Effects of inhaled nitric oxide during permissive hypercapnia in acute respiratory failure in piglets

Low-flow ECCO2R conjoined with renal replacement therapy platform to manage pulmonary vascular dysfunction with refractory hypercapnia in ARDS

Background

Hypercapnia worsens lung vascular dysfunction during acute respiratory distress syndrome (ARDS). We tested whether an extracorporeal carbon dioxide removal (ECCO2R) device based on a renal replacement therapy platform (Prismalung®) may reduce PaCO2 and alleviate lung vascular dysfunction in ARDS patients with refractory hypercapnia.

Methods

We planned to prospectively include 20 patients with moderate-to-severe ARDS, pulmonary vascular dysfunction on echocardiography, and PaCO2 ≥ 48 mmHg despite instrumental dead space reduction and the increase in respiratory rate. Hemodynamics, echocardiography, respiratory mechanics, and arterial blood gases were recorded at 2 (H2), 6 (H6) and 24 (H24) hours as ECCO2R treatment was continued for at least 24 h.

Results

Only eight patients were included, and the study was stopped due to worldwide shortage of ECCO2R membranes and the pandemic. Only one patient fulfilled the primary endpoint criterion (decrease in PaCO2 of more than 20 %) at H2, but this objective was achieved in half of patients (n = 4) at H6. The percentage of patients with a PaCO2 value < 48 mmHg increased with time, from 0/8 (0 %) at H0, to 3/8 (37.5 %) at H2 and 4/8 (50 %) at H6 (p = 0.04). There was no major change in hemodynamic and echocardiographic variables with ECCO2R, except for a significant decrease in heart rate. ECCO2R was prematurely discontinued before H24 in five (62.5 %) patients, due to membrane clotting in all cases.

Conclusions

This pilot study testing showed a narrow efficacy and high rate of membrane thrombosis with the first version of the system. Improved versions should be tested in future trials.

Trial registration

Registered at clinicaltrials.gov, identifier: NCT03303807, Registered: October 6, 2017, https://clinicaltrials.gov/ct2/show/NCT03303807.

Extracorporeal CO2 removal: Technical and physiological fundaments and principal indications

In recent years, technological improvements have reduced the complexity of extracorporeal membrane oxygenation devices. This have enabled the development of specific devices for the extracorporeal removal of CO2. These devices have a simpler configuration than extracorporeal membrane oxygenation devices and uses lower blood flows which could reduce the potential complications. Experimental studies have demonstrated the feasibility, efficacy and safety of extracorporeal removal of CO2 and some of its effects in humans. This technique was initially conceived as an adjunct therapy in patients with severe acute respiratory distress syndrome, as a tool to optimize protective ventilation. More recently, the use of this technique has allowed the emergence of a relatively new concept called "tra-protective ventilation"whose effects are still to be determined. In addition, the extracorporeal removal of CO2 has been used in patients with exacerbated hypercapnic respiratory failure with promising results. In this review we will describe the physiological and technical fundamentals of this therapy and its variants as well as an overview of the available clinical evidence, focused on its current potential.

Impact of acute hypercapnia and augmented positive end-expiratory pressure on right ventricle function in severe acute respiratory distress syndrome

PURPOSE

To evaluate the effects of acute hypercapnia induced by positive end-expiratory pressure (PEEP) variations at constant plateau pressure (P (plat)) in patients with severe acute respiratory distress syndrome (ARDS) on right ventricular (RV) function.

METHODS

Prospective observational study in two academic intensive care units enrolling 11 adults with severe ARDS (PaO(2)/FiO(2) <150 mmHg at PEEP >5 cmH(2)O). We compared three ventilatory strategies, each used for 1 h, with P (plat) at 22 (20-25) cmH(2)O: low PEEP (5.4 cmH(2)O) or high PEEP (11.0 cmH(2)O) with compensation of the tidal volume reduction by either a high respiratory rate (high PEEP/high rate) or instrumental dead space decrease (high PEEP/low rate). We assessed RV function (transesophageal echocardiography), alveolar dead space (expired CO(2)), and alveolar recruitment (pressure-volume curves).

