In vivo murine left ventricular pressure-volume relations by miniaturized conductance micromanometry

The mouse is the species of choice for creating genetically engineered models of human disease. To study detailed systolic and diastolic left ventricular (LV) chamber mechanics in mice in vivo, we developed a miniaturized conductance-manometer system. alpha-Chloralose-urethan-anesthetized animals were instrumented with a two-electrode pressure-volume catheter advanced via the LV apex to the aortic root. Custom electronics provided time-varying conductances related to cavity volume. Baseline hemodynamics were similar to values in conscious animals: 634 +/- 14 beats/min, 112 +/- 4 mmHg, 5.3 +/- 0.8 mmHg, and 11,777 +/- 732 mmHg/s for heart rate, end-systolic and end-diastolic pressures, and maximum first derivative of ventricular pressure with respect to time (dP/dtmax), respectively. Catheter stroke volume during preload reduction by inferior vena caval occlusion correlated with that by ultrasound aortic flow probe (r2 = 0.98). This maneuver yielded end-systolic elastances of 79 +/- 21 mmHg/microliter, preload-recruitable stroke work of 82 +/- 5.6 mmHg, and slope of dP/dtmax-end-diastolic volume relation of 699 +/- 100 mmHg.s-1.microliter-1, and these relations varied predictably with acute inotropic interventions. The control normalized time-varying elastance curve was similar to human data, further supporting comparable chamber mechanics between species. This novel approach should greatly help assess cardiovascular function in the blood-perfused murine heart.

Genetic control of differential baseline breathing pattern

The purpose of the present study was to determine the genetic control of baseline breathing pattern by examining the mode of inheritance between two inbred murine strains with differential breathing characteristics. Specifically, the rapid, shallow phenotype of the C57BL/6J (B6) strain is consistently distinct from the slow, deep phenotype of the C3H/HeJ (C3) strain. The response distributions of segregant and nonsegregant progeny were compared with the two progenitor strains to determine the mode of inheritance for each ventilatory characteristic. The BXH recombinant inbred (RI) strains derived from the B6 and C3 progenitors were examined to establish strain distribution patterns for each ventilatory trait. To establish the mode of inheritance, baseline breathing frequency (f), tidal volume, and inspiratory time (TI) were measured five times in each of 178 mature male animals from the two progenitor strains and their progeny by using whole body plethysmography. With respect to f and TI, the two progenitor strains were consistently distinct, and segregation analyses of the inheritance pattern suggest that the most parsimonious genetic model for response distributions of f and TI is a two-loci model. In similar experiments conducted on 82 mature male animals from 12 BXH RI strains, each parental phenotype was represented by one or more of the RI strains. Intermediate phenotypes emerged to confirm the likelihood that parental strain differences in f and TI were determined by more than one locus. Taken together, these studies suggest that the phenotypic difference in baseline respiratory timing between male B6 and C3 mice is best explained by a genetic model that considers at least two loci as major determinants.

Three-dimensional structure of the bronchial microcirculation in sheep

BACKGROUND

The bronchial circulation affects both pulmonary vascular and airway activity. Fundamental to understanding the role of the bronchial microcirculation in health and disease is understanding its anatomy. This study sought to identify specific structural elements that might contribute to the drop that occurs between the systemic blood pressure of the bronchial artery and the low pressure of the pulmonary bed into which the bronchial circulation flows and to better describe the connections of the bronchial and pulmonary circulations.

METHODS

To do this, the lungs of five sheep were cast by injecting a resin through bronchial and pulmonary arteries. After taking samples for light microscopy, the tissue was digested and the casts were viewed with a scanning electron microscope.

