Anesthesia, Mechanical Ventilation, and Hypoxic Pulmonary Vasoconstriction




Anesthesia, Mechanical Ventilation, and Hypoxic Pulmonary Vasoconstriction



Andres Hagerman, Marc Licker



Abstract


The inhalational anesthetic (InhA) technique offers a simple and reliable way to control the depth of anesthesia, to protect the organs against ischemia-reperfusion injuries, to potentiate the effects of neuromuscular blocking agents, and to decrease the airway resistance. In experimental models of sepsis, the exposure to halogen-containing hydrocarbons has been shown to improve survival, whereas propofol anesthesia is associated with increased mortality along with larger systemic release of endotoxins. Likewise, in thoracic surgery with one-lung ventilation, the early pulmonary protective effect of InhA is accompanied by a lower level of alveolar inflammatory mediators, as compared with intravenous anesthetic (IVA). In contrast, regarding the long-term effects, the impairment in cell-based immunity induced by InhA may promote “seeding” of tumor cells at the time of cancer resection and could explain a higher rate of cancer recurrence and a reduction in long-term survival, when compared with IVA. Because hypoxic pulmonary vasoconstriction is inhibited by volatile anesthetics, some anesthesiologists preferentially select IVA to avoid (or to treat) hypoxemia because of intrapulmonary blood shunting. Another reason to select an intravenous agent is the need to continue mechanical ventilation in the intensive care unit, a place where InhAs are usually not used. Patients with a higher preoperative risk profile caused by advanced respiratory, cardiac, or renal diseases would be the best candidates to receive volatile anesthetics given their preconditioning and protective organ effects. Conversely, switching from InhA to IVA could be indicated if patients present oxygen desaturation because of intrapulmonary shunting and/or if they experience intraoperative complications that justify the continuation of mechanical ventilation after surgery.


Keywords


pulmonary flow; hypoxemia; pulmonary hypertension; intrapulmonary shunt; ventilation-perfusion mismatch; intravenous anesthesia; volatile anesthetics



Introduction


One-lung ventilation (OLV) is a standard procedure required in thoracic surgery for cancer resection, treatment of bronchopulmonary fistula or bleeding, and lung transplantation, as well as for intrathoracic diagnostic investigations.1 Over the past 3 decades, the incidence of intraoperative hypoxemia has decreased and clinical outcome after thoracic surgery has largely been improved owing to the mastering of lung isolation techniques, better understanding of physiology related to OLV, application of protective lung ventilation, and implementation of minimally invasive surgical techniques.2


Hypoxic pulmonary vasoconstriction (HPV) represents an intrinsic property of pulmonary vascular smooth muscle cells (VSMCs), and an important homeostatic mechanism that helps to ensure correct gas exchange during OLV. In 1946, von Euler and Liljestrand first described the opposing hypoxic responses in pulmonary and systemic circulations expressed by vasoconstriction and vasodilatation, respectively.3 Ontogenetically, the HPV evolves in mammals from the need to ensure fetal oxygenation when the amniotic fluid-filled lungs are hypoxic.4 Actually, sensing low oxygen (O2) tension in pulmonary VSMC triggers HPV, which maintains high pulmonary vascular resistance (PVR) and diverts the oxygenated placental blood through the foramen ovale and the ductus arteriosus directly into the systemic arteries. Upon the first breath at birth, sensing the rising alveolar partial oxygen pressure (PO2) or PAO2 in pulmonary VSMCs interrupts the HPV response, allowing the deoxygenated blood flow to be distributed to all ventilating lung areas through a low resistive pulmonary circulation, whereas the foramen ovale and the ductus arteriosus both obliterate to prevent right-left shunting.


