PULMONARY VASOCONSTRICTION IN RESPONSE TO PRECAPILLARY HYPOXEMIA

Perioperative Management of Emergency Craniotomes in Children With Cyanotic Congenital Heart Disease: A Case Series

While congenital heart disease is not uncommon, cyanotic congenital heart disease (CCHD) accounts for a minor fraction of them. However, when cyanosis is present, it usually indicates a severe or critical illness. Tetralogy of Fallot (TOF) is one of the common CCHDs, representing 7-10% of all congenital cardiac malformations. Double-outlet right ventricle (DORV) is another CCHD similar to the TOF and associated with decreased pulmonary flow, ventricular septal defect (VSD), and aorta receiving blood from both ventricles. Reduced oxygen arterial saturation and increased viscosity by polycythemia induce focal cerebral ischemia, often in the area supplied by the middle cerebral artery leading to brain abscess. Brain abscesses require craniotomy, which is a major surgery. These patients also often show features of sepsis and increased intracranial pressure. The presence of CCHD further complicates the situation, making perioperative management even more challenging. There are studies in the literature on the management of similar cases, and they report successful management in most of them. However, not all such cases need intensive postoperative management. We present four pediatric cases who had either TOF or DORV and had to undergo craniotomy for brain abscess or ventriculoperitoneal shunt placement. We describe case management and highlight the critical features and cases that require prolonged postoperative critical care management.

Diastolic dysfunction precedes hypoxia-induced mortality in dystrophic mice

Duchenne muscular dystrophy (DMD) is a progressive striated muscle disease that is characterized by skeletal muscle weakness with progressive respiratory and cardiac failure. Together respiratory and cardiac disease account for the majority of mortality in the DMD patient population. However, little is known regarding the effects of respiratory dysfunction on the dystrophic heart. The studies described here examine the effects of acute hypoxia on cardiac function. These studies demonstrate, for the first time, that a mouse model of DMD displays significant mortality following acute exposure to hypoxia. This mortality is characterized by a steady decline in systolic function. Retrospective analysis reveals that significant decreases in diastolic dysfunction, especially in the right ventricle, precede the decline in systolic pressure. The initial hemodynamic response to acute hypoxia in the mouse is similar to that observed in larger species, with significant increases in right ventricular afterload and decreases in left ventricular preload being observed. Significant increases in heart rate and contractility suggest hypoxia-induced activation of the sympathetic nervous system. These studies provide evidence that while hypoxia presents significant hemodynamic challenges to the dystrophic right ventricle, global cardiac dysfunction precedes hypoxia-induced mortality in the dystrophic heart. These findings are clinically relevant as the respiratory insufficiency evident in patients with DMD results in significant bouts of hypoxia. The results of these studies indicate that hypoxia may contribute to the acceleration of the heart disease in DMD patients. Importantly, hypoxia can be avoided through the use of ventilatory support.

Hypoxia-induced inflammation in the lung: a potential therapeutic target in acute lung injury?

Acute lung injury (ALI) is a severe form of hypoxic lung disease responsible for a large number of deaths worldwide. Despite recent advances in supportive care, no reduction in mortality has been evident since the introduction of a standard consensus definition almost two decades ago. New strategies are urgently required to help design effective therapies for this condition. A key pathological feature of ALI involves regional alveolar hypoxia. Because alveolar hypoxia in isolation, such as that encountered at high altitude, causes an inflammatory pulmonary phenotype in the absence of any other pathogenic stimuli, these regions may not be passive bystanders but may actually contribute to the pathogenesis and progression of lung injury. Unique transcriptional responses to hypoxia in the lung apparently allow it to express an inflammatory phenotype at levels of hypoxia that would not produce such a response in other organs. We will review recent advances in our understanding of these unique transcriptional responses to moderate levels of alveolar hypoxia, which may provide new insights into the pathogenesis of ALI.