RESULTS

Casts of extrapulmonary bronchial arteries were structurally similar to other systemic arteries. Tortuous ones spiraled around bronchi and large blood vessels. Intrapulmonary bronchial arteries, about 100-300 microns in diameter, had sharp branching and deep focal constrictions with great rugosity that completely shut off the flow of the resin. These vessels correspond to the Sperrarterien described by von Hayek (and could cause the resistance associated with the pressure drop). Vasa vasorum ran in the walls of intrapulmonary pulmonary arteries for a variable distance before they entered the lumens of the pulmonary arteries. The smallest blood vessel found that was supplied with vasa vasorum was a bronchial artery 42 microns in diameter. Capillary-like networks with large luminal diameters were found on the pleural surface.

CONCLUSIONS

Scanning electron microscopy of microvascular casts provides a fresh description of the bronchial circulation, further delineates the communications of these two circulations, and may structurally account for some pressure drop between the bronchial and pulmonary circulations.

Contribution of pulmonary versus systemic perfusion of airway smooth muscle

Recent studies suggest a significant contribution of the pulmonary circulation to the perfusion of large airways. In this study we used anesthetized ventilated sheep (n = 19) to determine the functional contribution of the pulmonary circulation to airway smooth muscle. We performed sequential intravenous challenge with methacholine chloride (MCh; 0.25-2.5 mg/ml) to determine airway resistance (Raw) changes in the intact animal, after bronchial artery cannulation that essentially removed bronchial arterial delivery of MCh, and in an isolated lung preparation. After blocking the vagal reflex component of this response, we found that intravenous MCh in the intact preparation resulted in an average 2.2 +/- 0.5 cmH2O.l-1.s increase (181%) in Raw. After prevention of bronchial arterial delivery of MCh, Raw increased by 0.8 +/- 0.3 cmH2O.l-1.s (64%; P < 0.01 compared with intact preparation). In the isolated lung preparation, Raw increased by 0.6 +/- 0.2 cmH2O.l-1.s (63%; P < 0.01 compared with intact preparation). These results demonstrate that in sheep, the bronchial artery provides the major route for delivery of intravenously administered agonists to airway smooth muscle. Considering the large dilutional effect of an intravenously administered agonist by the time it reaches the bronchial artery, we conclude that the pulmonary component of agonist delivery to large airways is < 10% and unlikely to play a major physiological role.

Proposed nomenclature for quantifying subdivisions of the bronchial wall

There is increasing interest in the structural components of the airway wall because of the airway remodeling that is observed in conditions such as asthma and chronic obstructive pulmonary disease and because of their contribution to changes in airway mechanics. This interest has stimulated several groups to make morphometric measurements on airway cross sections, and their results have been reported using a variety of nomenclature. We propose the adoption of a standard system of nomenclature that is based on accepted terms for subdivisions of the airway wall and has been agreed to by several groups working in this field.

Hypercapnic ventilatory responses in mice differentially susceptible to acute ozone exposure

Susceptibility to ozone (O3)-induced pulmonary inflammation is greater in C57BL/6J (B6) than in C3H/HeJ (C3) strain of mice. We tested the hypothesis that altered ventilatory control occurs in B6 mice to a greater extent than in C3 mice after acute O3 exposure. Age-, sex-, and weight-matched C3 and B6 mice were exposed for 3 h to either 2 ppm O3 or filtered air. One and 24 h after O3 or air exposure, whole body plethysmography was used to measure breathing frequency (f), tidal volume (VT), and minute ventilation (VE). To assess changes in ventilatory control, mice were challenged by the elevation of fractional concentration of inspired CO2 levels to 5 and 8% in air for 10 min. After air exposure, there were significantly (P < 0.01) greater changes in VE in B6 than in C3 mice. Hypercapnia-induced changes in VE were significantly (P < 0.01) attenuated in B6 mice 1 h after O3 exposure. VT was significantly (P < 0.01) reduced 1 h after O3 in B6 and C3 mice; however, C3 mice increased f to sustain the hypercapnic VE response similar to air exposure. In contrast, the diminished VT in B6 mice 1 h after O3 occurred coincident with significantly (P < 0.01) reduced f, mean inspiratory flow, and slope of VE-to-%CO2 relationship compared with air exposure. Altered hypercapnic VE in B6 mice was partially reversed 24 h after O3 relative to air-exposed levels. These data suggest that control of ventilation during phenotypic response to CO2 is governed, in part, by genetic factors in inbred strains of mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral chemoreceptor modulation of the pulmonary vasculature in the cat

