Nonsustained pulmonary vasoconstriction during acute hypoxia in anesthetized dogs

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Analysis of responses to the Rho-kinase inhibitor Y-27632 in the pulmonary and systemic vascular bed of the rat

Responses to the Rho kinase inhibitor Y-27632 were investigated in the anesthetized rat. Under baseline conditions intravenous injections of Y-27632 decreased pulmonary and systemic arterial pressures and increased cardiac output. The decreases in pulmonary arterial pressures were enhanced when baseline tone was increased with U-46619, and under elevated tone conditions Y-27632 produced similar percent decreases in pulmonary and systemic arterial pressures. Injections of Y-27632 prevented and reversed the hypoxic pulmonary vasoconstrictor response. The increase in pulmonary arterial pressure in response to ventilation with a 10% O(2)-90% N(2) gas mixture was not well maintained during the period of hypoxic exposure. Treatment with the nitric oxide (NO) synthase (NOS) inhibitor nitro-l-arginine methyl ester (l-NAME) increased pulmonary arterial pressure and prevented the decline or fade in the hypoxic pulmonary vasoconstrictor response. The hypoxic pulmonary vasoconstrictor response was reversed by Y-27632 in control and in l-NAME-treated animals. The Rho kinase inhibitor attenuated increases in pulmonary arterial pressures in response to intravenous injections of serotonin, angiotensin II, and Bay K 8644. Y-27632, sodium nitrite, and BAY 41-8543, a guanylate cyclase stimulator, decreased pulmonary and systemic arterial pressures and vascular resistances in monocrotaline-treated rats. These data suggest that Rho kinase is involved in the regulation of baseline tone and in the mediation of pulmonary vasoconstrictor responses. The present data suggest that the hypoxic pulmonary vasoconstrictor response is modulated by the release of NO that mediates the nonsustained component of the response in the anesthetized rat. These data suggest that Rho kinase and NOS play important roles in the regulation of vasoconstrictor tone in physiological and pathophysiological states and that monocrotaline-induced pulmonary hypertension can be reversed by agents that inhibit Rho kinase, generate NO, or stimulate soluble guanylate cyclase.

Clinical perspective of hypoxia-mediated pulmonary hypertension

Pulmonary hypertension is a condition associated with a variety of pulmonary disorders whose common denominator is alveolar hypoxia. Such disorders include chronic obstructive pulmonary disease, pulmonary fibrosis, sleep-disordered breathing, and exposure to high altitude. Acute hypoxia is characterized by vasoconstriction of small pulmonary arteries, a phenomenon called hypoxic pulmonary vasoconstriction. With prolonged hypoxia, thickening of the smooth vascular layer of the small pulmonary arteries occurs, a phenomenon described as pulmonary vascular remodeling. Although the core mechanisms of both vasoconstriction and remodeling are thought to reside in the smooth muscle cell layer, the endothelium modulates these two processes. The purpose of this review is briefly to (a) discuss the mechanisms of hypoxic pulmonary hypertension as it pertains to certain disease states, and (b) examine the pathways that have potential therapeutic applications for this condition.

Graded response of K+ current, membrane potential, and [Ca2+]i to hypoxia in pulmonary arterial smooth muscle

Many studies indicate that hypoxic inhibition of some K+ channels in the membrane of the pulmonary arterial smooth muscle cells (PASMCs) plays a part in initiating hypoxic pulmonary vasoconstriction. The sensitivity of the K+ current (I(k)), resting membrane potential (E(m)), and intracellular Ca2+ concentration ([Ca2+]i) of PASMCs to different levels of hypoxia in these cells has not been explored fully. Reducing PO2 levels gradually inhibited steady-state I(k) of rat resistance PASMCs and depolarized the cell membrane. The block of I(k) by hypoxia was voltage dependent in that low O2 tensions (3 and 0% O2) inhibited I(k) more at 0 and -20 mV than at 50 mV. As expected, the hypoxia-sensitive I(k) was also 4-aminopyridine sensitive. Fura 2-loaded PASMCs showed a graded increase in [Ca2+]i as PO2 levels declined. This increase was reduced markedly by nifedipine and removal of extracellular Ca2+. We conclude that, as in the carotid body type I cells, PC-12 pheochromocytoma cells, and cortical neurons, increasing severity of hypoxia causes a proportional decrease in I(k) and E(m) and an increase of [Ca2+]i.