[Magnetic resonance imaging of pulmonary perfusion. Technical requirements and diagnostic impact]

With technical improvements in gradient hardware and the implementation of innovative k-space sampling techniques, such as parallel imaging, the feasibility of pulmonary perfusion MRI could be demonstrated in several studies. Dynamic contrast-enhanced 3D gradient echo sequences as used for time-resolved MR angiography have been established as the preferred pulse sequences for lung perfusion MRI. With these techniques perfusion of the entire lung can be visualized with a sufficiently high temporal and spatial resolution. In several trials in patients with acute pulmonary embolism, pulmonary hypertension and airway diseases, the clinical benefit and good correlation with perfusion scintigraphy have been demonstrated. The following review article describes the technical prerequisites, current post-processing techniques and the clinical indications for MR pulmonary perfusion imaging using MRI.

Anesthetic management of children with pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is associated with significant perioperative risk for major complications, including pulmonary hypertensive crisis and cardiac arrest. Several mechanisms of hemodynamic deterioration, including acute increases in pulmonary vascular resistance (PVR), alterations of ventricular contractility and function and coronary hypoperfusion can contribute to morbidity. Anesthetic drugs exert a variety of effects on PVR, some of which are beneficial and some undesirable. The goals of balanced and cautious anesthetic management are to provide adequate anesthesia and analgesia for the surgical procedure while minimizing increases in PVR and depression of myocardial function. The development of specific pulmonary vasodilators has led to significant advances in medical therapy of PAH that can be incorporated in anesthetic management. It is important that anesthesiologists caring for children with PAH be aware of the increased risk, understand the pathophysiology of PAH, form an appropriate anesthetic management plan and be prepared to treat a pulmonary hypertensive crisis.

Pulmonary Vascular Disease

The pulmonary vasculature is an anatomic compartment that is frequently overlooked in the histologic review of lung biopsy samples, other than those obtained specifically to assess pulmonary vascular disease.1 Though often of a nonspecific nature, the histologic pattern of vascular remodeling may at times suggest its underlying pathogenesis and provide clues to the cause of pulmonary hypertension.2 Disproportionately severe vascular pathology may further indicate alternate disease processes, such as congestive heart failure or thromboemboli, contributing to the patient’s overall respiratory condition.

Pathological changes in metabolism of poultry related to increasing production levels

A continuously increasing production level in poultry breeding has resulted in changes in metabolism. Selection procedures in breeding programmes are focused on an increase in growth rate and on a decrease in feed conversion ratio (less feed intake per unit of deposited tissue). These procedures do not pay attention to the maintenance requirements of birds. Imbalances between production (protein and fat deposition) and supply of energy for maintenance requirements lead to homeostatic dysregulation and to diseases of organs which supply the energy for production and maintenance. The alarming increase in metabolic diseases, such as heart failure syndrome, ascites, and oedema in the lungs and heart, can be directly related to an insufficient oxygen supply. A low oxygen consumption and heat production is one of the mechanisms by which a low feed conversion ratio can be achieved, as is induced hypothyroidism by which physical activity and thus heat production is reduced. Other diseases, such as liver cirrhosis, malabsorption syndrome, sudden death syndrome in broilers, and fatty liver-hemorrhage syndrome, which is nowadays the most important disease in laying hens in the Netherlands, can be related to an imbalance between the production rate and maintenance requirements. A continued selection on the basis of retained energy (in protein and fat) without paying attention to the maintenance requirements of birds will be detrimental for the health and welfare of poultry. These undesirable developments in poultry husbandry should be a challenge for sciences focused on welfare and stress in animals. Such a scientific approach to animals suffering from dysgenic changes in metabolism is needed to solve serious problems in poultry breeding.

