Restored hypoxic pulmonary vasoconstriction by peripheral chemoreceptor agonists in dogs

Almitrine for COVID-19 critically ill patients – a vascular therapy for a pulmonary vascular disease: Three case reports

BACKGROUND

Several reports with clinical, histological and imaging data have observed the involvement of lung vascular function to explain the severe hypoxemia in coronavirus disease 2019 (COVID-19) patients. It has been hypothesized that an increased pulmonary blood flow associated with an impairment of hypoxic pulmonary vasoconstriction is responsible for an intrapulmonary shunt. COVID-19 may lead to refractory hypoxemia (PaO₂/FiO₂ ratio below 100 mmHg) despite mechanical ventilation and prone positioning. We hypothesized that the use of a pulmonary vasoconstrictor may help decrease the shunt and thus enhance oxygenation.

CASE SUMMARY

We report our experience with three patients with refractory hypoxemia treated with almitrine to enhance oxygenation. Low dose almitrine (Vectarion^®; Servier, Suresnes, France) was started at an infusion rate of 4 μg × kg/min on a central line. The PaO₂/FiO₂ ratio and total respiratory system compliance during almitrine infusion were measured. For the three patients, the PaO₂/FiO₂ ratio time-course showed a dramatic increase whereas total respiratory system compliance was unchanged. The three patients were discharged from the intensive care unit. The intensive care unit length of stay for patient 1, patient 2 and patient 3 was 30 d, 32 d and 31 d, respectively. Weaning from mechanical ventilation was performed 13 d, 18 d and 15 d after almitrine infusion for patient 1, 2 and 3, respectively. We found no deleterious effects on the right ventricular function, which was similar to previous studies on almitrine safety.

CONCLUSION

Almitrine may be effective and safe to enhance oxygenation in coronavirus disease 2019 patients. Further controlled studies are required.

Physiology of One-Lung Ventilation

The understanding of the physiology and management of one-lung ventilation (OLV) has advanced over the last two decades. OLV induces an obligatory shunt through the nonventilated lung that causes varying degrees of arterial hypoxemia. Shunt may also occur in the venti lated lung. The optimal mode of ventilation of the dependent lung has not been well defined. The optimal tidal volume, respiratory rate, inspired oxygen concen tration, and positive end-expiratory pressure (PEEP) during OLV are not known. Functional residual capacity (FRC) of the ventilated lung can be lower than during two-lung ventilation, causing atelectasis and arterial hypoxemia. Patients who desaturate might be expected to show improvement in oxygenation with dependent lung PEEP, because of increased FRC and reduced V/Q mismatch. Not all patients have low lung volumes, and not all patients who have low lung volumes will desatu rate. Therefore, prophylactic PEEP is not usually neces sary or appropriate. Because the predominant cause of hypoxemia during OLV is shunt in the nondependent lung, therapies to improve arterial oxygenation during OLV should be primarily directed toward the nondepen dent lung. Partial reinflation of the nondependent lung with O2will reduce the physiological shunt fraction of the lung. Continuous positive airways pressure (CPAP) is an effective prophylactic and therapeutic treatment for hypoxemia. All studies examining CPAP have found it to be effective, provided it is preceded by lung reinflation.