Chronic hypoxia attenuates nitric oxide-dependent hemodynamic responses to acute hypoxia

Alterations in the nitric oxide (NO) pathway have been implicated in the pathogenesis of chronic hypoxia-induced pulmonary hypertension. Chronic hypoxia can either suppress the NO pathway, causing pulmonary hypertension, or increase NO release in order to counteract elevated pulmonary arterial pressure. We determined the effect of NO synthase inhibitor on hemodynamic responses to acute hypoxia (10% O(2)) in anesthetized rats following chronic exposure to hypobaric hypoxia (0.5 atm, air). In rats raised under normoxic conditions, acute hypoxia caused profound systemic hypotension and slight pulmonary hypertension without altering cardiac output. The total systemic vascular resistance (SVR) decreased by 41 +/- 5%, whereas the pulmonary vascular resistance (PVR) increased by 25 +/- 6% during acute hypoxia. Pretreatment with N(omega)-nitro-L-arginine methyl ester (L-NAME; 25 mg/kg) attenuated systemic vasodilatation and enhanced pulmonary vasoconstriction. In rats with prior exposure to chronic hypobaric hypoxia, the baseline values of mean pulmonary and systemic arterial pressure were significantly higher than those in the normoxic group. Chronic hypoxia caused right ventricular hypertrophy, as evidenced by a greater weight ratio of the right ventricle to the left ventricle and the interventricular septum compared to the normoxic group (46 +/- 4 vs. 28 +/- 3%). In rats which were previously exposed to chronic hypoxia (half room air for 15 days), acute hypoxia reduced SVR by 14 +/- 6% and increased PVR by 17 +/- 4%. Pretreatment with L-NAME further inhibited the systemic vasodilatation effect of acute hypoxia, but did not enhance pulmonary vasoconstriction. Our results suggest that the release of NO counteracts pulmonary vasoconstriction but lowers systemic vasodilatation on exposure to acute hypoxia, and these responses are attenuated following adaptation to chronic hypoxia.

Hypoxic constriction of porcine distal pulmonary arteries: endothelium and endothelin dependence

To determine the role of endothelium in hypoxic pulmonary vasoconstriction (HPV), we measured vasomotor responses to hypoxia in isolated seventh-generation porcine pulmonary arteries < 300 microm in diameter with (E+) and without endothelium. In E+ pulmonary arteries, hypoxia decreased the vascular intraluminal diameter measured at a constant transmural pressure. These constrictions were complete in 30-40 min; maximum at PO(2) of 2 mm Hg; half-maximal at PO(2) of 40 mm Hg; blocked by exposure to Ca(2+)-free conditions, nifedipine, or ryanodine; and absent in E+ bronchial arteries of similar size. Hypoxic constrictions were unaltered by indomethacin, enhanced by indomethacin plus N(G)-nitro-L-arginine methyl ester, abolished by BQ-123 or endothelial denudation, and restored in endothelium-denuded pulmonary arteries pretreated with 10(-10) M endothelin-1 (ET-1). Given previous demonstrations that hypoxia caused contractions in isolated pulmonary arterial myocytes and that ET-1 receptor antagonists inhibited HPV in intact animals, our results suggest that full in vivo expression of HPV requires basal release of ET-1 from the endothelium to facilitate mechanisms of hypoxic reactivity in pulmonary arterial smooth muscle.