Although exerting minimal effects in healthy lungs under normoxic conditions, HPV plays an important role during OLV in thoracic surgery and during pneumologic procedures to reduce intrapulmonary shunt and preserve blood oxygenation. In healthy volunteers undergoing awake fiberoptic bronchoscopy, the HPV response has been nicely demonstrated using lung scintigraphy and intravenous administration of 99mTc-labeled albumin.5 After inflation of a balloon-tipped catheter in the left lower bronchus, PAO2 and alveolar carbon dioxide partial pressure (PACO2) (distal to the tip of the occluding balloon) approached mixed venous levels within 80 to 300 seconds, and blood flow decreased by 50% in the lower lobe, with redistribution to other parts of the lungs.


In patients with pneumonia, atelectasis, asthma, chronic obstructive bronchopulmonary disease (COPD), and acute respiratory distress syndrome (ARDS), HPV represents a key homeostatic mechanism to attenuate hypoxemia by adapting ventilation to the perfusion (V/Q) ratio in various parts of the lungs.6 In contrast, when the whole lung is exposed to hypoxia (e.g., high altitude, sleep apnea, pulmonary fibrosis), HPV can be detrimental, resulting in pulmonary hypertension and acute cor pulmonale owing to a progressive increase in right ventricular afterload.


Anesthesiologists should understand the basics of the HPV response, its role in various physiologic and pathologic conditions, including OLV, and the way perioperative drugs, including anesthetic agents, may interfere with this protective reflex.



Physiology of Hypoxic Pulmonary Vasoconstriction



The Hypoxic Signal in Different Tissues


Phylogenetically, the mammalian homeostatic O2 sensing system (HOSS) has developed a multicomponent defense mechanism to hypoxic environments aimed to prevent cellular injuries, mitigate the decline in O2 tension, and adjust the energetic metabolism. The HOSS implicates a synchronized transcriptional regulation of a variety of genes, and it is coordinated, in large part, by the hypoxic-inducible factor (HIF) and transcription factors such as nuclear factor-kB ­(NF-kB) and activated protein-1, which are “master regulators” for oxygen homeostasis and inflammatory processes.7 The HOSS entails a network of specialized cells such as VSMCs (pulmonary and placental arteries, ductus arteriosus), endothelial cells (ECs), glomic cells in the carotid body, renal myofibroblasts, adrenomedullary chromaffin cells, and neuroepithelial bodies in the airways. When exposed to hypoxia, these specialized tissues detect low O2 tension in their local environments through mitochondrial redox-based or cytoplasmic HIF, and they orchestrate coordinated responses to optimize oxygen uptake and delivery (Fig. 15.1).8 In type I glomus cells from the carotid and aortic bodies, the low O2 tension leads to inhibition of voltage-gated potassium channels (Kv), resulting in membrane depolarization and influx of extracellular Ca2+ via voltage-gated Ca2+ channels (VGCC), which triggers the release of neuromediators and a faster ventilatory drive. In the kidney, the low O2 content (e.g., difference between O2 delivery and O2 utilization) leads to inhibition of prolyl-4-hydroxylase domain proteins, which activates HIF signaling to generate erythropoietin and in turn to enhance erythropoiesis. Hypoxia-mediated release of catecholamines, vascular endothelial growth factors, and erythropoietin all tend to enhance tissue O2 delivery owing to increased cardiac output (CO), angiogenesis, and higher hemoglobin (Hb) concentrations.


image
• Fig. 15.1 Oxygen sensing and cellular response. HIF, Hypoxic inducible factor; HO, heme oxygenase; iNOS, inducible nitric oxide synthase; VSMC, vascular smooth muscle cell.

The HPV response chiefly operates in arteriolar and venular pulmonary vessels (diameter < 900 μm), veins accounting for 20% of the total increase in PVR. By distorting the alveolar wall, contractile interstitial cells located within the alveolar septa slightly contribute to reduce capillary blood flow and to increase PVR.9 The ECs modulate the HPV through the local release of vasodilators (nitric oxide [NO], prostacyclin [PGI2]) and vasoconstrictors (endothelin [ET], epoxyeicosatrienoic acid [EET], thromboxane A2 [TXA2], and platelet activating factor), with pulmonary veins exhibiting greater sensitivity to vasoconstrictor stimuli than arteries.10,11


From ex vivo ventilated/perfused lungs in which alveolar and blood O2 tension were modified independently, Marshall et al. have demonstrated that acute elevation in PVR was predominantly triggered by sensing low PAO2and less by sensing deoxygenated blood in mixed venous blood (PmvO2).12,13 The stimulus of HPV (PO2S) has been modeled according to the following equation: PO2 stimulus = PAO2 0. 62 + PmvO2 0. 38.


