Nitric oxide dose response during moderate and severe hypoxia in swine

Inhaled nitric oxide has pulmonary vasodilator efficacy both in the immediate and prolonged phase of acute pulmonary embolism

BACKGROUND

Inhaled nitric oxide (iNO) effectively reduces right ventricular afterload when administered in the immediate phase of acute pulmonary embolism (PE) in preclinical animal models. In a porcine model of intermediate-risk PE, we aimed to investigate whether iNO has pulmonary vasodilator efficacy both in the immediate and prolonged phase of acute PE.

METHODS

Anesthetized pigs (n = 18) were randomized into three subgroups. An acute PE iNO-group (n = 6) received iNO at 40 ppm at one, three, six, nine and 12 hours after onset of PE. Vehicle animals (n = 6) received PE, but no active treatment. A third group of sham animals (n = 6) received neither PE nor treatment. Animals were evaluated using intravascular pressures, respiratory parameters, biochemistry and intracardiac pressure-volume measurements.

RESULTS

The administration of PE increased mean pulmonary artery pressure (mPAP) (vehicle vs sham; 33.3 vs 17.7 mmHg, p < 0.0001), pulmonary vascular resistance (vehicle vs sham; 847.5 vs 82.0 dynes, p < 0.0001) and right ventricular arterial elastance (vehicle vs sham; 1.2 vs 0.2 mmHg/ml, p < 0.0001). Significant mPAP reduction by iNO was preserved at 12 hours after the onset of acute PE (vehicle vs iNO; 0.5 vs -3.5 mmHg, p < 0.0001). However, this response was attenuated over time (p = 0.0313). iNO did not affect the systemic circulation.

CONCLUSIONS

iNO is a safe and effective pulmonary vasodilator both in the immediate and prolonged phase of acute PE in an in-vivo porcine model of intermediate-risk PE.

Riociguat, sildenafil and inhaled nitric oxide reduces pulmonary vascular resistance and improves right ventricular function in a porcine model of acute pulmonary embolism

BACKGROUND

Pulmonary vasodilators as add-on to current treatment strategies in acute pulmonary embolism may improve right ventricular unloading and hence improve patient outcome. We aimed to investigate whether stimulation of the nitric oxide (NO)-soluble guanylate cyclase (sGC)-cyclic guanosine monophosphate (cGMP) pathway with riociguat, sildenafil or inhaled NO causes pulmonary vasodilation and improves right ventricular function in a porcine model of acute intermediate risk pulmonary embolism.

METHODS

Two large autologous blood clots were administered to the pulmonary circulation of 28 pigs (60 kg). Animals were randomized to four increasing, clinically equivalent doses of riociguat (n=6), sildenafil (n=6), inhaled NO (n=6) or vehicle (n=6). Sham animals (n=4) did not receive pulmonary embolism or treatment. Haemodynamic responses were evaluated at baseline, after pulmonary embolism and after each dose using invasive pressure measurements, transoesophageal echocardiography, respiratory parameters and blood analysis.

RESULTS

Pulmonary embolism caused a three-fold increase in pulmonary vascular resistance compared with baseline (pulmonary embolism: 352±29 vs. baseline: 107±6 dynes, p<0.0001). All treatments lowered pulmonary vascular resistance compared with vehicle (riociguat: -158±35, sildenafil: -224±35, inhaled NO: -156±35 dynes, p<0.0001). Sildenafil, but neither inhaled NO nor riociguat, caused a decrease in systemic vascular resistance (sildenafil 678±41 vs. vehicle 1081±93 dynes, p=0.02) and increased cardiac output (sildenafil 8.8±0.8 vs. vehicle: 5.9±0.2 L/min, p<0.001). Systemic blood pressure was unaltered in all treatment groups.

CONCLUSION

Stimulation of the NO-sGC-cGMP pathway by riociguat, sildenafil and inhaled NO reduces pulmonary vascular resistance in a porcine model of acute pulmonary embolism without lowering systemic blood pressure.

Generation of purified nitric oxide from liquid N2O4 for the treatment of pulmonary hypertension in hypoxemic swine

Inhaled nitric oxide (NO) selectively dilates pulmonary blood vessels, reduces pulmonary vascular resistance (PVR), and enhances ventilation-perfusion matching. However, existing modes of delivery for the treatment of chronic pulmonary hypertension are limited due to the bulk and heft of large tanks of compressed gas. We present a novel system for the generation of inhaled NO that is based on the initial heat-induced evaporation of liquid N2O4 into gas phase NO2 followed by the room temperature reduction to NO by an antioxidant, ascorbic acid cartridge just prior to inhalation. The biologic effects of NO generated from liquid N2O4 were compared with the effects of NO gas, on increased mean pulmonary artery pressure (mPAP) and PVR in a hypoxemic (FiO2 15%) swine model of pulmonary hypertension. We showed that NO concentration varied directly with the fixed cross sectional flow of the outflow aperture when studied at temperatures of 45, 47.5 and 50°C and was independent of the rate of heating. Liquid N2O4-sourced NO at 1, 5, and 20 ppm significantly reduced the elevated mPAP and PVR induced by experimental hypoxemia and was biologically indistinguishable from gas source NO in this model. These experiments show that it is feasible to generate highly purified NO gas from small volumes of liquid N2O4 at concentrations sufficient to lower mPAP and PVR in hypoxemic swine, and suggest that a miniaturized ambulatory system designed to generate biologically active NO from liquid N2O4 is achievable.

