Negative Transmural Pressure Research Articles

Elastic properties of collapsing and expanding trachea.

The compliance of the canine trachea under positive and negative transmural pressures was measured in an in-vivo preparation. The average compliance values found in eight animals were 11.7 X 10(-6) (dynes/cm2)-1 at zero transmural pressure and 4.9 and 6.9 X 10(-6) (dynes/cm2)-1 at -20 and +20 X 10(3) dynes/cm2 transmural pressure, respectively. These compliances were significantly lower than those measured by others in excised preparations. Stress relaxation was noted at all pressure levels.

Characteristics of pressure-wave propagation in a compliant tube with a fully collapsed segment

In order to model the fluid dynamics of Korotkoff sound generation when the artery under the cuff is fully collapsed during most of the heart cycle, the characteristics of pressure-wave propagation in a long silicone-rubber tube were studied experimentally. The central portion of this tube was designed to collapse to zero cross-sectional area as a result of high negative transmural pressure, thus simulating a collapsed artery. Propagation of a single half-sinusoidal pressure wave in and around this segment was studied in detail by pressure, velocity and tube-longitudinal-shape measurements.A very steep wave front (shock wave) capable of producing a short tapping sound was formed by an overtaking phenomenon in the fully collapsed tube segment and it propagated into the inflated tube distal to the collapsed segment. An empirical equation relating the flow rate penetrating into the collapsed segment, the incident-wave pressure and the external pressure Pc over the collapsed segment was obtained. This equation predicts that the pressure-wave propagation in a fully collapsed segment depends only on the flow rate into the collapsed segment.The initial internal pressure of the tube distal to the collapsed segment Pd is one independent variable in the high-cuff-pressure condition. The amplitude of the steep wave front and the shape of the pressure wave in the inflated tube distal to the collapsed segment are governed by Pc–Pd and the flow rate penetrating the collapsed segment. For the same flow rate, if Pc–Pd is lower than a critical value, the amplitude of the pressure in the distal tube decreases with increasing Pd because of positive pressure-wave reflection at the exit of the collapsed segment. On the other had, if Pc–Pd is higher than that value, no wave reflection occurs and the amplitude of the pressure wave is independent of Pd. In the latter case a severe constriction exists near the distal end of the collapsed segment, and flow occurs as two thin high-speed jets.

Pharyngeal compliance in snoring subjects with and without obstructive sleep apnea.

Recent studies have demonstrated a reduction in pharyngeal cross-sectional area and in upper airway muscle tone in patients with obstructive sleep apnea. These findings suggest that the pharynx in such patients may be more compliant than normal even in the awake state. We have tested this hypothesis by examining the pressure-area relationship of the pharynx in 13 patients and in 7 control subjects. Measurements were performed during wakefulness, with the subject seated, and at a constant lung volume near functional residual capacity. Pharyngeal area was measured by an acoustic reflection technique. Pharyngeal pressure was varied by having the subject perform gradual inspiratory and expiratory isovolume maneuvers against a distally occluded airway while mouth pressure was recorded. Specific compliance of the pharynx was calculated as the fractional change in pharyngeal area between a pressure of 0 and -10 cm H2O and and between 0 and 10 cm H2O. Specific pharyngeal compliance was 0.036 +/- 0.004 cm H2O-1 (mean +/- SE) in the control group and 0.094 +/- 0.012 cm H2O-1 in patients with OSA (p less than 0.01). These findings indicate that patients with obstructive sleep apnea have increased pharyngeal compliance. This abnormality predisposes to pharyngeal occlusion during sleep when negative transmural pressures are generated in the pharynx.