Role of EDRF in pulmonary circulation during sustained hypoxia

The pulmonary artery pressure (PAP) response to hypoxia is characterized by an initial vasoconstriction followed by vasodilation. Pulmonary vessels can release endothelium-derived relaxing factor (EDRF), which is considered to be nitric oxide (NO), but the role of EDRF in the regulation of normal and hypoxic pulmonary vascular tone is still uncertain. We designed this study to address the in vivo role of EDRF in vasodilation during sustained hypoxia. We studied the effects of an EDRF-synthesis inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME), on the pulmonary vascular response to sustained hypoxia (10% O2, 20 min) in normoxic (N) and chronically hypoxic (CH) rats. Biphasic PAP response was observed in N rats, whereas PAP was unchanged in CH rats during sustained hypoxic exposure. The L-NAME-induced PAP increase during normoxia was greater in CH than in N rats, suggesting that basal EDRF plays an important role in attenuating the severity of pulmonary hypertension in CH rats. Administration of L-NAME increased the initial increment in PAP by acute hypoxia and shifted the PAP response upward throughout sustained hypoxia, while still showing the biphasic pattern, in N rats. In contrast, PAP increased acutely and remained elevated with little recovery in the late phase in CH rats. The inducible NO synthase messenger RNA (mRNA) expression and protein showed greater increases in the lungs of CH than in N rats. These results suggest that EDRF release during sustained hypoxia may partly contribute to the roll-off in PAP response during sustained hypoxia in N rats, and that augmented EDRF may prevent a further increase in PAP during chronic hypoxia.

Effects of hypoxia, mechanical and chemical endothelium denudation on guinea-pig isolated pulmonary arteries

1. The isolated unstimulated main trunk, extralobar and intralobar branches of the pulmonary artery of the guinea-pig developed well-sustained contractions upon exposure to hypoxia (95% N2-5% CO2 gas mixture; PO2 11-15 mm Hg). The contractions were readily reversible by reoxygenation (95% O2-5% CO2). 2. Mechanical removal of the endothelium did not significantly affect the magnitude of the hypoxia-induced contractions in rings obtained from the main trunk of the pulmonary artery but reduced those of rings obtained from the proximal and distal extralobar branches. 3. Mechanical removal of the endothelium also did not affect the magnitude of contractions induced by BaCl2 in the main but significantly reduced contractions induced by the same agent in the proximal and distal extralobar branches of the pulmonary artery, suggesting that the reduction of hypoxia-induced contractions in the endothelium-denuded rings is due to impairment of vascular reactivity. 4. Pretreatment with L-N-nitro arginine, an inhibitor of the synthesis of the endothelium-derived relaxing factor, did not significantly affect the hypoxia-induced contractions but increased the magnitude of BaCl2-induced contractions in the main and the extralobar branches. 5. These observations demonstrate that isolated pulmonary artery rings of the guinea-pig develop slow contractions in response to hypoxia without prior contraction with an agonist, and that the endothelium plays little role in the hypoxia-induced contractions of guinea-pig isolated large pulmonary arteries. 6. Furthermore, these observations suggest that the effect of mechanical endothelium denudation or pharmacological manipulation, such as EDRF inhibition, on vascular reactivity should be considered when the effect of hypoxia is studied in isolated pulmonary arteries.

Is hypoxic pulmonary vasoconstriction important during single lung ventilation in the lateral decubitus position?

Hypoxic pulmonary vasoconstriction (HPV) has not been demonstrated in human single lung anaesthesia in the lateral decubitus position (LDP). The purpose of this study was to determine whether (1) HPV occurs in the non-dependent, non-ventilated lung, and (2) if the infusion of sodium nitroprusside (SNP) inhibits HPV. During intravenous anaesthesia the tracheas of seven patients were intubated with double lumen endotracheal tubes. Standard monitors plus radial and pulmonary arterial catheters were placed. Patients were positioned in the LDP and haemodynamic and gas exchange data were recorded for each of three stages; I: two-lung ventilation, II: single, dependent lung ventilation (1 LV) and III: 1LV with infusion of SNP. In stage II the PaO2 decreased from 531 +/- 42 mmHg to 285 +/- 42 mmHg (P < 0.05) and Qs/Qt increased from 12.3 +/- 2.7 to 29.0 +/- 6.3% (P < 0.05). With SNP infusion there was a 30% increase in cardiac index (CI) (P < 0.05). The SNP infusion was not associated with changes in Qs/Qt or PaO2. This model demonstrates changes in Qs/Qt and PaO2 associated with single-lung ventilation in ASA I and II patients in the LDP but we were unable to demonstrate inhibition of HPV by SNP.

