprehension, as well as been valuable for teaching purposes.