When pulmonary VSMCs or isolated lungs are exposed to decremental levels of local PO2, the HPV develops at a threshold of 85 to 90 mm Hg, to reach a maximum at 65 to 70 mm Hg (about halfway between PAO2 and PmvO2) and declining thereafter to stop around 5 mm Hg (Fig. 15.2).14


image
• Fig. 15.2 Thresholds for physiologic responses to low oxygen tension. HIF, Hypoxic inducible factor; HPV, hypoxic pulmonary vasoconstriction; HV, hypoxic ventilation.

The O2 sensing area is thought to extend from precapillary arterioles to upstream pulmonary arteries with different vasoconstrictive responses according to the size of the vessels.15 The small arteries (diameter < 0.5 mm) are closely exposed to PAO2 (from outside) and to PmvO2 (from inside), whereas larger arteries (diameter of 0.5–2 mm) that are supplied by systemic blood through the vasa vasorum are exposed to both PmvO2 (from inside) and to arterial O2 pressure (PaO2 from outside). Hence, variable diffusion O2 gradients are generated through the wall of pulmonary arteries that determine the true stimulus for HPV (PO2S), being predominant in small arteries. In larger arteries, the HPV response is less profound and mainly activated when systemic hypoxemia develops (PaO2 < 40–50 mm Hg), as bronchial arteries uniformly dilate like systemic vessels.



Time Course of Hypoxic Pulmonary Vasoconstriction


Hypoxic exposure of pulmonary VSMCs results in a biphasic vasoconstrictive response. The early phase starts immediately, peaking at 5 to 20 minutes and is associated with transient elevations of intracellular calcium concentrations. Phase 2 develops more gradually to reach a second peak at 60 to 120 minutes, as a result of myofibrillar calcium-sensitization coupled with enhanced release of vasoconstrictive compounds (ET-1 and EET) and decreased production of NO from the ECs (Fig. 15.3).14 A sustained vasoconstrictive response may not persist in case of severe hypoxia. If normoxia is restored after or during the early hypoxic phase (<20 minutes), the HPV stops allowing normalization of PVR and pulmonary blood flow with improved V/Q ratios. In contrast, after a prolonged period of hypoxia, the HPV disappears over several hours despite full reoxygenation, resulting in greater ventilation-perfusion (V/Q) mismatch and consequently a larger alveolar-to-arterial O2 pressure gradient (A-aPO2). A second hypoxic challenge (in ipsilateral lung) will result in an enhanced HPV response.


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• Fig. 15.3 Time course and factors influencing hypoxic pulmonary vasoconstriction (HPV). V/Q, ventilation/perfusion.

Both atelectasis formation and persistent HPV are incriminated in causing hypoxemia in the early hours after thoracic anesthesia. Likewise, in bilateral thoracic interventions and after starting the second stage, a V/Q mismatch may develop in the dependent ventilated lung, which is attributed to the residual HPV response that lags behind the time of reoxygenation.



The Oxygen Sensing and Effector Mechanisms of Hypoxic Pulmonary Vasoconstriction


In eukaryotic cells, mitochondria are acting as powerhouses using mainly O2 to convert nutrients into chemical energy in the form of adenosine triphosphate (ATP) that is used in processes including muscle contraction, ion transport, nerve impulse propagation, and chemical synthesis. A series of oxidation-reduction reactions involves the transfer of electrons from nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2) across the electron transport chain (ETC). Because mitochondrial respiration continues even at very low O2 levels (∼7 mm Hg) to generate ATP, it is unlikely that variations in ATP concentrations trigger the hypoxic response.16 Ultimately, in the process of oxidative phosphorylation, O2 serves as the terminal electron acceptor of the ETC to form H2O, while, at multiple sites along the ETC, some leaking electrons react with O2, forming reactive oxygen species (ROSs), such as superoxide and hydrogen peroxide.