Absence of parasympathetic control of pulmonary vascular pressure-flow plots in hyperoxic and hypoxic dogs

Hypoxic stimulation of the peripheral chemoreceptors has been reported to inhibit hypoxic pulmonary vasoconstriction (HPV). This has been explained by a reflex vagal (Chapleau et al., 1988) or sympathetic (Naeije et al., 1989) pulmonary vasodilation. We therefore investigated the effects of bilateral cervical vagotomy and of muscarinic block (atropine sulfate 0.1 mg.kg-1 i.v.) on multipoint pulmonary arterial pressure (Ppa)-cardiac index (Q) plots in 16 sodium pentobarbital-anesthetized dogs ventilated alternately in hyperoxia (fraction of inspired O2, FIO2, 0.4) and in hypoxia (FIO2 0.1). Over the range of Q studied, 2 to 5 L.min-1.m-2, hypoxia increased Ppa and did not change pulmonary capillary wedge pressure (Ppw). After bilateral cervical vagotomy or after atropine, Ppa and Ppw at all levels of Q were not modified either during hyperoxia or during hypoxia. These results show that the parasympathetic system does not affect the global hypoxia-induced pulmonary vasopressor response and thus suggest that the depressor effect of chemoreceptor stimulation on HPV is not vagally mediated.

Time-dependent response of fetal pulmonary blood flow to an increase in fetal oxygen tension

We describe the temporal characteristics of the response of the fetal pulmonary circulation to the vasodilatory stimulus of a sustained increase in fetal PO2 (5.1 +/- 0.7 Torr) in 13 chronically prepared fetal sheep. Left pulmonary artery blood flow was measured by electromagnetic flow transducer. Fetal PO2 was increased by delivery of 100% oxygen to the ewe and did not significantly change during the 2 h period of oxygen administration. Fetal left pulmonary artery blood flow slowly increased to a peak approximately 2.7 times the control value 40-50 min after the onset of increased PO2. It then steadily declined toward baseline over the next hour of increased PO2. Maximal pulmonary blood flow in response to the increase in PO2 increased with gestational age. Pulmonary arterial, aortic, and left atrial blood pressures did not change significantly in the animals in which measurements were made. We conclude that the changes in fetal pulmonary blood flow with increased fetal PO2 depend upon the time after the PO2 is increased. The adaptation seen during the second hour suggests the existence of mechanisms that tend to keep the fetal pulmonary circulation chronically constricted at any PO2 likely to be encountered in fetal life.

Hypoxic pulmonary vasoconstriction in the human lung: the effect of prolonged unilateral hypoxic challenge during anaesthesia

The influence of time on the pulmonary vasoconstrictor response to hypoxia was studied in six subjects during general anaesthesia and artificial ventilation prior to elective surgery. The lungs were intubated separately with a double-lumen bronchial catheter. After preoxygenation of both lungs for 30 min, the test lung was rendered hypoxic for 60 min by ventilation with 5% O2 in N2, with the control lung still being ventilated with 100% O2. Cardiac output was determined by thermodilution, and the distribution of blood flow between the lungs was assessed from the excretion of a continuously infused poorly soluble gas (SF6). The fractional perfusion of the test lung decreased from 53% to 25% of cardiac output within the first 15 min of unilateral hypoxia. The pulmonary artery mean pressure increased by 14% and the pulmonary vascular resistance (PVR) of the test lung increased by 54%. Venous admixture increased from 21% to 39% of cardiac output, while the "true" shunt was maintained at about 15%. Arterial oxygen tension (Pao2) fell from 45 kPa to 12 kPa. Prolonging the unilateral hypoxic challenge caused no further change in the redistribution of the pulmonary blood flow, but cardiac output and pulmonary artery mean pressure continued to increase to 40%-50% above control values after 1 h of hypoxia. The PVR of the test lung remained unchanged. The findings suggest that there is an immediate vasoconstrictor response to hypoxia in the human lung and that there is no further potentiation or diminution, of the response during a 60-min period of hypoxia.