The vein utilizes different sources of energy than the artery during pulmonary hypoxic vasoconstriction

Recent studies have shown that the contractile response to hypoxia is much greater in the pulmonary vein than in the artery. The purpose of this study was to investigate the effects of substrate utilization and oxidative phosphorylation on the responses of the pulmonary vein and artery to acute hypoxia. Isolated rat pulmonary arterial and venous rings were placed in tissue baths containing Earle's balanced salt solution (37 degrees C, 95% O2/5% CO2, pH 7.4), and attached to force transducers. The vascular rings were equilibrated for 1 h and then contracted maximally with 80 mM KCl to establish maximum active tension development (Po). Following washout and complete relaxation, the rings were incubated with the following substrates or metabolic inhibitors for 30-40 min: varying concentrations of glucose (0, 5.5, 10, or 20 mM), or glycolytic intermediates (4 mM pyruvate or 4 mM lactate), or inhibitors of glycolysis (50 mM 2-deoxyglucose or 0.1 mM iodoacetate), or an inhibitor of oxidative phosphorylation (0.1 microM rotenone). Vascular rings were then made hypoxic by lowering the bath Po2 to 30 torr. The pulmonary vein responded with a single contraction while the artery responded biphasically as previously reported. The pulmonary venous hypoxic response was not affected by the absence of glucose but was inhibited by high glucose concentrations. Neither glucose metabolic intermediates (pyruvate or lactate) nor the glycolysis inhibitor 2-deoxyglucose had any effect on the pulmonary venous response to hypoxia. However, inhibition of oxidative phosphorylation by rotenone inhibited the venous hypoxic response. In contrast, the pulmonary arterial phase 1 contraction to hypoxia was inhibited and phase 2 contraction was abolished in glucose-free solution. This effect was not due to the decreased production of glucose metabolic intermediates, since addition of pyruvate or lactate did not reverse the decreased arterial hypoxic response in glucose-free solution. Increasing the glucose concentration did not affect phase 1 contraction, but 20 mM glucose inhibited the phase 2 contraction. Inhibition of glycolysis with 2-deoxyglucose or iodoacetate decreased phase 1 contraction and abolished the phase 2 contraction. Inhibition of oxidative ATP production with rotenone abolished phase 1 but not phase 2 contraction. In conclusion, (1) the pulmonary venous response to hypoxia is unaffected by inhibition of glycolysis but is inhibited by high glucose and by inhibition of oxidative ATP production; (2) the pulmonary arterial hypoxic phase 1 contraction is dependent on oxidative ATP production; and (3) the phase 2 contraction of the pulmonary arterial hypoxic response depends on glycolytic ATP production but not on oxidative ATP production. These results indicate that the pulmonary vein and artery preferentially utilize different sources of energy for hypoxic contractions.

The site and mechanism of oxygen sensing for the pulmonary vessels

Lung vessels are unique in the body in that they react to hypoxia with constriction rather than dilatation. Whether this characteristic is inherent in the lung vessel or is due to an influence from a sensor in the surrounding lung parenchyma is not resolved. Recent data, however, showing that vascular hypoxia as well as airway hypoxia can produce pulmonary vasoconstriction and that the sensor for alveolar hypoxia is upstream in the precapillary vessels, allows but does not prove the precapillary pulmonary artery itself to be the O2 sensor. In addition, with the elimination of the mast cell as a necessary extravascular sensor for hypoxia at least in the mouse, there is no good candidate for an extravascular sensor for hypoxic pulmonary vasoconstriction.