In pulmonary VSMCs, the most plausible mechanism todetect hypoxia is the mitochondrial PO2-sensitive ability tovary the production of diffusible ROSs and/or to alter the cytosolic redox state. Although controversies surround the issue on whether ROSs generated by the ETC decrease or increase in response to hypoxia, there is strong evidence that HPV is triggered by mitochondrial signals that launch a coordinated response involving membranar and sarcolemnal potassium and calcium channel receptors to increase cytoplasmic calcium concentrations.17 As proposed by Smith and Schumacker,17 mitochondrial ROSs (see Fig. 15.4) move to the cytosol, where they activate several pathways leading to intracellular calcium mobilization: (1) inhibition of Kv channels causing membrane depolarization, which promotes the opening of VGCC; (2) activation of phospholipase C (PLC) to generate diacyl glycerol (DAG) and inositol 1,4,5-triphosphate (IP3) that stimulate calcium release, respectively, from membranal receptor-operating channels (ROCs) and sarcolemnal IP3 receptors; and (3) activation of ryanodine receptors (RyR) by oxidation of cysteine residues. In addition, by increasing the ratio of adenosine monophosphate (AMP) to ATP, low O2 tension stimulates AMP-kinase (AMPK), resulting in activation of the RyRs and further Ca2+ release from the sarcoplasmic reticulum (SR). Together, the elevation of intracellular Ca2+ concentrations induces calcium binding to calmodulin, subsequent activation of myosin light chain (MLC) kinase, myosin phosphorylation, and ultimately contractions of VSMCs.


After activation of Rho kinase and MLC phosphatase, myofibrillar proteins become more sensitized to the effects of calcium sensitization, resulting in sustained contractions of pulmonary VSMCs. Besides acting directly on calcium channels, ROSs influence the cytoplasmic redox state, namely, the ratio of oxidized to reduced proteins, glutathione (GSSG/GSH), and nicotinamide adenine dinucleotide (NAD/NADH). According to the redox hypothesis, hypoxic conditions are associated with lesser release of ROSs from the mitochondria that promotes a shift from an oxidized to reduced cellular redox state in the cytoplasm, which in turn triggers Ca2+ mobilization through inhibition of Kv channels and opening of VGCCs.


When exposed to hypoxia, both pulmonary and systemic VSMCs share the same effector mechanisms of calcium-mediated changes in vascular tone, whereas pulmonary VSMCs present the unique property to detect low levels of O2 tension and transmit signals to mobilize Ca2+ close to myofibrillar proteins. Mitochondria are located closer to the plasmalemmal membrane in pulmonary VSMCs compared with systemic VSMCs.18 Therefore a greater structural and functional coupling between mitochondria and Kv channels in pulmonary VSMCs could explain the specific O2 sensitivity and HPV response.



Neurohumoral Modulation of Hypoxic Pulmonary Vasoconstriction


Exposures to intermittent or chronic hypoxia (e.g., pulmonary diseases, altitude) represent stressful challenges that, by stimulating the sympathetic nervous and renin-angiotensin-aldosterone systems (SNS and RAAS), result in elevation of systemic and pulmonary arterial pressures (PAPs).