Canine nonresponders to alveolar hypoxic vasoconstriction and quantitative restoration of the response by aspirin I-3

Hypoxic pulmonary vasoconstriction is characterized by considerable variability in rate of response (pulmonary vascular resistance [PVR] as a function of time under hypoxia). To further define this response, forty-five closed chest dogs were anaesthetized (pentobarbitone sodium), intubated, and mechanically ventilated. Constant left lower lobar pulmonary artery flow was maintained through a balloon tipped 14F catheter via an extracorporeal pump at a rate to achieve lobar pulmonary artery pressure (Plobar) equal to main pulmonary artery pressure (PPA) and thereafter held constant. Left ventricular end diastolic pressure (PLVED) was measured by left ventricular catheter and lobar pulmonary artery flow rate (Q) by flow meter. Lobar PVR (mmHg/min per 1) was calculated every 15 min. Ventilation with 10% oxygen (O2) separated two groups based on the increase in PVR over time: twenty-two rapid hypoxic responders [HR] (slope= delta PVR/delta min greater than 0.3) and twenty-three slow or nonresponders [NR] (slope less than 0.1). The twenty-three NR were divided into two groups. Ten NR dogs (NR control) had no change in mean PVR (23.9, s.d. = 8.2, to 24.1, s.d. = 9.6) over a mean of 78 min and were used as controls. Thirteen NR dogs (NR ASA) had no change in mean PVR (32.9, s.d. = 9.5, to 32.3, s.d. = 9.8) over 75 min and were given aspirin (ASA), 10-15 mg/kg intra-arterially. The NR ASA group mean PVR then increased from 32.3 (s.d. = 9.8) to 59.1 (s.d. = 23.9, 82.9% increase, P less than 0.01) over a mean of 54 min. The mean PVR for the twenty-two HR rose from 39.8 (s.d. = 34.0) to 64.5 (s.d. = 36.6, 62.1% increase, P less than 0.01) over a mean of 72 min. The slopes of rate of response for HR (0.66) and for NR ASA (0.88) were not significantly different. The absolute values of PVR reached after plateau for HR and for NR ASA (after ASA) were also not different. Aspirin restored the NR capability to develop pulmonary vasoconstriction in response to alveolar hypoxia. The rate of response and the absolute level of response reached were also restored by aspirin.

[Technics for artificial ventilation of a single lung during thoracotomy]

Different means of limiting the fall in arterial PO2 produced by single artificial ventilation were studied in 60 patients during thoracotomy. Changing from ventilating both lungs to the one healthy lung in the lateral recumbent position, without modifying tidal volume and frequency, brought about a fall in arterial PO2 from 180 +/- 56 to 67 +/- 40 mmHg. The alveolar to arterial oxygen gradient increased to 110 +/- 45 mmHg (the alveolar oxygen pressure being calculated). Reducing the tidal volume so as to keep the inflation pressure at its initial level did not improve the arterial PO2 but slightly increased the arterial PCO2 (2.3 mmHg). The use of 6 to 8 cm H2O positive end-expiratory pressure did not significantly modify the arterial PO2 or PCO2. Increasing the inspired oxygen fraction from 0.5 to 0.7 increased the arterial PO2 from 100 +/- 89 mmHg to 165 +/- 59 mmHg, whilst the alveolar to arterial oxygen gradient increased to 118 +/- 60 mmHg. Clamping the pulmonary artery increased the arterial PO2 and dual lung ventilation restored it to its initial value. Therefore, the only effective means of increasing oxygenation was to increase the inspired oxygen fraction. Unilateral continuous positive airway pressure was not used so as not to impair surgery. Dual lung ventilation may be necessary if the arterial PO2 remains low.

Hypoxia stimulates prostacyclin generation by dog lung in vitro

We have investigated the possibility that pulmonary biosynthesis of prostacyclin and thromboxane A2 (TXA2) may be affected by variations in PO2. Fresh lung homogenated from 8 dogs were incubated for 1 hour at 37 degrees C in Krebs-Ringer solution, at high (492 mmHg) or low (53 mmHg) PO2. After incubation, the stable metabolites of prostacyclin (6-keto-PGF1) and of TXA2 (TXB2) were measured by radioimmunoassay. The basal ("pre-incubation") levels of these metabolites, measured in tubes in which biosynthesis was arrested by the addition of indomethacin (10 g/ml), were 0.76 +/- 0.15 and 0.76 +/- 0.11 ng/mg wet wt. for 6-keto-PGF1 alpha, in high and low PO2, respectively, and 0.97 +/- 0.09 and 0.82 +/- 0.15 x 10(-1) ng/mg wet wt. for TXB2, in high and low PO2 respectively. Synthesis during incubation in other tubes was estimated by subtracting basal values from those measured at the end of incubation. More 6-keto-PGF1 alpha was formed in homogenates exposed to low PO2 (3.01 +/- 0.45) than in those exposed kept at high PO2 (1.89 +/- 0.37, p less than 0.001), but equal amounts of TXB2 were bound under both conditions. These results suggest that hypoxia may stimulate pulmonary prostacyclin synthesis; a pulmonary vasodilator, prostacyclin may help modulate hypoxic vasoconstriction in the lung.