The present study was undertaken to determine whether stimulation of the carotid and aortic bodies (cb and ab) could affect the pulmonary vasculature. Our hypothesis was that each promoted vasodilation and thus could modulate the pulmonary vasoconstrictor response to hypoxia. The experimental design of the first set of experiments took advantage of the facts that 1) the ab, but not the cb, increases its neural output in response to CO, whereas both respond to a decreased arterial PO2 (hypoxic hypoxia, HH) and 2) the aortic nerves in cats are easily transected. Hence, both cb and ab sent neural activity to the brain stem when the intact cat was exposed to 10% O2 in N2. Only the ab sent information during CO hypoxia (COH intact). Only the cb did so during HH in the cat in which the aortic nerves had been transected, removing the aortic body (HH abr); neither ab nor cb did so during COH abr. Fifteen anesthetized paralyzed artificially ventilated cats were fit with catheters in the femoral artery and vein, right and left atria, left ventricle, and pulmonary artery and with an aortic flow probe. In the HH intact and HH abr conditions, there was a significant rise in cardiac output, whereas pulmonary arterial pressure (Ppa) rose initially but then leveled off while cardiac output continued to rise. During the 15-min exposure to HH, pulmonary vascular resistance [PVR = (Ppa - Pla)/cardiac output, where Pla is left atrial pressure] rose initially and then decreased significantly at 2-3 min. In response to COH, PVR showed only a significant decrease. In the second set of experiments, seven cats were instrumented as above and had loops placed in the common carotid arteries for selectively perfusing the cbs. In response to a brief infusion of venous blood mixed with 0.3-0.5 micrograms NaCN, which selectively stimulated only the cb, aortic flow remained relatively constant while heart rate and Ppa - alveolar pressure difference decreased significantly; so also did PVR. These data are consistent with the hypothesis that stimulation of the ab and cb singly or together can provoke a significant pulmonary vasodilation in the anesthetized paralyzed artificially ventilated cat.

Bronchial circulatory reversal of methacholine-induced airway constriction

Although a role for the bronchial circulation in clearance of bronchoactive agents has been frequently proposed, experimental evidence is limited. In this study, we determined the importance of bronchial blood flow (QBA) in the recovery from methacholine-(MCh) induced bronchoconstriction. In 10 pentobarbital-anesthetized ventilated sheep, the bronchial branch of the bronchoesophageal artery was cannulated and perfused (0.7 ml.min-1.kg-1) with blood pumped from the femoral artery. MCh was infused directly into the bronchial artery at increasing concentrations (10(-7) to 10(-5) M). MCh infusion caused a concentration-dependent increase in airway resistance at constant QBA. However, the time constant of recovery (TC) from airway constriction after cessation of the MCh infusion was not dependent on the MCh concentration or the magnitude of the increases in airway resistance. When QBA was at 50, 100, and 200% of control level, with constant MCh concentration, TC was 44 +/- 6, 25 +/- 2, and 24 +/- 2 (SE) s at each flow level, respectively. TC at 50% of control QBA was significantly greater than at control QBA (P less than 0.01). Thus the magnitude of QBA can alter the time course of recovery from MCh-induced increases in airway resistance. These results document the importance of QBA in reversing agonist-induced constriction and suggest that an impaired bronchial circulation may contribute to the mechanism of airway hyperreactivity.

