Theoretical and experimental comparison of constant inspired concentration and pulsed delivery in NO therapy

In vitro validation and characterization of pulsed inhaled nitric oxide administration during early inspiration

PURPOSE

Admixture of nitric oxide (NO) to the gas inspired with mechanical ventilation can be achieved through continuous, timed, or pulsed injection of NO into the inspiratory limb. The dose and timing of NO injection govern the inspired and intrapulmonary effect site concentrations achieved with different administration modes. Here we test the effectiveness and target reliability of a new mode injecting pulsed NO boluses exclusively during early inspiration.

METHODS

An in vitro lung model was operated under various ventilator settings. Admixture of NO through injection into the inspiratory limb was timed either (i) selectively during early inspiration ("pulsed delivery"), or as customary, (ii) during inspiratory time or (iii) the entire respiratory cycle. Set NO target concentrations of 5-40 parts per million (ppm) were tested for agreement with the yield NO concentrations measured at various sites in the inspiratory limb, to assess the effectiveness of these NO administration modes.

RESULTS

Pulsed delivery produced inspiratory NO concentrations comparable with those of customary modes of NO administration. At low (450 ml) and ultra-low (230 ml) tidal volumes, pulsed delivery yielded better agreement of the set target (up to 40 ppm) and inspiratory NO concentrations as compared to customary modes. Pulsed delivery with NO injection close to the artificial lung yielded higher intrapulmonary NO concentrations than with NO injection close to the ventilator. The maximum inspiratory NO concentration observed in the trachea (68 ± 30 ppm) occurred with pulsed delivery at a set target of 40 ppm.

CONCLUSION

Pulsed early inspiratory phase NO injection is as effective as continuous or non-selective admixture of NO to inspired gas and may confer improved target reliability, especially at low, lung protective tidal volumes.

Effects of pulsed inhaled nitric oxide on arterial oxygenation during mechanical ventilation in anaesthetised horses undergoing elective arthroscopy or emergency colic surgery

BACKGROUND

Administration of pulsed inhaled nitric oxide (PiNO) improves arterial oxygenation in spontaneously breathing anaesthetised healthy horses and in horses undergoing colic surgery. However, because hypoventilation commonly occurs, horses are often mechanically ventilated to prevent hypercarbia.

OBJECTIVES

To evaluate the effects of PiNO on arterial oxygenation during anaesthesia in mechanically ventilated healthy horses and horses undergoing colic surgery.

STUDY DESIGN

Prospective nonblinded clinical trial.

METHODS

Fifty horses undergoing elective arthroscopy (Group A) and 30 horses undergoing colic surgery (Group C) in dorsal recumbency were included in the study. Every second horse in each group received PiNO (A-INO, C-INO), the others served as controls (A-CN, C-CN). All horses were mechanically ventilated and anaesthesia was maintained with isoflurane. PiNO was mechanically delivered at the proximal end of the endotracheal tube as a pulse during the first part of each inspiration. Data were collected at the start (baseline, before PiNO) and at the end of inhalation anaesthesia. The Tukey method was used to compare baseline and end values for each parameter.

RESULTS

Arterial oxygen tension (PaO₂ ) increased from (median [IQR]) 13.6 (9.3, 30.1) at baseline to 24.2 (18.6, 37.0) kPa at the end of anaesthesia in A-INO (P = 0.005) and from 7.7 (6.4, 8.5) to 15.5 (9.9, 26.9) kPa in C-INO (P = 0.007). Mean (95% CI) difference in F-shunt between baseline and end were -6 (-10; -1) and -11 (-22; -1) % in A-INO (P = 0.005) and C-INO (P = 0.04) respectively. There was no change in PaO₂ or F-shunt from baseline to end of anaesthesia in A-CN or C-CN.

MAIN LIMITATIONS

Cardiac output was not measured, thus O₂ delivery could not be calculated.

CONCLUSIONS

The combination of mechanical ventilation and PiNO improved pulmonary gas exchange during anaesthesia by a simultaneous decrease in F-shunt and improved alveolar ventilation.