Inhibitors of oxidative ATP production cause transient vasoconstriction and block subsequent pressor responses in rat lungs

We wondered if depression of oxidative adenosine triphosphate (ATP) production caused pulmonary vasoconstriction. If so, then several chemically different inhibitors of oxidative ATP production all should cause pulmonary pressor responses. The vascular reactivity of isolated, blood-perfused rat lungs was established by eliciting pressor responses to airway hypoxia and to intraarterial angiotensin II. Then, during normoxia, we added to perfusate one of five chemical inhibitors of oxidative ATP production: 10 mM azide, 1 mM cyanide, 1 mM dinitrophenol, 5 or 10 microM antimycin A, or 0.5 microM rotenone. Each of the five chemical inhibitors, but not their solvents, caused a transient pressor response, followed by loss of vascular reactivity to hypoxia, angiotensin II, and chemical inhibitors. The inhibitor pressor responses were not due to an effect on blood cells, since they also were seen in lungs perfused with plasma. The magnitudes of pressor responses to all metabolic inhibitors except azide correlated with the magnitudes of preceding pressor responses to hypoxia, but not to the preceding angiotensin II responses. When verapamil or calcium chloride was added to perfusate, the hypoxic and inhibitor pressor responses were blunted more than was the angiotensin II response. Thus, five chemically different substances, inhibiting different steps of oxidative ATP production, all caused pressor responses that were blocked readily by verapamil and by increased perfusate calcium chloride. These results support the possibility that depression of oxidative ATP production elicits pulmonary vasoconstriction that is dependent on influx of extracellular calcium. Hypoxia might also be sensed in the pulmonary circulation by decreased oxidative ATP production in some as yet unidentified lung cell.

Vascular resistance in atelectatic lungs: effects of inhalation anesthetics

We have investigated the relative contribution of mechanical obstruction and hypoxia-induced vasoconstriction to the increased pulmonary vascular resistance (PVR) in atelectatic lungs. For this purpose we have utilized the previous observation that inhalation anesthetics inhibit the vasoconstrictor response to pulmonary hypoxia. The effects of halothane, enflurane and ether on PVR in atelectatic lungs have been explored. Two pairs of isolated rat lungs were perfused in series at constant flow. One of the preparations was made atelectatic by airway occlusion subsequent to ventilation with a high PO2 gas (95% O2). Ventilation of the other preparation continued with hypoxic gas (2% O2), resulting in a gradual increase in PVR in both preparations. When maximum PVR was reached, one of the above inhalation anesthetics was administered to the atelectatic lungs via the ventilated lung preparation. This caused a dose-dependent, reversible reduction of PVR. The same effect was observed when pulmonary arterial PO2 was increased (greater than 66.5 kPa). Histological examination revealed that two out of four preparations were completely atelectatic 1 h after airway occlusion, whereas atelectasis was nearly complete in the other two. In two groups, airways were occluded for 1 h. In the first group PVR increased to 163% (median) above baseline level, as found during ventilation with high PO2. High arterial PO2 reduced PVR in the atelectatic lungs to 50% (median) above baseline, whereas papaverine induced a further PVR reduction, to 7% (median) above baseline. In the other group, papaverine was given before airway occlusion, and PVR increased to 10% (median) above baseline. Comparison of the two groups shows that mechanical obstruction accounts for about 6% (10/163) of the overall rise in PVR during atelectasis.