RESULTS

Compared to low PEEP, PaO(2)/FiO(2) ratio and alveolar recruitment were increased with high PEEP. Alveolar dead space remained unchanged. Both high-PEEP strategies induced higher PaCO(2) levels [71 (60-94) and 75 (53-84), vs. 52 (43-68) mmHg] and lower pH values [7.17 (7.12-7.23) and 7.20 (7.16-7.25) vs. 7.30 (7.24-7.35)], as well as RV dilatation, LV deformation and a significant decrease in cardiac index. The decrease in stroke index tended to be negatively correlated to the increase in alveolar recruitment with high PEEP.

CONCLUSIONS

Acidosis and hypercapnia induced by tidal volume reduction and increase in PEEP at constant P (plat) were associated with impaired RV function and hemodynamics despite positive effects on oxygenation and alveolar recruitment ( ClinicalTrials.gov #NCT00236262).

Hypercapnic acidosis and compensated hypercapnia in control and pulmonary hypertensive piglets

Low tidal volume/inspiratory pressure ventilator strategies result in hypercapnia, which has been shown to increase pulmonary vasomotor tone. This may be particularly detrimental in infants and children with preexistent pulmonary hypertension. In this study, a piglet model of chronic hypoxia-induced pulmonary hypertension was used to test the hypotheses that: 1) the effects of hypercapnic acidosis are exaggerated by preexistent pulmonary hypertension; and 2) the pulmonary hemodynamic effects of hypercapnic acidosis are attenuated by normalizing pH. Pulmonary hypertension was induced by 2 weeks of hypoxia. Hemodynamic responses were measured in control and pulmonary hypertensive piglets during both normoxia and hypoxia under normocapnic, hypercapnic acidotic, and compensated hypercapnic conditions. We found that: 1) hypercapnic acidosis increased both normoxic and hypoxic pulmonary vascular resistance index (PVRI) in control piglets; 2) the pressor effects of hypercapnia were not attenuated by infusing bicarbonate to normalize the pH; and 3) piglets with chronic hypoxia-induced pulmonary hypertension had elevated baseline normoxic and hypoxic PVRI, but responded to hypercapnic acidosis and compensated hypercapnia in a similar way to control piglets. These data suggest that acute hypercapnic acidosis may have deleterious effects on the pulmonary hemodynamics of normal and pulmonary hypertensive subjects which may not be acutely reversed by buffering the pH.

Effect of position, nitric oxide, and almitrine on lung perfusion in a porcine model of acute lung injury

In a porcine model of oleic acid-induced lung injury, the effects of inhaled nitric oxide (iNO) and intravenous almitrine bismesylate (ivALM), which enhances the hypoxic pulmonary vasoconstriction on the distribution of regional pulmonary blood flow (PBF), were assessed. After injection of 0.12 ml/kg oleic acid, 20 anesthetized and mechanically ventilated piglets [weight of 25 +/- 2.6 (SD) kg] were randomly divided into four groups: supine position, prone position, and 10 ppm iNO for 40 min followed by 4 microg x kg(-1) x min(-1) ivALM for 40 min in supine position and in prone position. PBF was measured with positron emission tomography and H(2)15O. The redistribution of PBF was studied on a pixel-by-pixel basis. Positron emission tomography scans were performed before and then 120, 160, and 200 min after injury. With prone position alone, although PBF remained prevalent in the dorsal regions it was significantly redistributed toward the ventral regions (P < 0.001). A ventral redistribution of PBF was also obtained with iNO regardless of the position (P = 0.043). Adjunction of ivALM had no further effect on PBF redistribution. PP and iNO have an additive effect on ventral redistribution of PBF.

Hypocapnia and hypercapnia in respiratory management of newborn infants

Recent experimental and clinical data demonstrate that both hypocapnia and hypercapnia during the neonatal period may result in beneficial or adverse consequences. Multiple retrospective studies report a strong association between PaCO2 levels less than 25 to 30 mm Hg and an increased incidence of cystic PVL and CP in preterm infants. Prolonged exposure to PaCO2 values less than 25 to 30 mm Hg is also associated with hearing loss in term and near-term infants. A low tidal volume strategy combined with permissive hypercapnia is potentially a strategy that could prevent lung injury. Clearly, more randomized, controlled trials are needed before this latter strategy or that of permissive hypercapnia can be recommended routinely for preterm, near-term, or term gestation infants with respiratory disorders.