Effect of left atrial pressure on bronchial vascular hemodynamics

We studied the bronchial vascular response to downstream pressure elevation by increasing left atrial pressure (Pla) and mean airway pressure (Paw) with positive end-expiratory pressure (PEEP). In seven pentobarbital-anesthetized ventilated sheep, we cannulated and perfused the bronchial branch of the bronchoesophageal artery. Steady-state bronchial artery pressure- (Pba) flow (Qba) relationships were obtained as Pla was increased by inflating a balloon catheter in the left atrium. Bronchial vascular resistance (BVR), determined by the inverse slope of the Pba-Qba relationship, increased significantly from 3.2 +/- 0.3 (SE) mmHg.ml-1.min-1 at a Pla of 2.9 +/- 0.7 mmHg to 5.1 +/- 0.5 mmHg.ml-1.min-1 at a Pla of 20.1 +/- 2.0 mmHg (P = 0.0007). Under control Qba (23.3 +/- 1.2 ml/min), these changes in BVR represent a 3.6 +/- 0.7-mmHg increase in Pba per mmHg increase in Pla. The zero-flow pressure increased 1.3 +/- 0.2 mmHg/mmHg increase in Pla. After infusion of papaverine, a smooth muscle paralytic agent, directly into the bronchial artery, BVR decreased significantly to 1.3 +/- 0.7 mmHg.ml-1.min-1 (P = 0.0004). Under these dilated conditions, BVR was unaltered by increases in Pla. After papaverine administration, Pba increased 0.9 +/- 0.1 and 1.2 +/- 0.1 mmHg/mmHg increase in Pla during control and zero-flow conditions, respectively. Thus the effect of Pla elevation on BVR appears to be dependent on active smooth muscle responses. Paw elevation had similar effects on Pba. Under control Qba, Pba increased 2.2 +/- 0.4 mmHg/mmHg increase in Paw.(ABSTRACT TRUNCATED AT 250 WORDS)

Diurnal variation of lung reactivity in normal and myopathic hamsters

In this study we measured the diurnal variation in lung reactivity in normal and myopathic hamsters. Lung reactivity to an intravenous bolus of 0.7 mg/kg acetylcholine (ACh) was measured as the change in peak airway pressure of anesthetized ventilated animals. In the normal hamsters, lung reactivity was 57% higher during the day (8 A.M., 12 P.M., and 4 P.M.) than during the night (p = 0.01). This increased reactivity was not associated with any changes in baseline pressures. The lung reactivity was correlated to body activity as measured on an electronic activity monitor over seven consecutive days. The hamster, being a nocturnal forager, gradually increased activity at about 6 P.M. and maintained intermittent activity until about 6 A.M. Sleep occurred between 6 A.M. and 6 P.M., and this was when the lung reactivity was greatest. In the myopathic hamsters, although the magnitude of the response to ACh was about 40% lower than that in normal animals, the lung reactivity during the daytime was still about 46% greater than that during the night (p = 0.007). The body activity records from the myopathic animals showed that these animals do not have a normal sleep pattern during the daytime; sleep is not continuous, showing intermittent periods of physical activity. Our results in the normal animals are consistent with observations in man, which show greater problems with airway obstruction and asthmatic attacks during the night.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hypoxia on bronchial circulation