Airways and pulmonary vessels are innervated by autonomic and sensory nerve fibers, the highest density of noradrenergic and cholinergic neural endings being found around extrapulmonary and hilar arteries and decreasing toward the periphery (artery size > 60–80 mcm).19 The vagal nerve of the parasympathetic pathway originates from the nucleus ambiguus in the brain stem, whereas sympathetic nerves emerge from the middle and inferior cervical and the first five thoracic ganglia. Vagal stimulation causes mild vasorelaxation mediated by the endothelial release of NO and the vasoactive intestinal factor (VIP). Stimulation of the sympathetic nervous system produces the release of norepinephrine (NE) from sympathetic neural endings close to the arterial wall and of epinephrine from the adrenal medulla in the blood stream. Stimulation of alpha-1–adrenergic-receptors (AAR-1) with NE causes vasoconstriction, owing to the inhibition of adenylate cyclase and the combined activation of Gq/11-proteins and PLC, that produce IP3 and DAG, which in turn enhance the internal/external release of Ca2+. In contrast, epinephrine causes potent vasodilatory effects by stimulation of beta-1 and beta-2 adrenergic receptors (BAR-1 and BAR-2) and consequent activation of the cAMP-dependent protein kinase pathway and the release of NO from ECs. Under normoxic and unstressed physiologic conditions, the sympathovagal systems play a negligible role in pulmonary vasomotor tone because PVR remains largely unchanged in animals subjected to a chemical, pharmacologic, or anesthetic sympathectomy. However, under hypoxic conditions, pulmonary BAR-mediated vasodilatation has been shown to predominate over AAR-1 vasoconstriction, partially blunting the potent acute HPV.20 For instance, in patients with sleep apnea, BAR-mediated relaxation of pulmonary arteries offsets the HPV, mitigates the elevation of PVR which in turn alleviates the workload of the right ventricle. Lending support to these clinical findings, experimental data have demonstrated that the HPV response is attenuated with alpha-adrenergic blockers and enhanced with beta-adrenergic blockers.21


Hypoxic challenge has been shown to increase the activity of renal adrenergic receptors (AAR-1 and BAR-1) in juxtaglomerular cells, resulting in increased renin secretion and release of angiotensin II (Ang II) by the action of an angiotensin converting enzyme (ACE) largely present in the endothelium of pulmonary capillaries and in other tissues (e.g., brain, heart). In VSMCs, Ang II exerts dual effects on angiotensin receptors type 1 and 2 (AT-1 and AT-2): (1) vasoconstriction mediated by stimulation of the G-protein of AT-1, leading to Ca2+ mobilization, owing to activation of PLC and subsequent formation of IP3 and DAG; and (2) vasodilation mediated by stimulation of AT-2, leading to increased cyclic guanosine monophosphate (cGMP) and subsequent release of NO.22 The attenuation of HPV in patients chronically treated with ACE inhibitors suggests the predominant constrictive effect of Ang II.23


Altogether, there is strong evidence supporting that hypoxia-induced stimulation of the SNS and RAAS results in systemic cardiovascular and renal changes (e.g., tachycardia, hypertension, oliguria), a blunted HPV response, owing to the predominant pulmonary BAR-2 vasodilatory effects, which partially offset the constricting effects on AAR-1 and AT-1 receptors.24 Interestingly, a preserved HPV response has been reported in patients after bilateral lung transplantation, despite the loss of the autonomic neural innervation.25



Modulation of Hypoxic Pulmonary Vasoconstriction by the Endothelium


Depending on experimental settings, denuding the endothelium from the pulmonary arteries may result in either suppression or enhancement of the HPV response, and the reasons for these opposing effects still remain unclear.26 Nevertheless, by sensing low O2 tension and releasing active vasodilatory and vasoconstrictor compounds, ECs contribute to regulate vascular tone.


Real-time fluorescence imaging in intact lungs demonstrated that hypoxia induces membrane depolarization in capillary ECs that propagates retrogradely to the feeding arterioles via gap junctions and connexin proteins (C×40). In parallel to this endothelial signal, a similar response is propagated upstream through precapillary VSMCs via other connexin proteins (C×43).27 A growing body of evidence indicates that the release of endothelial vasoactive compounds enhances the second phase of acute HPV, owing to decreased NO levels, increased ET-1 and EETs production, whereas the increased PG12-mediated relaxation is offset by the enhanced TXA2-mediated constriction of VSMCs.