Endothelium-independent and -dependent vasodilation in alkalotic and acidotic piglet lungs

Although significant pulmonary hypertension can occur in patients treated with either hypocapnic alkalosis or "permissive" hypercapnic acidosis, the effects of sustained alkalosis or acidosis on subsequent vasodilator responses have not been established. This study measured the effects of 60-100 min of sustained alkalosis or acidosis on endothelium-independent and -dependent vasodilation with inhaled nitric oxide (iNO) and acetylcholine (ACh) in isolated lungs from 1-week-old piglets. After stabilization, lungs were divided into control (pH 7.40, PaCO(2) 40 torr, n = 5), alkalotic (pH 7.60, PaCO(2) 25 torr, n = 6), or acidotic (pH 7.25, PaCO(2) 65 torr, n = 5) groups and ventilated with 21% O(2) for 40 min. Acute hypoxic pulmonary vasoconstriction (HPV) was then induced with 4-6% O(2). After a stable pressor response had occurred (approximately 20 min), pulmonary artery dose-response relationships to increasing concentrations of iNO were measured. The iNO was then stopped and after a stable hypoxic pressure had again been reestablished (approximately 20 min), dose-responses to increasing concentrations of ACh were measured. Hypoxic pulmonary vascular resistance (PVR) was similar in all groups. Pulmonary artery pressure dose-response relationships to iNO and ACh were blunted in the alkalosis group, suggesting that both endothelium-independent and -dependent vasodilation were reduced during sustained hypocapnic alkalosis. In contrast, sustained acidosis did not alter subsequent vasodilator responses. Future studies must elucidate the mechanisms underlying blunted pulmonary vasodilation during sustained alkalosis and examine the consequences of sustained alkalosis therapy on subsequent vasodilator responses in clinical practice.

Combined effects of inhaled nitric oxide and positive end-expiratory pressure during mechanical ventilation in acute respiratory distress syndrome

We studied the combined effects of inhaled nitric oxide (INO) and positive end expiratory pressure (PEEP) during mechanical ventilation in patients with acute respiratory distress syndrome (ARDS). Eleven patients received 0 and 4 parts per million of INO in random order for 30 min at PEEP levels of 0, 5, and 10 cm H2O. Respiratory and cardiovascular parameters were measured. The addition of INO and PEEP significantly improved arterial oxygenation (p < 0.005 and p < 0.0001, respectively). The combined effect of INO and PEEP on arterial oxygenation was remarkable during 10 cm H2O PEEP. There was synergistic effect on arterial oxygenation by combining INO and 10 cm H2O PEEP. The present study showed that the combination of INO and 10 cm H2O PEEP enhanced arterial oxygenation in patients with ARDS.

Respiratory failure: current status of experimental therapies

A number of advances in the treatment of infants and children with respiratory failure have been investigated in the laboratory with translation to clinical practice. Investigators have recognized that application of high ventilating pressures and failure to apply adequate levels of positive end-expiratory pressure (PEEP) can inflict injury to the already failing lung. Other interventions such as prone positioning and application of new ventilating strategies such as proportional assist ventilation (PAV), inverse ratio ventilation (IRV), high frequency ventilation, liquid ventilation, and intratracheal pulmonary ventilation (ITPV), continue to be developed and explored. Administration of inhaled nitric oxide (iNO) may improve pulmonary physiology and gas exchange in patients with respiratory insufficiency. Finally, the technique of extracorporeal life support (ECLS) is being simplified and refined. This report summarizes the status of these advances and describes the basic science and clinical research that brought them to clinical application.

Advances in ventilatory support of the pediatric surgical patient

Severe respiratory failure in newborn and pediatric patients is associated with significant morbidity and mortality. Basic science laboratory investigation has led to advances both in our understanding of ventilator-induced lung injury and in optimizing the supportive use of conventional ventilation strategies. Over the past few years, progress has been made in alternative therapies for ventilating both children and adults with severe respiratory failure. This review focuses on recent laboratory and clinical data detailing the techniques of permissive hypercapnia, high frequency oscillatory ventilation, inhaled nitric oxide, intratracheal pulmonary ventilation, and liquid ventilation. Some of these modalities are becoming commonplace, and others may have much to offer the clinician if their benefit is clearly demonstrated in future clinical trials.