We studied the effect of systemic hypoxia on the bronchial vascular pressure-flow relationship in anesthetized ventilated sheep. The bronchial artery, a branch of the bronchoesophageal artery, was cannulated and perfused with a pump with blood from a femoral artery. Bronchial blood flow was set so bronchial arterial pressure approximated systemic arterial pressure. For the group of 25 sheep, control bronchial blood flow was 22 ml/min or 0.7 ml.min-1.kg-1. During the hypoxic exposure, animals were ventilated with a mixture of N2 and air to achieve an arterial PO2 (PaO2) of 30 or 45 Torr. For the more severe hypoxic challenge, bronchial vascular resistance (BVR), as determined by the slope of the linearized pressure-flow curve, decreased acutely from 3.8 +/- 0.4 mmHg.ml-1.min to 2.9 +/- 0.3 mmHg.ml-1.min after 5 min of hypoxia. However, this vasodilation was not sustained, and BVR measured at 30 min of hypoxia was 4.2 +/- 0.8 mmHg.ml-1.min. The zero flow intercept, an index of downstream pressure, remained unaltered during the hypoxic exposure. Under conditions of moderate hypoxia (PaO2 = 45 Torr), BVR decreased from 4.6 +/- 0.3 to 3.8 +/- 0.4 mmHg.ml-1.min at 5 min and remained dilated at 30 min (3.6 +/- 0.5 mmHg.ml-1.min). To determine whether dilator prostaglandins were responsible for the initial bronchial vascular dilation under conditions of severe hypoxia (PaO2 approximately equal to 30 Torr), we studied an additional group of animals with pretreatment with the cyclooxygenase inhibitors indomethacin (2 mg/kg) and ibuprofen (12.5 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of airway pressure on bronchial blood flow

We studied the effects of increased airway pressure caused by increasing levels of positive end-expiratory pressure (PEEP) on bronchial arterial pressure-flow relationships. In eight alpha-chloralose-anesthetized mechanically ventilated sheep (23-27 kg), the common bronchial artery, the bronchial branch of the bronchoesophageal artery, was cannulated and perfused with a pump. The control bronchial blood flow (avg 12 +/- 1 ml/min or 0.48 ml X min-1 X kg-1) was set to maintain mean bronchial arterial pressure equal to systemic blood pressure. Pressure-flow curves of the bronchial circulation were measured by making step changes in bronchial blood flow, and changes in these curves were analyzed with measurements of the pressure at zero flow and the slope of the linearized curve. The zero-flow pressure represents the effective downstream pressure, and the slope represents the resistance through the bronchial vasculature. At a constant bronchial arterial pressure of 100 mmHg, an 8 mmHg increase in mean airway pressure caused a 40% reduction in bronchial blood flow. Under constant flow conditions, increases in mean airway pressure with the application of PEEP caused substantial increases in bronchial arterial pressure, averaging 4.6 mmHg for every millimeters of mercury increase in mean airway pressure. However, bronchial arterial pressure at zero flow increased approximately one-for-one with increases in mean airway pressure. Thus the acute sensitivity of the bronchial artery to changes in mean airway pressure results primarily from changes in bronchovascular resistance and not downstream pressure.

Peripheral circulatory alterations in canine anaphylactic shock

We have examined peripheral circulatory variables that might contribute to the decrease in cardiac output and arterial pressure characteristic of anaphylactic shock. In six dogs instrumented with a right heart bypass, the intravenous administration of Ascaris suum antigen caused a 53% decrease in cardiac output and a 58% decrease in arterial pressure. Resistance to venous return increased from 0.0038 +/- 0.0004 to 0.0056 +/- 0.0006 mmHg X ml-1 X min (P less than 0.05), mean systemic pressure decreased from 7.0 +/- 0.6 to 4.4 +/- mmHg (P less than 0.005), and vascular compliance did not change. Assuming a constant vascular volume, the decrease in mean systemic pressure could be explained by a rightward shift of the systemic pressure volume curve. This constant-volume assumption was tested in intact (n = 6) and splenectomized dogs (n = 7). Serial measurements of protein oncotic pressure and hematocrit were used to estimate plasma volume changes during anaphylaxis. Both methods for estimating volume showed small increases in plasma volume at the time of the largest decrease in arterial pressure in both groups of animals. These results suggest that the primary circulatory mechanisms responsible for anaphylactic shock are an increase in resistance to venous return and a shift of the systemic pressure volume curve and not an acute loss of plasma volume.