Pharmacological interventions in acute respiratory distress syndrome

Pharmacological interventions are commonly considered in acute respiratory distress syndrome (ARDS) patients. Inhaled nitric oxide (iNO) and neuromuscular blockers (NMBs) are used in patients with severe hypoxemia. No outcome benefit has been observed with the systematic use of iNO. However, a sometimes important improvement in oxygenation can occur shortly after starting administration. Therefore, its ease of use and its good tolerance justify iNO optionally combined with almitirne as a rescue therapy on a trial basis. Recent data from the literature support the use of a 48-h infusion of NMBs in patients with a PaO2 to FiO2 ratio <120 mmHg. No strong evidence exists on the increase of ICU-acquired paresis after a short course of NMBs. Fluid management with the goal to obtain zero fluid balance in ARDS patients without shock or renal failure significantly increases the number of days without mechanical ventilation. On the other hand, patients with hemodynamic failure must receive early and adapted fluid resuscitation. Liberal and conservative fluid strategies therefore are complementary and should ideally follow each other in time in the same patient whose hemodynamic state progressively stabilizes. At present, albumin treatment does not appear to be justified for limitation of pulmonary edema and respiratory morbidity. Aerosolized β2-agonists do not improve outcome in patients with ARDS and one study strongly suggests that intravenous salbutamol may worsen outcome in those patients. The early use of high doses of corticosteroids for the prevention of ARDS in septic shock patients or in patients with confirmed ARDS significantly reduced the duration of mechanical ventilation but had no effect or even increased mortality. In patients with persistent ARDS after 7 to 28 days, a randomized trial showed no reduction in mortality with moderate doses of corticosteroids but an increased PaO2 to FiO2 ratio and thoracopulmonary compliance were found, as well as shorter durations of mechanical ventilation and of ICU stay. Conflicting data exist on the interest of low doses of corticosteroids (200 mg/day of hydrocortisone) in ARDS patients. In the context of a persistent ARDS with histological proof of fibroproliferation, a corticosteroid treatment with a progressive decrease of doses can be proposed.

An anesthesiologist's guide to hypoxic pulmonary vasoconstriction: implications for managing single-lung anesthesia and atelectasis

PURPOSE OF THE REVIEW

Hypoxic pulmonary vasoconstriction is the pulmonary circulation's homeostatic mechanism for matching regional perfusion to ventilation and optimizing systemic PaO2. The role of hypoxic pulmonary vasoconstriction in anesthesiology is reviewed.

RECENT FINDINGS

In hypoxic pulmonary vasoconstriction, airway hypoxia causes resistance pulmonary arteries to constrict, diverting blood to better-oxygenated alveoli. Hypoxic pulmonary vasoconstriction optimizes O2 uptake in atelectasis, pneumonia, asthma, and adult respiratory distress syndrome. During single-lung anesthesia, hypoxic pulmonary vasoconstriction helps maintain systemic oxygenation. When hypoxic pulmonary vasoconstriction is weak, systemic hypoxemia is exacerbated. Although not widely used, the peripheral chemoreceptor agonist almitrine enhances hypoxic pulmonary vasoconstriction and improves PaO2 during single-lung anesthesia. The mechanism of hypoxic pulmonary vasoconstriction involves a redox-based O2 sensor within pulmonary artery smooth muscle cells. Pulmonary artery smooth muscle cells mitochondria vary production of reactive O2 species in proportion to PaO2. Hypoxic withdrawal of these redox second messengers inhibits voltage-gated potassium channels, depolarizing the pulmonary artery smooth muscle cells. Depolarization activates L-type calcium channels, increasing cytosolic calcium and triggering hypoxic pulmonary vasoconstriction.

SUMMARY

An understanding of hypoxic pulmonary vasoconstriction is clinically relevant for anesthesiologists. Randomized clinical trials with robust endpoints are required to assess strategies for enhancing hypoxic pulmonary vasoconstriction in thoracic surgery patients.

Right ventricular response to high-dose almitrine infusion in patients with severe hypoxemia related to acute respiratory distress syndrome

OBJECTIVE

To evaluate the effects of high-dose almitrine infusion on gas exchange and right ventricular function in patients with severe hypoxemia related to acute respiratory distress syndrome (ARDS).

DESIGN

Prospective study.

SETTING

Medicosurgical intensive care department (ten beds).

PATIENTS

Nine patients with ARDS and severe hypoxemia (PaO2/FIO2 ratio, <150 torr [20 kPa]).

INTERVENTION

High-dose almitrine infusion (16 microg/kg/min for 30 mins).

MEASUREMENTS AND MAIN RESULTS

Gas exchange and hemodynamic parameters were recorded before and after almitrine infusion. Right ventricular function was evaluated by using a fast response thermistor pulmonary artery catheter that allowed measurement of right ventricular ejection fraction and calculation of right ventricular end-diastolic and end-systolic volumes. Almitrine did not significantly alter arterial oxygenation and intrapulmonary shunt. Almitrine increased mean pulmonary arterial pressure (MPAP) from 31 +/- 4 to 33 +/- 4 mm Hg (p < .05), pulmonary vascular resistance index from 353 +/- 63 to 397 +/- 100 dyne x sec/ cm5 x m2 (p < .05), and right ventricular end-systolic volume index from 71 +/- 22 to 77 +/- 21 mL/m2 (p < .05); almitrine decreased right ventricular ejection fraction from 36% +/- 7% to 34% +/- 8% (p < .05). Stroke volume index and cardiac index did not change. The almitrine-induced changes in right ventricular ejection fraction were closely correlated with the baseline MPAP (r2 = .71, p < .01).