Regional alveolar gas composition and lung function in sheep

The influence of regional alveolar oxygen and carbon dioxide tensions on the distribution of lung blood flow and gas exchange was studied in unanaesthetised sheep. Right apical lobe (RAL) hypoxia, induced by administering nitrogen or nitrogen/oxygen mixtures to the lobe, stimulated a prompt, graded and well sustained reduction in lobar blood flow. Maximum hypoxia was accompanied by an approximate 65% reduction in perfusion, a significant fall in RAL carbon dioxide tension and output, a reversal of lobar oxygen flux and an average 13 Torr fall in arterial oxygen tension. The reduction in perfusion and gas exchange persisted in the face of elevated systemic oxygen tensions produced by giving pure oxygen instead of air to the remainder of the lung (RL). Mild RAL hypercapnia potentiated the hypoxia-induced change in perfusion and gas exchange. During lobar hypoxia RL blood flow and gas exchange increased to maintain total pulmonary gas exchange at an essentially constant level. RAL hyperoxia did not significantly alter the distribution of perfusion or gas exchange.

Chemoreceptor stimulation and hypoxic pulmonary vasoconstriction in conscious dogs

Dogs with electromagnetic flow probes implanted on their left (QL) and main (QT) pulmonary arteries, catheters in their left atria and external jugular veins, and chronic tracheostomies were trained to accept Carlens dual-lumen endotracheal tubes into their tracheostomies, thus allowing separate ventilation of the two lungs. Swan-Ganz catheters were inserted through the jugular vein catheters. Pneumotachographs measured air flow to each lung. During bilateral ventilation with room air or O2, QL was about 36% of QT. When the left lung was ventilated with N2 while the right remained on O2, PAO2 was above 90 mmHg and QL fell to about 25% of QT. When the left lung was ventilated with N2 and the right with room air, PAO2 fell below 40 mm Hg and QL increased to control levels. This increase in perfusion of the hypoxic lung during systemic hypoxemia was not seen in dogs after surgical deafferentation of the systemic arterial chemoreceptors, indicating that stimulation of the arterial chemoreceptors may interfere with the hypoxic pulmonary vasoconstriction.

Variability of the pulmonary vascular response to hypoxia and relation to gas exchange in dogs

Anaesthetized and mechanically ventilated dogs were subjected to five minutes of alveolar hypoxia with Fio2 ranging from 0.08 to 0.20 while pulmonary artery pressure (Ppa), pulmonary wedge pressure (Pwp) and cardiac output (Q) were measured. Hypoxia increased Ppa in all dogs whereas Pwp and Q did not change significantly. Thus pulmonary vascular resistance (PVR) increased by a mean for all dogs of 56 per cent. There was great variation in the absolute increase in Ppa and PVR between animals and these were not statistically correlated with arterial Po2, but highly significant and reproduceable inverse relationships were found for Ppa and PVR against arterial oxygen per cent saturation. The slopes of these responses varied between dogs from -0.013 to -0.144 for PVR and from -0.038 to -0.561 for Ppa. The alveolar-arterial oxygen gradient and the pulmonary shunt fraction were reduced in a similar way with decreasing arterial oxygen per cent saturation, but dead space/tidal volume ratio remained unchanged. Thus the slope of PVR response to hypoxia against arterial oxygen per cent saturation is unique for individual animals. This may reflect functional and likely structural adaptations of the pulmonary vascular smooth muscle.

Chemoreceptor influence on pulmonary blood flow during unilateral hypoxia in dogs

Dogs anesthetized with 30 mg/kg pentobarbital were artificially respired after differential cannulation of the main stem bronchi. Following median sternotomy, blood flow was monitored by electromagnetic flow probes on the left pulmonary artery (QL) and on the pulmonary trunk or aorta, QT. Following 10 min of bilateral 100% O2, QL was 42.5 +/- 7% of QT. When 6% O2, was substituted as the gas mixture inspired by the left lung while the right lung remained on 100% O2, PaO2 was above 70 mm Hg and QL fell to 24.5 +/- 5% of QT. Room air was then used to ventilate the right lung while the left lung remained on 6% O2. This caused PaO2 to fall to 42.3 +/- 3 MM Hg and QL to rise to 38.3 +/- 6% QT. This increase in blood flow to the unilaterally hypoxic lung during systemic hypoxemia did not occur in dogs after peripheral chemoreceptor denervation. Therefore, interference with the local response to alveolar hypoxia during systemic hypoxemia appears to be mediated by the arterial chemoreceptors.