Steady, supercritical flow in collapsible tubes. Part 1. Experimental observations

Experimental results are presented for steady, supercritical flow of a liquid in a thin-walled compliant tube which is in a state of partial collapse due to a negative transmural pressure. Particular attention is paid to the effects of longitudinal tension.With a constant external pressure, friction acts to increase the area in the down-stream direction. With the tube tilted downward, friction may be so balanced by gravity forces as to result in an asymptotic approach to an equilibrium situation in which the area and velocity remain constant. When the downstream pressure is increased sufficiently, shock-like transitions to subcritical inflated states, with positive transmural pressure, occur. The longitudinal length scale of the shock is on the order of one to several tube diameters. The pressure rise across the shock lies between that for loss-free pressure recovery and that given by the Borda-Carnot sudden-expansion theory.The presence of longitudinal tension causes a train of standing waves of area to appear upstream of a local area disturbance, such as a shock-like transition. The standing waves are superimposed upon the more gradual area changes associated with friction and gravity. The wave amplitude grows in the downstream direction. Theoretical interpretations of the observations are presented in the companion paper (part 2).

Reflex influences from the extrathoracic trachea during airway occlusion

We have performed experiments in 26 dogs anesthetized with pentobarbital and fitted with an endotracheal tube. The inflatable cuff of this tube was positioned either at the level of the cricoid cartilage or at the thoracic inlet. In this latter situation the extrathoracic trachea (E.T.T.) is not subjected to any change in transmural pressure both during breathing and airway occlusion. We have compared the inspiratory output in term of the integrated phrenic discharge during airway occlusion at FRC with the tracheal tube positioned at either one of the two levels. In most of the experiments (16 out of 26) the inspiratory output during airway occlusion is significantly greater (157%) when the E.T.T. is not by-passed and this different disappears after bilateral vagotomy. We interpret these results by the asymmetrical response of the tracheal stretch receptors to psitive and negative transmural pressure (Pt); most of these receptors are active at FRC and decrease their activity at low negative Pt, as that attained in the first occlued breath. These results seem to suggest that the reflex influences from the extrathoracic tracheal receptors on the inspiratory output are similar to those originating from the intrathoracic airway stretch receptors.

Mechanics of the trachea and behaviour of its slowly adapting stretch receptors.

1. The trachea is constructed by a series of U-shaped cartilaginous rings supporting a membranous posterior wall. We have studied separately the pressure-volume relationships of the two components. 2. The motion of the membranous posterior wall contributes most to the tracheal volume change caused by any given transmural pressure change; the cartilaginous rings provide a semi-rigid support to the posterior wall and have a far greater compliance with negative than positive transmural pressure. 3. The response of tracheal stretch receptors to transmural pressure can be explained by the mechanical coupling between cartilages and posterior wall. They respond both to positive and negative transmural pressure, they are active at zero transmural pressure and have a point of least activity with small negative transmural pressures. 4. The stress-strain relationship of the posterior wall has been studied in static and dynamic conditions in control situations and after removal of either the tunica fibrosa or the trachealis muscle. Each of these two components contributes to the stiffness of the posterior wall, with the trachealis muscle providing most of its viscosity. 5. The response of tracheal stretch receptors to transverse traction of the posterior membranous wall has been studied in both static and dynamic conditions before and after removal of the tunica fibrosa. The behaviour of these receptors reflects the visco-elastic properties of the trachealis muscle in which they have been localized.

Stretch receptors of the trachea.

The slowly adapting stretch receptors of the trachea are uniquely located in its membraneous posterior wall (Figure l) and, more precisely, within the trachealis muscle and are activated by its transversal traction1. The posterior wall of the trachea is supported by the quasi-rigid U-shaped cartilages and the mechanical coupling between the two structures allows the application of tension to the membranous wall and therefore the activation of the stretch receptors when either a positive or a negative transmural pressure is applied across the air (Figure 2). The slowly adapting stretch receptors localized in the trachea represent about 45% of the total number of so-called pulmonary stretch receptors in the dog2 and 33% in the cat3.KeywordsPosterior WallTransmural PressureInspiratory MuscleInspiratory EffortStretch ReceptorThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Steady Flow in Collapsible Tubes