During prolonged hypoxia, proliferation of ECs is associated with the release of mitogenic factors for VSMCs, and generation of ET-1 dose-dependently mediates the overexpression of Kv and transient receptor potential channels, thereby contributing to the development of pulmonary hypertension (PH).28



Physiologic Variations in Hypoxic Pulmonary Vasoconstriction Responses


In healthy individuals breathing air at sea level, pulmonary vessels are almost fully dilated, and inhalation of NO—a selective pulmonary vasodilator—does not improve oxygenation, suggesting that the HPV response is nonactivated under normoxic conditions.



Pulmonary Circulation


Compared with the systemic circulation, pulmonary vessels offer greater distensibility and recruitability to accommodate higher blood flow and volemia while blunting the elevation in pulmonary blood pressure. In contrast with the systemic circulation, the pressure-flow relationship in the pulmonary circulation is not linear because of the greater distensibility and potential recruitability of its vasculature. Therefore the pulmonary circulation may adapt to changing blood flow conditions, while minimizing the changes in PAP.29 For instance, a reduction in CO (e.g., head-up position, blood loss) leads to a larger drop in pulmonary vascular driving pressure, even though the vasomotor tone remains per se unchanged.30 Conversely, as pressure along the entire circuit increases, the pressure-flow curve becomes more linear because the fully distended vessels are by definition no longer distensible. Hence, when left atrial pressure increases with fluid loading or head-down position, the ability of hypoxic constriction to either redirect blood flow or reduce conductance is curtailed.


Lung volume also influences PVR, as described by a parabolic U-shaped curve. The PVR reaches a nadir at the functional residual capacity (FRC), and it increases at higher inflating pressures as alveolar capillaries are “squeezed” by elevated alveolar pressures, or when breathing operates below the FRC, as the extraalveolar vessels become distorted and partially obstructed, offering greater resistance to blood flow. Complete lung collapse, as occurs during OLV in thoracic surgery, results in mechanical vascular obstruction that contributes to blood flow diversion to the ventilated dependent lung.


Besides the PAO2 level, the strength of HPV also depends on the basal vascular tone and the volume of the lung exposed to hypoxia. A more potent HPV response is elicited when PVR is low (near FRC), and it decreases when the lung is overdistended or totally collapsed. From animal studies, we know that if exposure to hypoxia exceeds 40% of the whole lung, the redistribution of blood flow to nonhypoxic pulmonary areas is decreased, suggesting that the increased PAP tends to overcome the increased local arteriolar resistance generated by HPV.31



Physical Factors: Calcium, CO2, pH, Temperature


Pulmonary arteries are minimally sensitive to changes in plasma calcium concentrations.32 In the isolated blood perfused canine lung, severe hypocalcemia is required to induce vasodilation and fails to influence the HPV response. Conversely, acute hypercalcemia fails to modify the pressure-flow curves and PVR, suggesting that calcium chloride can be safely administered intraoperatively through a right-sided venous catheter.


With the development of whole body hypothermia (from 37°C –27°C), both systemic and PVR are increased mainly as a result of cardiac depression, lower CO, and enhanced vasomotor tone,33 whereas the HPV decreases linearly with the drop in central temperature.34 Blunting of HPV is also observed with hypocapnia and metabolic alkalosis. Conversely, hypercapnia has been associated with decreased exhaled NO and enhanced HPV response in isolated ventilated-perfused lungs.35



Senescence


With aging, oxidative stress and proinflammatory processes are the culprit mechanisms involved in atherogenesis, thickening and stiffening of systemic arteries that manifest clinically by hypertension, coronary artery disease, and heart failure. Likewise, the low pressure pulmonary circulation presents age-related reductions in arterial and venous distensibility, along with decreased left ventricle compliance leading to increased PAP. The reduction in antioxidant capacity with increased oxidative stress is incriminated in endothelial dysfunction and vascular extramatrix deposition of collagen.36 Accordingly, PAP increases disproportionately more during exercise in older than in younger healthy humans, given lesser vasodilating mediators released from the ECs, stiffening of the vascular wall, and left ventricular diastolic dysfunction. In the elderly, the lesser increase in CO is associated with higher O2 extraction in skeletal muscles, together with a greater fall in PmvO2 that may act as a stimulus for enhanced HPV and further elevation in PAP during intense aerobic exercise.