CONCLUSION

In patients with severe hypoxemia related to ARDS, high-dose almitrine infusion did not improve arterial oxygenation and impaired the loading conditions of the right ventricle. The decrease in right ventricular ejection fraction induced by almitrine was correlated with the baseline MPAP. Thus, high-dose almitrine infusion may be harmful in ARDS patients with severe hypoxemia and pulmonary hypertension.

Impact of sleep in COPD

Sleep has well-recognized effects on breathing, including changes in central respiratory control, airways resistance, and muscular contractility, which do not have an adverse effect in healthy individuals but may cause problems in patients with COPD. Sleep-related hypoxemia and hypercapnia are well recognized in COPD and are most pronounced in rapid eye movement sleep. However, sleep studies are usually only indicated in patients with COPD when there is a possibility of sleep apnea or when cor pulmonale and/or polycythemia are not explained by the awake PaO(2) level. Management options for patients with sleep-related respiratory failure include general measures such as optimizing therapy of the underlying condition; physiotherapy and prompt treatment of infective exacerbations; supplemental oxygen; pharmacologic treatments such as bronchodilators, particularly ipratropium bromide, theophylline, and almitrine; and noninvasive positive pressure ventilation.

Intravenous almitrine combined with inhaled nitric oxide for acute respiratory distress syndrome. The NO Almitrine Study Group

Inhaled nitric oxide (iNO), a selective pulmonary vasodilator and intravenously administered almitrine, a selective pulmonary vasoconstrictor, have been shown to increase PaO2 in patients with acute respiratory distress syndrome (ARDS). This prospective study was undertaken to assess the cardiopulmonary effects of combining both drugs. In 48 consecutive patients with early ARDS, cardiorespiratory parameters were measured at control, after iNO 5 ppm, after almitrine 4 micrograms. kg-1. min-1, and after the combination of both drugs. In 30 patients, dose response to 2, 4, and 16 micrograms. kg-1. min-1 of almitrine with and without NO was determined. Almitrine and lactate plasma concentrations were measured in 17 patients. Using pure O2, PaO2 increased by 75 +/- 8 mm Hg after iNO, by 101 +/- 12 mm Hg after almitrine 4 micrograms. kg-1. min-1, and by 175 +/- 18 mm Hg after almitrine combined with iNO (p < 0.001). In 63% of the patients, PaO2 increased by more than 100% with the combination of both drugs. Mean pulmonary artery pressure (Ppa) increased by 1.4 +/- 0.2 mm Hg with almitrine 4 micrograms/kg/ min (p < 0.001) and decreased by 3.4 +/- 0.4 mm Hg with iNO and by 1.5 +/- 0.3 mm Hg with the combination (p < 0.001). The maximum increase in PaO2 was obtained at almitrine concentrations <= 4 micrograms. kg-1. min-1, whereas almitrine increased Ppa dose-dependently. Almitrine plasma concentrations also increased dose-dependently and returned to values close to zero after 12 h. In many patients with early ARDS, the combination of iNO 5 ppm and almitrine 4 micrograms. kg-1. min-1 dramatically increases PaO2 without apparent deleterious effect allowing a rapid reduction in inspired fraction of O2. The long-term consequences of this immediate beneficial effect remain to be determined.