Vascular actions of histamine H1- and H2-receptor agonists in dogs and cats

Systemic and pulmonary vascular responses to infusions of histamine, 2-methylhistamine (2-MeH), and 4-methylhistamine (4-MeH) were determined. In dogs, H1-receptor stimulation with 2-MeH induced pulmonary vasoconstriction, and a decrease in cardiac output. H2-receptor stimulation with 4-MeH induced pulmonary and systemic vasodilation, tachycardia, and an increase in cardiac output. In cats, 2-MeH caused slight pulmonary vasoconstriction and 4-MeH caused vasodilation. Both H1- and H2-receptors mediated the systemic vasodilation, tachycardia, and increase in cardiac output observed in cats.

Histamine H1- and H2-receptors in the cat and their roles during alveolar hypoxia

We sought to define the roles of H1-and H2-receptors in the cat and to evaluate the roles of these receptors during alveolar hypoxia. In pentobarbital anesthetized cats, we found that histamine infusion (1.1 microgram/kg/min for 3 min) increased cardiac output and decreased pulmonary and systemic vascular resistances. However, when cardiac output was held constant, histamine infusion induced pulmonary vasoconstriction. Histamine infusions after H1-and H2-receptor blockade (chlorpheniramine and metiamde, respctively) indicated that H2-receptors mediated systemic vasodilatation. In the lung, H1-receptors mediated vasoconstriction, and H2-receptors mediated vasodilatation. Hypoxia (10% O2) caused large increases in pulmonary vascular resistance which were not blocked by H1-, H2-, or combined H1- and H2-receptor blockade. In the intact cat, histamine does not appear to mediate hypoxic pulmonary hypertension.

Hypoxia-induced vasoconstriction in isolated perfused lungs exposed to injectable or inhalation anesthetics

Investigations during the last two decades have revealed a tendency to inpaired pulmonary gas exchange in patients during general anesthesia. In the awake state, arterial hypoxemia is counteracted by a mechanism which tends to normalize the ventilation/perfusion ratio of the lungs by way of a hypoxia-induced vasoconstriction in poorly ventilated areas. This results in a redistribution of perfusion to more adequately ventilated lung regions. Recent observations suggest, however, that this beneficial mechanism is blunted by some commonly used inhalation anesthetics. In the present study the effect of inhalation anesthetics and injectable anesthetics on the vasoconstrictor response to acute alveolar hypoxia have been compared in isolated blood-perfused rat lungs. The experiments showed that the response was unaffected by N2O and injectable anesthetics, while a reversible, dose-dependent damping effect was demonstrated for the volatile inhalation anesthetics, ether, halothane and methoxyflurance. The effect could be demonstrated at blood concentrations comparable to those used in clinical anesthesia, and it was not due to a general paralysis of the vascular smooth muscle. The findings might, at least in part, explain the occurrence of arterial hypoxemia during general inhalation anesthesia.

Pulmonary microembolism: attenuated pulmonary vasoconstriction with prostaglandin inhibitors and antihistamines

The mechanism(s) involved in the pulmonary vascular and airway responses to pulmonary microembolism have not been clearly defined. Therefore, we determined the effects of specific prostaglandin and histamine blockade on the hemodynamic and arterial blood gas tension responses to particulate microembolism (200 mu glass beads) in intact anesthetized dogs. The marked increases in pulmonary arterial pressure and pulmonary vascular resistance observed in the untreated dogs were attenuated, but not abolished, following both prostaglandin blockade (with either meclofenamate or polyphloretin phosphate) and histamine blockade (with chlorpheniramine and metiamide) at 5 minutes, and were still attenuated 30 minutes post embolization. Combined prostaglandin and histamine blockade further attenuated, but again did not abolish, the pulmonary vascular responses. Cardiac outputs and systemic arterial pressures were unchanged from control by embolism. The alveolar hypoventilation (decreased arterial oxygen tension and increased carbon dioxide tension) observed in the untreated embolized dogs was prevented only with the prostaglandin inhibitors. Pulmonary microembolism in intact dogs, therefore, appears to induce vasoconstriction mediated partially by prostaglandin and histamine action, and alveolar hypoventilation mediated by prostaglandin, but not histamine, action.