Middle-aged subjects exposed to moderate alveolar hypoxia (PAO2 ∼50 mm Hg) over 20 minutes exhibit a greater increase in mean systolic PAP compared with younger individuals (+9 mm Hg) and similar changes in heart rate and in CO. These age-related enhanced HPV responses could be explained by the greater sensitivity of the O2 sensing apparatus, greater propensity for Ca2+ cell entry, and greater activity of the calcium-calmodulin-dependent MLC kinase that contribute to sustained contractile response of pulmonary VSMCs.37



Altitude


In contrast with regional HPV acting in case of atelectasis or pneumonia, generalized HPV in response to hypobaric hypoxia causes various degrees of PH that may be associated with interstitial and alveolar edema in susceptible patients.38 The so-called “high-altitude pulmonary edema” (HAPE) occurs in nonacclimatized healthy individuals within 1 to 5 days after arrival at altitudes higher than 2500 m but also in well-acclimatized high-landers returning from low altitudes.39 Lung imaging in symptomatic HAPE shows a patchy peripheral and nodular distribution of edema, whereas bronchoalveolar lavage reveals a protein-rich exudate with mild hemorrhage, neutrophilia, and proinflammatory cytokines consistent with the heterogeneous distribution of HPV and the involvement of inflammation secondary to alveolar-capillary barrier disruption. In addition, the alveolar fluid clearance mechanisms are impaired by hypoxia, owing to the reduced expression and activity of epithelial sodium channels and sodium/potassium (Na+/K+) ATPase proteins linked to (BAR-2) signaling.



Genetics


Epidemiologic studies have shown an association between early hypoxic exposure and later development of hyperreactivity in pulmonary vascular response.40 For instance, transient perinatal hypoxia was found to predispose to exaggerated HPV when low-altitude dwellers were exposed to acute hypobaric hypoxic conditions. Likewise, the occurrence of preeclampsia in Bolivian high-altitude dwellers was shown to predispose their offspring to greater sensitivity of HPV response, owing to augmented oxidative stress and impaired NO bioavailability.


Different adaptive phenotypes have been identified in populations living in high altitudes. Compared with Han Chinese low-landers, Tibetans high-landers show hyporesponsiveness of the HIF transcriptional system characterized by a blunted HPV response associated with high exhaled NO concentrations, few hypertrophic changes in pulmonary arteries, and minimal increase in Hb concentrations.41 Andean highlanders exhibit other physiologic characteristics, namely, a blunted hypoxic ventilatory response, polycythemia, larger lung volumes, and better pulmonary diffusion compared with Tibetans. These variable adaptive solutions reflect different evolutionary pathways selected to survive in hypoxic environments.42



Variations of Hypoxic Pulmonary Vasoconstriction in Pathologic Conditions


In the clinical settings, the strength of the HPV response can be estimated by comparing physiologic markers under hypoxic versus normoxic/hyperoxic conditions: (1) the ratio of PAP under constant flow conditions, (2) the changes in PaO2, (3) measurements or calculation of intrapulmonary shunt (QS/QT), and (4) the dispersion of the V/Q ratio before/after the administration of drugs affecting the HPV.


The multiple inert gas elimination technique (MIGET) uses the relationships between arterial, expired, and mixed venous concentrations of trace amounts of six marker gases dissolved in saline and infused intravenously.43 The distribution of ventilation and perfusion is expressed as a function of VA/Q ratio and provides an index of overall VA/Q mismatch as characterized by the standard deviation of the distributions of ventilation and perfusion (LogSD Q and LogSD V). According to Riley and Cournand, the bell curve of V/Q scatter originally supported the theory of the three-lung units: one unit with ideal gas exchange (V/Q ratios close to 0.8–1), one consisting of true shunt (V/Q < 0.005), and one of “dead space ventilation” (V/Q > 100).44 Currently, the MIGET model includes 50 units, each with a predefined V/Q ratio, and it provides a conceptually practical approach to understand the changes in pulmonary gas exchange, taking also into account participation of very low V/Q units in “venous admixture.”