Induced low P50 in anesthetized rats: blood gas, circulatory and metabolic adjustments

In anesthetized, normoxic or hypoxic rats the hemodynamic, metabolic and O2 transport characteristics following exchange transfusion with human erythrocytes containing a high O2 affinity hemoglobin (Hemoglobin Creteil, beta 89 Ser----Asp) have been studied. The in vivo oxygen partial pressure at 50% oxygen hemoglobin saturation (P50) decreased from 37.4 +/- 2.1 to 12.7 +/- 0.7 mm Hg; the arterial oxygen tension was reduced significantly from 109.9 +/- 7.7 to 87.3 +/- 12.0 mm Hg. There was a decrease in right ventricular partial pressure of oxygen (PvO2), (P less than 0.001), oxygen consumption (VO2), (P less than 0.001), arterio-venous difference, (P less than 0.001), and peripheral vascular resistance index, (P less than 0.01). Exchange transfusion with normal rat blood (P50 = 37.2 +/- 2.4 mm Hg) or with 2,3-diphosphoglycerate-enriched human red blood cells (P50 = 34.7 +/- 2.2 mm Hg), did not modify these variables in normoxic rats. In hypoxia, the reduction in P50 was associated with a further decrease in PvO2 an increase in serum lactate concentration and a VO2 decrease.

Relation between mixed venous oxygen tension and pulmonary vascular tone during normoxic, hyperoxic and hypoxic ventilation in dogs

To evaluate the relation between the oxygen tension of pulmonary arterial blood (PvO2) and pulmonary vascular resistance (PVR), an extracorporeal circuit was used to vary the PvO2 of blood perfusing the left lung in 3 groups of open-chest dogs mechanically ventilated with an inspired fraction of oxygen of 0.35 (group I), 0.21 (group II), or 0.10 (group III). Left lung pulmonary blood flow, left atrial pressure and perfusate, and systemic pH and PCO2 did not change significantly as PvO2 was varied over a range of 11 to 52 mm Hg in groups I and II. There was no correlation between decreases in PvO2 and the percent change in left lung PVR compared with baseline (PvO2 approximately 40 mm Hg) in group I (r = 0.12, difference not significant). In contrast, there was a significant correlation between decreases in PvO2 and the percent change from baseline in PVR in group II (r = -0.50, p less than 0.02). Left lung alveolar oxygen tension (PAO2) decreased significantly when PvO2 was at its lowest in group II (p less than PAO2 0.05). Both groups I and II had a significant pulmonary vasoconstrictor response to hypoxic ventilation, indicating that vasoreactivity was preserved in this model. The increase in PVR with hypoxia was significantly blunted in dogs perfused with blood that had a high PvO2 (41 +/- 12.4 mm Hg, group IIIa) compared with dogs perfused with blood that had a low PvO2 (25 +/- 6 mm Hg, group IIIb), despite comparable values for PAO2.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Transient unilateral hypoperfusion of the lung following mediastinoscopy

Two cases of pulmonary hypoperfusion occurring after mediastinoscopic examination were demonstrated on lung scans. In one case, this finding on the ninth day required a pulmonary angiographic study that yielded normal findings. Repeat lung scans were normal. We propose that localized mediastinal edema or bleeding after mediastinoscopic examination can produce defects of hypoperfusion, and we urge caution in the interpretation of lung scans up to nine days after mediastinoscopic examination.

Increased pulmonary vascular resistance with systemic hypertension. Effect of minoxidil and other antihypertensive agents

Recent case reports suggest that pulmonary hypertension could be caused by minoxidil, a new potent vasodilating antihypertensive drug. Therefore, we evaluated the incidence and severity of pulmonary hypertension in 110 patients with systemic hypertension. Fourteen patients were treated with minoxidil for 2 to 35 months (mean 19.9 months), 15 were treated with no drugs, and the remaining 81 patients received conventional antihypertensive agents of several types. Pulmonary vascular resistance correlated positively (P is less than 0.05) with systemic vascular resistance. Minoxidil-treated patients with hypertension previously refractory to conventional therapy had slightly lower pulmonary vascular resistance than other hypertensive subjects. There was no correlation between pulmonary vascular resistance and duration of minoxidil therapy or other types of antihypertensive regimens. The positive correlation between pulmonary and systemic vascular resistance suggests the possibility of a causal hypertension relation in the two vascular beds.