Inhaled nitric oxide, oxygen, and alkalosis: dose-response interactions in a lamb model of pulmonary hypertension

Inhaled nitric oxide (NO) is currently used as an adjuvant therapy for a variety of pulmonary hypertensive disorders. In both animal and human studies, inhaled NO induces selective, dose-dependent pulmonary vasodilation. However, its potential interactions with other simultaneously used pulmonary vasodilator therapies have not been studied. Therefore, the objective of this study was to determine the potential dose-response interactions of inhaled NO, oxygen, and alkalosis therapies. Fourteen newborn lambs (age 1-6 days) were instrumented to measure vascular pressures and left pulmonary artery blood flow. After recovery, the lambs were sedated and mechanically ventilated. During steady-state pulmonary hypertension induced by U46619 (a thromboxane A2 mimic), the lambs were exposed to the following conditions: Protocol A, inhaled NO (0, 5, 40, and 80 ppm) and inspired oxygen concentrations (FiO2) of 0.21, 0.50, and 1.00; and Protocol B, inhaled NO (0, 5, 40, and 80 ppm) and arterial pH levels of 7.30, 7.40, 7.50, and 7.60. Each condition (in randomly chosen order) was maintained for 10 min, and all variables were allowed to return to baseline between conditions. Inhaled NO, oxygen, and alkalosis produced dose-dependent decreases in mean pulmonary arterial pressures (P < 0.05). Systemic arterial pressure remained unchanged. At 5 ppm of inhaled NO, alkalosis and oxygen induced further dose-dependent decreases in mean pulmonary arterial pressures (P < 0.05). At inhaled NO doses > 5 ppm, alkalosis induced further dose-independent decreases in mean pulmonary arterial pressure, while oxygen did not. We conclude that in this animal model, oxygen, alkalosis, and inhaled NO induced selective, dose-dependent pulmonary vasodilation. However, when combined, a systemic arterial pH > 7.40 augmented inhaled NO-induced pulmonary vasodilation, while an FiO2 > 0.5 did not. Therefore, weaning high FiO2 during inhaled NO therapy should be considered, since it may not diminish the pulmonary vasodilating effects. Further studies are warranted to guide the clinical weaning strategies of these pulmonary vasodilator therapies.

Combined inhaled nitric oxide and inhaled prostacyclin during experimental chronic pulmonary hypertension

Inhaled nitric oxide (NO) and inhaled prostacyclin (PGI2) produce selective reductions in pulmonary vascular resistance (PVR) through differing mechanisms. NO decreases PVR via cGMP, and PGI2 produces pulmonary vasodilation via cAMP. As a general pharmacological principle, two drugs that produce similar effects via different mechanisms should have additive or synergistic effects when combined. We designed this study to investigate whether combined inhaled NO and PGI2 therapy results in additive effects during chronic pulmonary hypertension in the rat. Monocrotaline injected 4 wk before study produced pulmonary hypertension in all animals. Inhaled NO (20 parts/million) reversibly and selectively decreased pulmonary artery pressure (Ppa) with a mean reduction of 18%. Four concentrations of PGI2 were administered via inhalation (5, 10, 20, and 80 microg/ml), both alone and combined with inhaled NO. Inhaled PGI2 alone decreased Ppa in a dose-dependent manner with no change in mean systemic arterial pressure. Combined inhaled NO and PGI2 selectively and significantly decreased Ppa more did than either drug alone. The effects were additive at the lower concentrations of PGI2 (5, 10, and 20 microg/ml). The combination of inhaled NO and inhaled PGI2 may be useful in the management of pulmonary hypertension.

Cardiopulmonary effects of inhaled nitric oxide in normal dogs and during E. coli pneumonia and sepsis

We investigated the effect of inhaled nitric oxide (NO) at increasing fractional inspired O2 concentrations (FIO2) on hemodynamic and pulmonary function during Escherichia coli pneumonia. Thirty-eight conscious, spontaneously breathing, tracheotomized 2-yr-old beagles had intrabronchial inoculation with either 0.75 or 1.5 x 10(10) colony-forming units/kg of E. coli 0111:B4 (infected) or 0.9% saline (noninfected) in one or four pulmonary lobes. We found that neither the severity nor distribution (lobar vs. diffuse) of bacterial pneumonia altered the effects of NO. However, in infected animals, with increasing FIO2 (0.08, 0.21, 0.50, and 0.85), NO (80 parts/million) progressively increased arterial PO2 [-0.3 +/- 0.6, 3 +/- 1, 13 +/- 4, 10 +/- 9 (mean +/- SE) Torr, respectively] and decreased the mean arterial-alveolar O2 gradient (0.5 +/- 0.3, 4 +/- 2, -8 +/- 7, -10 +/- 9 Torr, respectively). In contrast, in noninfected animals, the effect of NO was significantly different and opposite; NO progressively decreased mean PO2 with increasing FIO2 (2 +/- 1, -5 +/- 3, -2 +/- 3, and -12 +/- 5 Torr, respectively; P < 0.05 compared with infected animals) and increased mean arterial-alveolar O2 gradient (0.3 +/- 0.04, 2 +/- 2, 1 +/- 3, 11 +/- 5 Torr; P < 0.05 compared with infected animals). In normal and infected animals alike, only at FIO2 < or = 0.21 did NO significantly lower mean pulmonary artery pressure, pulmonary artery occlusion pressure, and pulmonary vascular resistance index (all P < 0.01). However, inhaled NO had no significant effect on increases in mean pulmonary artery pressure associated with bacterial pneumonia. Thus, during bacterial pneumonia, inhaled NO had only modest effects on oxygenation dependent on high FIO2 and did not affect sepsis-induced pulmonary hypertension. These data do not support a role for inhaled NO in bacterial pneumonia. Further studies are necessary to determine whether, in combination with ventilatory support, NO may have more pronounced effects.