The one-dimensional theory of steady flow in a thin-walled tube, partially collapsed by a negative transmural pressure difference, is developed in a general way. The mechanics of the flow is closely coupled to the mechanics of the tube. The latter is characterized by a “tube law”: the relationship between cross-sectional area and transmural pressure difference. Features analogous to those in gas dynamics and free-surface flow may manifest themselves: a characteristic wave propagation speed; opposite phenomena at flow speeds, respectively, less than and greater than the wave speed; choking; and shocklike transitions. There are many practical examples of such flows, mainly in physiology and medicine. The one-dimensional, steady analysis includes the effects of friction, lengthwise variations in external pressure, variations in elevation, resting area, wall stiffness, and mechanical properties. The speed index S (ratio of flow speed to wave speed), analogous to the Mach and Froude numbers, appears naturally in the results as a controlling parameter of behavior. Various practical ways of passing continuously from subcritical (S < 1) to supercritical (S > 1)speed are suggested. A preliminary theory of shocklike, dissipative transitions is developed, the results of which depend sensitively on the tube law. Explicit working formulas are developed for several simple types of flow (friction alone; changes in rest area alone; changes in external pressure or elevation alone) for a simple, approximate tube law. Various modes of flow behavior for a flow affected by both friction and gravity are explored.

Transduction properties of tracheal stretch receptors.

1. Using single fibre vagal afferent recording, we have studied the behavior of slowly adapting stretch receptors located in an isolated, in situ, segment of the trachea in dogs. Responses to positive and negative steady and oscillating transmural pressures were investigated. 2. Seventy-eight per cent of the receptors studied were tonically active at resting tracheal volume. Ninety per cent showed a more pronounced response to positive than to negative transmural pressures. 3. During pressure oscillations the majority of the receptors had a higher discharge frequency at any given pressure during the ascending phase of the pressure wave than at the same pressure under static conditions. During most of the ensuing descent of pressure toward zero the discharge frequency led transmural pressure. 4. With increasing frequency of oscillation the differences from the static responses increased (dP/dt sensitivity), especially during the ascending limb of the pressure oscillation (rectifying behavior). 5. In a small number of receptors, discharge frequency lagged behind transmural pressure or was in phase with it ("no loop" pattern). 6. In three cases the same receptor exhibited dP/dt sensitivity during positive pressure oscillations, whereas discharge frequency lagged behind pressure during negative pressure oscillations. This indicates that the lack of dP/dt sensitivity exhibited under negative pressure conditions does not represent an intrinsic property of these receptors, but reflects some aspect of their mechanical arrangement within the airway wall. 7. THESE PATTERNS OF RESPONSE ARE DISCUSSed in terms of intrinsic and extrinsic characteristics of the receptors. 8. The physiological implications of stretch receptor behaviour are also considered.

Atrial transmural pressures during experimental pericardial tamponade.

To elucidate the mechanism responsible for the occurrence of negative transmural atrial pressures in pericardial tamponade, saline or air was progressively injected into the pericardial sac. In 14 anesthetized, open-chest dogs, left atrial (LA), left ventricular (LV), and pericardial pressures (PP) were measured with catheter tip manometers. In beating and fibrillated hearts, measurements were carried out at normovolemia, hypovolemia and hypervolemia. Transmural atrial pressure (TAP = LA - PP) became negative when the PP reached 130 mm H 2 O whether the heart was beating or not. In normovolemia the relation between TAP and PP was: TAP (mm H 2 O) = 38.1--0.237PP--0.00042PP 2 . In anesthetized, closed-chest dogs, air inflation of pleural space produced pressures around the heart similar to those obtained in pericardial tamponade. With such pneumothorax, no negative TAP occurred either with beating or quiescent hearts. It is hypothesized that pericardial filling stiffens the pliable structures surrounding the heart and thereby diminishes the collapsibility of the atrial walls which form the inner part of the pericardial sac. Veins entering the atria through the filled pericardial sac are held open like the hole in an inflated inner tube. Through these mechanisms some cardiac filling is assured even with marked pericardial tamponade.

Ventricular Volume of Nonbeating Excised Dog Hearts in the State of Elastic Equilibrium

The volume in the left ventricle and that in the right ventricle were measured statically starting from zero transmural pressure, when the ventricle is in the state of elastic equilibrium, to a negative transmural pressure of 30 mm Hg. Freshly excised dog hearts were submerged in Ringer's solution at 10°C to establish zero transmural pressure, to offset the contribution of the weight of the ventricular walls on the shape of the ventricle, and to retard the onset of postmortem changes. The volume was approximated by an equation which included a power function of the body weight and another of the ventricular weight. For male dogs, the left ventricle when in the state of elastic equilibrium contained a mean volume of 14.5 ml for a body weight of 10 kg and a ventricular weight of 58 g; the corresponding figure for the right ventricle was 12.2 ml. In a quiescent heart below a certain ventricular volume, potential energy will be stored in structural elements of the ventricle wall that will facilitate the filling of the ventricle.