The Utility of Nitric Oxide in the Postoperative Period

Inhaled nitric oxide (iNO) is a pulmonary-selective vaso dilator with minimal bronchodilator activity in humans. NO also inhibits platelet and neutrophil activation and adhesion and inhibits ischemia-reperfusion injury. The pulmonary vasodilatory property of iNO causes a reduc tion in pulmonary vascular resistance and improvement in arterial oxygenation in a wide spectrum of diseases characterized by pulmonary hypertension and hypox emia. Promising examples of diseases for which NO may provide beneficial physiologic effects are primary and secondary pulmonary hypertension, right ventricu lar failure, cardiac transplantation, pulmonary embo lism, protamine reactions, acute respiratory distress syndrome, lung transplantation and, perhaps, chronic obstructive airways disease. The usefulness of iNO may be improved by concomitant therapy with pulmonary- selective intravenous vasoconstrictors (eg, Almitrine; Vectarian, Neuilly, France) and cGMP phosphodiester ase V inhibitors (eg, Zaprinast; Research Biochemicals International, Natick, MA). Almitrine improves oxygen ation, synergistically with iNO, and may be useful in disease states characterized primarily by hypoxemia. Zaprinast may be useful for weaning iNO and avoidance of rebound pulmonary hypertension.

Metabolic, ventilatory and circulatory effects of doxapram in anaesthetized pigs during normoxia and hypoxia

BACKGROUND

In a previous clinical study doxapram was found to improve ventilatory efficacy postoperatively, presumably via effects on hypoxic pulmonary vasoconstriction (HPV). The present study was designed to see whether doxapram induced any changes of arterial oxygenation and pulmonary vascular resistance during normoxia or hypoxia and whether the changes were influenced by the anaesthetic agents.

METHODS

Seventeen piglets were anaesthetized by combinations of either midazolam + fentanyl + pancuronium + pentobarbital (TIVA, n = 9), or by midazolam + fentanyl + pancuronium + halothane, 0.5% in end-tidal gas (Hal, n = 8). Analyses of expired gas and mixed venous and arterial blood in combination with determinations of central blood flow and pressures allowed for calculations of standard metabolic, ventilatory and circulatory data. Values were obtained at normoventilation using normoxic (FIO2 = 0.3) and hypoxic (FIO2 = 0.08) gas mixtures at calculated doxapram plasma concentrations of 1, 2 and 4 micrograms.ml-1.

RESULTS

With few exceptions doxapram administration affected the investigated variables only moderately during normoxia. In group Hal, PVR and SVR showed a biphasic raise (P < 0.05), CO fell (P < 0.05-P > or = 0.05) and C (a-v)O2 rose (P < 0.05). In group TIVA, PaO2 fell (P < 0.01-0.05) despite unchanged PVR, CO and VD/VT. Hypoxia affected a moderate increase in PVR in group TIVA (P < 0.05), which was slightly lower at the lowest and highest plasma levels of doxapram (P < 0.05). In group Hal, the induction of hypoxia induced a more pronounced rise in PVR (P < 0.05) which showed a biphasic response to increasing dose levels of doxapram, the lowest dose affecting a further rise (P < 0.05) and the highest a reduction to values below hypoxia control levels (P < 0.05). Pronounced differences between the two groups with respect to values for metabolic and circulatory variables make the interpretation of data difficult.

CONCLUSIONS

Doxapram administration to anaesthetized animals did not induce any effects indicative of augmentation of the HPV response.

Effect of modulators of hypoxic pulmonary vasoconstriction on the response to inhaled nitric oxide in a neonatal model of severe pulmonary atelectasis

Hypoxic pulmonary vasoconstriction (HPV) is an intrinsic mechanism that facilitates ventilation to perfusion matching and preservation of oxygenation. We investigated the neonatal HPV response from extensive atelectasis and tested the hypothesis that (I) the resulting hypoxemia is corrected by inhaled nitric oxide (NO); (2) the "pulmonary steal" of blood away from hypoxic area is further improved by modulators of the HPV. Intratracheal injection of steel beads in 32 piglets (7 to 20 days) resulted in atelectasis of 50% to 75% of the lungs. The piglets were then randomized to receive saline (control), indomethacin (IND) 2 mg/kg, doxapram (DOX) 0.5 mg/kg/h or almitrine (ALM) 4 micrograms/kg/min. After 30 minutes, all animals were subjected to NO at 40 ppm. Atelectasis resulted in severe impairment in oxygenation (PaO2 - 105 +/- 6 mm Hg, AaDO2 = 536 +/- 9 mm Hg; shunt fraction = 31% +/- 2%) and moderate pulmonary hypertension. Mean pulmonary artery pressure (PAP) increased to 35 +/- 0.8 mm Hg. NO reduced pulmonary vascular resistance (PVR) from 128 +/- 14 mm Hg/kg/mL/min to 74 +/- 9 mm Hg/kg/mL/min and improved gas exchange (PaO2 = 180 +/- 50 and AaDO2 = 438 +/- 50 mm Hg). Following the development of atelectasis, the peripheral chemoreceptor agonists (ALM and DOX) did not modify gas exchange and had no significant cardiovascular effect. ALM and DOX failed to enhance the response to NO. IND did not alter HPV, but prevented the improvement in gas exchange associated with NO-induced pulmonary vasodilation.