The role of lung imaging in pulmonary embolism

The advantages of lung scanning in suspected pulmonary embolism are its diagnostic sensitivity, simplicity and safety. The ability to delineate regional pulmonary ischaemia, to quantitate its extent and to follow its response to therapy provides valuable clinical data available by no other simple means. The negative scan effectively excludes pulmonary embolism but, although certain of its features favour the diagnosis of embolism, the positive scan inherently lacks specificity and requires angiographic confirmation when embolectomy, caval plication or infusion of a thrombolytic agent are contemplated. The addition of simple ventilation imaging techniques with radioxenon overcomes this limitation by providing accurate analog estimation or digital quantitation of regional ventilation: perfusion (V/Q) ratios fundamental to understanding the pathophysiologic consequences of embolism and other diseases of the lung.

Conditions governing the pulmonary vascular response to ventilation hypoxia and hypoxaemia in the dog

1. Isolated lung lobes of the dog perfused through the pulmonary circulation only with atropinized autologous blood obtained by bleeding out the animal under general anaesthesia or following premedication with morphine hydrochloride were subjected to repetitive tests of ventilation hypoxia, the control and test gas mixtures containing similar concentrations of CO(2).2. The pulmonary vasomotor response to ventilation hypoxia depended upon the temperature of the perfusate and the time which had elapsed from the death of the animal to the start of perfusion, termed the ;ischaemic period'. The higher blood temperatures and shorter ischaemic periods favoured a pulmonary vasopressor response to hypoxia, and the lower temperatures and longer ischaemic periods a vasodepressor response or an absence of response.3. The vasopressor responses to hypoxia were associated with a rise in the pH (average, 0.09 in 5 experiments) and a fall in the P(CO2) of the blood. There were no consistent changes in the pH and P(CO2) of the blood accompanying vasodepressor responses.4. The vasopressor responses could be obtained over periods of perfusion lasting 4 hr or longer.5. It is suggested that changes in the composition of blood equilibrated in the isolated perfused lung cannot be predicted from in vitro dissociation curves.

Humoral agent from calf lung producing pulmonary arterial vasoconstriction

Saline washings obtained in vivo from the lung of young calves produce pulmonary hypertension upon intrayascular (systemic or pulmonary) injection into either the dog or the calf. This pulmonary hypertension is produced by vasoconstriction of small, precapillary pulmonary vessels. The active agent, pulmonary arterial constrictor substance, differs chemically and physiologically from other substances which have been investigated with respect to vasomotor activity in the pulmonary circulation. Although the chemical nature of the active agent is not known it appears to have a relatively large molecular weight. Whether this agent plays a role in the physiological regulation of the pulmonary circulation is not known.

Pulmonary Capillary Flow Pulse and the Site of Pulmonary Vasoconstriction in the Dog

A nitrous oxide-plethysmographic method was used to measure simultaneously the instantaneous pulmonary capillary blood flow and the pressure gradient across the pulmonary vascular bed. The method entailed the use of lightly anesthetized, curarized dogs for the moment-to-moment measurement of nitrous oxide uptake and the corresponding pulmonary arterial and wedge pressures. After establishing the usual patterns of pulmonary capillary blood flow, the flow patterns were modified by atropine, norepinephrine, and acute hypoxia. Before the exhibition of these agents, pulmonary capillary flow was pulsatile, with peak rates of flow of approximately twice the mean. After atropine, the amplitude of the flow pulse decreased passively as stroke volume and the pulmonary arterial pulse pressure decreased. In contrast to atropine, acute hypoxia elicited precapillary vasoconstriction as manifested by an increase in pulmonary arterial pressure and an unchanged pulmonary arterial wedge pressure in conjunction with an unchanged, or diminished and blunted, pulmonary capillary flow pulse. The predominant effect of norepinephrine appeared to be exerted distal to the pulmonary capillary bed since pulmonary arterial and wedge pressures increased in parallel while the capillary flow pulse changed passively according to changes in stroke volume.