Chronic Lung Diseases


In chronic lung diseases (e.g., COPD, interstitial fibrosis, cystic fibrosis, bronchiectasis), hypoxic stress is known to induce pulmonary vascular cell proliferation (fibroblasts, VSMCs) and vascular remodeling (cellular hypertrophia, differentiation of fibroblast into myoblasts, extracellular deposits of collagen and elastin), resulting in eccentric wall thickening and the development of PH (∼10% of COPD patients). The contribution of HPV to PH is greatest in the early disease stages of PH, when O2 therapy alleviates the hypoxia-induced vasoconstriction, whereas at later stages, PH is resistant O2 therapy because of structural “fixed” vascular remodeling.


The severity of airflow obstruction in COPD patients does not correlate with hypoxemia, which is largely determined by the heterogeneity of V/Q distribution (shunt and low V/Q units). In COPD patients, Melot et al.45 demonstrated that almitrine besylate was associated with increased PaO2 (+13%) and reduced V/Q inequalities, whereas nifedipine, a Ca2+ channel blocker, resulted in lower PaO2 (‒15%) by inhibition of HPV and consequent blood flow shifting from normal V/Q to low V/Q alveolar units.


Distinct phenotypes have been identified with regards to HPV that are linked to the expression of a regulatory subunit of NADPH oxidase (p22phox) in EC and in pulmonary VSMs.46 In a small cohort of 71 patients with end-stage COPD, a positive correlation was found between the vascular expression of a regulatory subunit of NADPH oxidase (p22phox), mean pulmonary artery pressure, and the oxygenation index (PaO2/FiO2 ratio).47 These findings suggest distinct phenotypes: the low expression of p22phox was associated with diminished HPV and low PAP. In contrast, higher expression of p22phox was associated with preserved HPV that allowed better V/Q matching but at the cost of PH and later cor pulmonale.




Asthma


Studies using MIGET and positron emission tomographic/computed tomographic lung imaging in patients with asthma have shown that the uneven distribution of bronchoconstriction was paralleled by heterogeneous regional hypoxia and V/Q mismatch with consequent hypoxemia.48 During methacholine-induced bronchoconstriction in patients breathing room air, blood flow was reduced by almost 30% in hypoventilating areas and increased by 10% in normal-/high-ventilated areas. In the same patients, breathing 80% O2 was found to attenuate the averaged blood flow redistribution by 50%. Given the efficacy of HPV, blood flow was shifted from hypoxic low V/Q areas to normal and high V/Q areas. In addition, blood flow was also redistributed from nonhypoxic lung regions (low V/Q areas) as a result of mechanical and biologic interactions between the constricted airways, pulmonary vessels, and lung parenchyma.



Obstructive Sleep Apnea


In the absence of chronic cardiopulmonary diseases, about 30% of patients with obstructive sleep apnea (OSA) present PH.49 Although presenting normal O2 saturation while awake, these patients experience repeated short-lasting episodes of desaturation during sleep, often associated with arousals. The cumulative effects of these intermittent hypoxic/hypercapnic challenges result in activation of the SNS, polycythemia, and vascular remodeling in both systemic and pulmonary arteries. The mechanisms of PH are caused by an increased left ventricular preload (postcapillary PH), a sustained HPV response, and vascular changes characterized by proliferation of VSMCs and asymmetric neointimal hyperplasia (precapillary PH). Repeated short-lasting hypoxic episodes augment the pulmonary vasoconstrictor reactivity by triggering endothelial release of vasoconstrictor mediators (e.g., ET-1), as well as mitochondrial formation of ROSs with myofibrillar Ca2+ sensitization dependent on the protein kinase C-beta in VSMCs.

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Oct 6, 2021 | Posted by in ANESTHESIA | Comments Off on Anesthesia, Mechanical Ventilation, and Hypoxic Pulmonary Vasoconstriction
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