Additive effect on gas exchange of inhaled nitric oxide and intravenous almitrine bismesylate in the adult respiratory distress syndrome

OBJECTIVE

To assess the additive effect of inhaled nitric oxide (NO) and intravenous almitrine bismesylate (ALM) on gas exchange.

DESIGN

Prospective self-controlled study.

SETTING

3 medico-surgical intensive care units.

PATIENTS

17 patients with severe hypoxemia (PaO2/FIO2 ratio: 88 +/- 30 mmHg, venous admixture: 47 +/- 7%) and elevated mean pulmonary artery pressure (MPAP: 30 +/- 5 mmHg) due to adult respiratory distress syndrome (ARDS).

INTERVENTIONS

5 conditions were studied: 1) baseline, 2) 5 to 10 ppm of NO during 30 min, 3) discontinuation of NO during 30 min, 4) ALM infusion (0.5 mg/kg) during 30 min, 5) ALM infusion (0.5 mg/kg) during 30 min in combination with 5 to 10 ppm of NO.

MEASUREMENT AND RESULTS

The PaO2/FIO2 ratio rose from 88 +/- 30 to 98 +/- 37 mmHg (NS) with NO alone, and from 92 +/- 25 to 130 +/- 56 mmHg (p < 0.01) with NO + ALM (p < 0.05 vs NO alone). Seven patients were considered as "NO-responders" (rise in PaO2/FIO2 ratio of 10 mmHg or more with NO); in this subgroup the PaO2/FIO2 ratio rose from 87 +/- 30 to 128 +/- 39 mmHg (p < 0.05) with NO alone, and from 93 +/- 20 to 169 +/- 51 mmHg (p < 0.01) with NO + ALM (p < 0.05 versus NO alone). MPAP decreased from 30 +/- 5 to 26 +/- 5 mmHg (p < 0.01) with NO alone, increased slightly from 28 +/- 5 to 31 +/- 5 mmHg (NS) with ALM alone and decreased to 27 +/- 5 mmHg (p < 0.05) with NO + ALM.

CONCLUSIONS

NO + ALM had additive effects on gas exchange while decreasing MPAP in patients with ARDS. The effects of NO alone were small and non significant, except in a subgroup of 7 patients in whom the combination of both therapies had the more pronounced results.

Doxapram improves pulmonary function after upper abdominal surgery

The effects of doxapram on postoperative pulmonary function were studied in 40 ASA I and II patients randomly allocated to receive either doxapram 1.8 mg.kg-1.h-1 or placebo for 2 h immediately after elective cholecystectomy. The two groups displayed similar reductions of carbon dioxide production at 2 h and 6 h postoperatively, whereas oxygen consumption remained at preoperative levels for 24 h. Minute ventilation was similarly reduced in the two groups at 2 h and 6 h postoperatively, with corresponding increases in PaCO2. PaO2 was similarly and significantly decreased in both groups postoperatively, whereas P(A-a)O2 remained unchanged at 2 h and 6 h in doxapram-treated patients. FRC was reduced postoperatively in both groups, significantly more so in the control group at 6 h. Various indices of intrapulmonary gas distribution, including the functional (nitrogen) dead space, underwent similar changes in the two groups. By contrast, the physiological dead space was reduced in doxapram-treated patients at 2 h, 6 h and 24 h postoperatively, whereas no significant changes were seen in the control group. The ventilatory equivalent for CO2 was significantly lower in the doxapram-treated group, implying higher ventilatory efficiency. Our findings indicate that infusion of doxapram postoperatively attenuates the impairment of pulmonary function postoperatively, chiefly via effects on V'A/Q' ratios. No side effects of doxapram were observed.