Mechanical forces and pulmonary vascular conductance

Vascular conductance (flow rate per unit of arteriovenous pressure gradient) was studied in the degassed lung lobe in which saline solution was used to fill the airway and the pleural space, and to perfuse the blood vessels at selected arterial and venous pressures. This preparation eliminated effects due to surface, osmotic, and hydrostatic forces, those due to geometric relationships resulting from distention of the lung, and those which result from oscillations of pressures in the airway and in the vessels. All the data obtained could be treated in a consistent manner by considering the airway pressure to be the zero reference level. The phenomenon of critical closing could be demonstrated as a mechanical effect due to negative transmural capillary pressure; thus, when airway pressure exceeded arterial and venous pressures, conductance was zero. As arterial pressure was raised above airway pressure, fluid was displaced from the intralobar (airway) compartment. Conductance was correlated directly with the volume of displacement of fluid from the intralobar compartment. These data indicated that intralobar vessels, presumably the alveolar capillaries, opened when transmural pressure was positive and closed when it was negative. Variations in conductance occurred with changes in transmural pressure. Positive transmural pressures were produced by elevations of arterial or venous pressures, or by a lowering of airway pressures. When intralobar capillary transmural pressure was about +7 cm. of saline, conductance was maximal. Changes in the degree of filling of the lung lobe, or in the volume of fluid in the vessels exposed to the pleural surface, had no significant effects on conductance. Recurrent oscillations of the system were observed at transmural pressure of approximately zero. These oscillations were similar to the patterns of recurrent closure and opening of soft-walled vessels (flitter) noted in previous studies. Conductance was proportional to the period during which the vessel was open (duty cycle). It is shown that marked changes in pulmonary vascular conductance can be produced by mechanical effects at the collapsible intra-alveolar capillary. Before changes in conductance can be attributed to humoral, neurogenic, or pharmacologic factors, the potential mechanical effects on conductance of changes in transmural pressure at the capillary must be controlled.

Ventilatory effects on left atrium and pulmonary vascular resistance.

Pulmonary vascular resistance was studied in thoracotomized dogs enclosed in a respirator. The lung was inflated by respirator pressure, the trachea being open to the atmosphere. Blood flow through the lung was held constant. “Intrapleural” and left atrial pressures fell equally during inspiration; the pulmonary arterial pressure fell to a lesser extent. Calculated resistance increased during the midinspiratory phase and became maximal at end-inspiration and early expiration. Negative-pressure lung inflation had only small effects on resistance at minimal negative respirator pressures; resistance increased markedly as “intrapleural” pressure became progressively more negative. Resistance returned to control values at each end-expiration, regardless of the previous degree of negative-pressure inflation. Resistance increased in isolated lungs under constant flow perfusion when pulmonary venous and “intrapleural” pressures oscillated together; resistance fell when pulmonary venous pressure was held constant with respect to atmospheric pressure. The results suggest that previous disagreements on pulmonary vascular resistance may have been due to failure to evaluate the effect of “intrapleural” pressure on left atrial pressure. The increased resistance during negative-pressure inflation may result from a negative transmural pressure in the collapsible vessels of the lung. Submitted on July 18, 1962

Relation of negative intraventricular pressure to ventricular volume.

Elastic forces of the ventricular walls produce a subatmospheric (negative transmural) pressure in the empty quiescent ventricle of open chest dogs. As fluid is added to the ventricle the negative intraventricular trausmural pressures become 0 and finally positive. The resulting ventricular pressure-volume curves are S-shaped, having one limb in the negative and another in the positive pressure range. It is concluded that previously reported suction and negative intraventricular transmural pressures lasting throughout the entire diastole in ventricles with small residual volumes are caused by elastic forces which also prevail under static conditions.