Assessment of fluid balance in isolated sheep lungs

In this study we demonstrate the validity and utility of an isolated lung preparation developed for the study of pulmonary fluid balance. Lungs of 2- to 3-mo-old sheep were perfused in situ with autologous blood treated with indomethacin (20 micrograms/ml). Lung lymph flow (QL), uncontaminated by systemic lymph, was measured from either the efferent duct of the caudomediastinal lymph node or the thoracic duct in the superior mediastinum. Lung weight change (delta W) was measured as the opposite of the change in weight of the extracorporeal blood reservoir. A unique feature of this experimental model is the ability to assess lung fluid balance from simultaneous measurements of delta W and QL. In addition, hemodynamic and blood gas variables can be tightly controlled. Our results show that changes in QL and the lymph-to-plasma oncotic pressure ratio caused by an increase in microvascular pressure were comparable with those seen previously in intact sheep. When microvascular pressure was returned to control levels, QL fell despite a sustained increase in the amount of extravascular lung water, suggesting compartmentalization of the filtrate and/or effects of intravascular volume on lymph-driving pressure or resistance. Lymph flow was directly proportional to respiratory frequency over the range of 0-30 min-1 when the change in frequency was maintained for periods as long as 30 min. This preparation should prove useful in the study of lung fluid balance, particularly when it is desired to use interventions which are precluded or difficult in intact animals.

Effects of imidazole and indomethacin on fluid balance in isolated sheep lungs

We determined the effects of extracorporeal perfusion with a constant flow (75 ml . min-1 . kg-1) of autologous blood on hemodynamics and fluid balance in sheep lungs isolated in situ. After 5 min, perfusate leukocyte and platelet counts fell by two-thirds. Pulmonary arterial pressure (Ppa) increased to a maximum of 32.0 +/- 3.4 Torr at 30 min and thereafter fell. Lung lymph flow (QL), measured from the superior thoracic duct, and perfusate thromboxane B2 (TXB2) concentrations followed similar time courses but lagged behind Ppa, reaching maxima of 4.1 +/- 1.2 ml/h and 2.22 +/- 0.02 ng/ml at 60 min. Lung weight gain, measured as the opposite of the weight change of the extracorporeal reservoir, and perfusate 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) concentration increased rapidly during the first 60 min and then more gradually. After 210 min, weight gain was 224 +/- 40 g and 6-keto-PGF1 alpha concentration, 4.99 +/- 0.01 ng/ml. The ratio of lymph to plasma oncotic pressure (pi L/pi P) at 30 min was 0.61 +/- 0.06 and did not change significantly. Imidazole (5 mM) reduced the changes in TXB2, Ppa, QL, and weight and platelet count but did not alter 6-keto-PGF1 alpha, pi L/pi P, or leukocyte count. Indomethacin (0.056 mM) reduced TXB2, 6-keto-PGF1 alpha, and the early increases in weight, Ppa, and QL but did not alter the time courses of leukocyte or platelet counts. Late in perfusion, however, Ppa and QL were greater than in either untreated or imidazole-treated lungs.(ABSTRACT TRUNCATED AT 250 WORDS)

The beneficial and harmful effects of positive end expiratory pressure

The effects of increasing levels of positive end expiratory pressure on gas exchange and pulmonary mechanics were determined utilizing an ex vivo ventilated perfused canine pulmonary lobe. When zero positive end expiratory pressure was used, shunting, weight gain and a decrease in compliance occurred over the four and one-half hour experiment. Shunting was eliminated when 5, 10 or 15 centimeters of water of positive end expiratory pressure were used. However, increasing extravascular fluid sequestration and decreasing pulmonary compliance occurred progressively with increasing levels of positive end expiratory pressure above 5 centimeters of water. Pulmonary artery pressure increased immediately along with end inspiratory pressure, an amount approximately equal to the increase in positive end expiratory pressure, and this is thought to be the primary cause of the increased rate of fluid sequestration. These experiments suggest that an optimal level of positive end expiratory pressure exists when the shunt can be reduced and oxygenation improved without increasing the rate of extravascular fluid accumulation to the point where long time deleterious effects could outweigh immediate benefits.