A comparative study of the effects of almitrine bismesylate and lateral position during unilateral bacterial pneumonia with severe hypoxemia

The management of patients with unilateral pneumonia and severe hypoxemia often represents a therapeutic challenge. Mechanical ventilation with the diseased lung uppermost may improve gas exchange, but it is not devoid of adverse effects. No hemodynamic measurements have been reported in patients ventilated in this manner; therefore, whether or not the improvement in PaO2 is counterbalanced by hemodynamic deterioration remains unknown. Almitrine bismesylate is a drug that seems able to improve gas exchange in patients with chronic obstructive pulmonary disease or the adult respiratory distress syndrome. The increase in PaO2 after its administration has been attributed to an improvement in ventilation-perfusion relationships. Its use has never been reported during unilateral pneumonia with severe hypoxemia. We therefore compared its effects with those of lateral position in eight consecutive mechanically ventilated patients with unilateral pneumonia. Blood gas and hemodynamic measurements were performed both at maintenance FIO2 and at an FIO2 of 1.0. Almitrine (1 mg/kg over 1 h) had no effect on PaO2 under either FIO2 condition. Cardiac output remained unchanged, but mean pulmonary artery pressure increased from 22.5 +/- 1.2 to 26.5 +/- 1.3 mm Hg (p < 0.02). By contrast, lateral position had striking effects on PaO2, which increased from 100 +/- 14 mm Hg in supine position to 156 +/- 23 mm Hg (p < 0.01) when the abnormal lung was placed uppermost at maintenance FIO2 and from 207 +/- 21 (supine) to 300 +/- 28 mm Hg (lateral) (p < 0.01) at FIO2 1.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Almitrine and doxapram in experimental lung injury

Almitrine and doxapram, two structurally unrelated peripheral chemoreceptor agonists, have been shown to enhance hypoxic pulmonary vasoconstriction in anesthetized dogs. We hypothesized that these drugs would increase pulmonary vascular tone and improve gas exchange in canine lung injury caused by oleic acid (OA). Pulmonary hemodynamics and gas exchange were investigated in pentobarbital-anesthetized dogs before and after intravenously administered OA 0.09 ml/kg and again after placebo (n = 6), almitrine 2 micrograms/kg/min (n = 6), or doxapram 20 micrograms/kg/min (n = 6) in a randomized order. Cardiac output (Q) was manipulated using a femoral arteriovenous bypass and an inferior vena cava balloon catheter to construct mean pulmonary artery pressure (Ppa)-Q plots in order to discriminate active from passive changes in Ppa. Gas exchange was assessed by measuring arterial PO2 and intrapulmonary shunt, determined using a sulfur hexafluoride infusion. OA increased Ppa over the range of Q studied, and it deteriorated gas exchange by an increase in intrapulmonary shunt. After OA, placebo had no effect on Ppa, arterial PO2, or intrapulmonary shunt. Both almitrine and doxapram further increased Ppa at all levels of Q studied, but they did not affect indices of gas exchange after OA. We conclude that in this experimental model of acute lung injury, almitrine and doxapram induce pulmonary vasoconstriction without, however, diverting blood flow toward better oxygenated lung regions.

The pulmonary circulation in acute lung injury: a review of some recent advances

1. According to the Starling resistor model of the pulmonary circulation, the pulmonary hypertension of oleic acid lung injury, an experimental model close to the early stage of clinical ARDS, primarily results from an increased vascular closing pressure which exceeds Pla and becomes the effective outflow pressure of the pulmonary circulation. Therefore, calculated pulmonary vascular resistance should be interpreted cautiously during haemodynamic investigations in patients with ARDS. 2. Part of this increased vascular closing pressure is functional. During acute lung injury pulmonary vasomotor tone can be reduced by vasodilators, or increased by cyclooxygenase inhibitors and almitrine. 3. Pulmonary vasodilation due to infused vasodilators usually impairs gas exchange in ARDS. 4. There is evidence that HPV is altered during ARDS. Drugs capable of enhancing the efficacy of HPV could improve gas exchange. If proven safe in the future, cyclooxygenase inhibitors and almitrine are interesting compounds to be tested in ARDS.