Pulmonary Delivery of Therapeutic and Diagnostic Gases

The 21st Congress for the International Society for Aerosols in Medicine included, for the first time, a session on Pulmonary Delivery of Therapeutic and Diagnostic Gases. The rationale for such a session within ISAM is that the pulmonary delivery of gaseous drugs in many cases targets the same therapeutic areas as aerosol drug delivery, and is in many scientific and technical aspects similar to aerosol drug delivery. This article serves as a report on the recent ISAM congress session providing a synopsis of each of the presentations. The topics covered are the conception, testing, and development of the use of nitric oxide to treat pulmonary hypertension; the use of realistic adult nasal replicas to evaluate the performance of pulsed oxygen delivery devices; an overview of several diagnostic gas modalities; and the use of inhaled oxygen as a proton magnetic resonance imaging (MRI) contrast agent for imaging temporal changes in the distribution of specific ventilation during recovery from bronchoconstriction. Themes common to these diverse applications of inhaled gases in medicine are discussed, along with future perspectives on development of therapeutic and diagnostic gases.

Variability in uptake efficiency for pulsed versus constant concentration delivery of inhaled nitric oxide

Background

Nitric oxide (NO) is currently administered using devices that maintain constant inspired NO concentrations. Alternatively, devices that deliver a pulse of NO during the early phase of inspiration may have use in optimizing NO dosing efficiency and in extending application of NO to long-term use by ambulatory, spontaneously breathing patients. The extent to which the amount of NO delivered for a given pulse sequence determines alveolar concentrations and uptake, and the extent to which this relationship varies with breathing pattern, physiological, and pathophysiological parameters, warrants investigation.

Methods

A mathematical model was used to analyze inhaled nitric oxide (NO) transport through the conducting airways, and to predict uptake from the alveolar region of the lung. Pulsed delivery was compared with delivery of a constant concentration of NO in the inhaled gas.

Results

Pulsed delivery was predicted to offer significant improvement in uptake efficiency compared with constant concentration delivery. Uptake from the alveolar region depended on pulse timing, tidal volume, respiratory rate, lung and dead space volume, and the diffusing capacity of the lung for NO (D_LNO). It was predicted that variation in uptake efficiency with breathing pattern can be limited using a pulse time of less than 100 ms, with a delay of less than 50 ms between the onset of inhalation and pulse delivery. Nonlinear variation in uptake efficiency with D_LNO was predicted, with uptake efficiency falling off sharply as D_LNO decreased below ~50-60 ml/min/mm Hg. Gas mixing in the conducting airways played an important role in determining uptake, such that consideration of bulk convection alone would lead to errors in assessing efficiency of pulsed delivery systems.

Conclusions

Pulsed NO delivery improves uptake efficiency compared with constant concentration delivery. Optimization of pulse timing is critical in limiting intra- and inter-subject variability in dosing.

Clinical perspectives with long-term pulsed inhaled nitric oxide for the treatment of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a chronic, progressive disease of the pulmonary vasculature with a high morbidity and mortality. Its pathobiology involves at least three interacting pathways – prostacyclin (PGI₂), endothelin, and nitric oxide (NO). Current treatments target these three pathways utilizing PGI₂ and its analogs, endothelin receptor antagonists, and phosphodiesterase type-5 (PDE-5) inhibitors. Inhaled nitric oxide (iNO) is approved for the treatment of hypoxic respiratory failure associated with pulmonary hypertension in term/near-term neonates. As a selective pulmonary vasodilator, iNO can acutely decrease pulmonary artery pressure and pulmonary vascular resistance without affecting cardiac index or systemic vascular resistance. In addition to delivery via the endotracheal tube, iNO can also be administered as continuous inhalation via a facemask or a pulsed nasal delivery. Consistent with a deficiency in endogenously produced NO, long-term pulsed iNO dosing appears to favorably affect hemodynamics in PAH patients, observations that appear to correlate with benefit in uncontrolled settings. Clinical studies and case reports involving patients receiving long-term continuous pulsed iNO have shown minimal risk in terms of adverse events, changes in methemoglobin levels, and detectable exhaled or ambient NO or NO₂. Advances in gas delivery technology and strategies to optimize iNO dosing may enable broad-scale application to long-term treatment of chronic diseases such as PAH.

Pulsed delivery of inhaled nitric oxide counteracts hypoxaemia during 2.5 hours of inhalation anaesthesia in dorsally recumbent horses

OBJECTIVE

The study aimed to investigate the effect of varying pulse lengths of inhaled nitric oxide (iNO), and 2.5 hours of continuous pulse-delivered iNO on pulmonary gas exchange in anaesthetized horses.

STUDY DESIGN

Experimental study.

ANIMALS

Six Standardbred horses.

METHODS

Horses received acepromazine, detomidine, guaifenesin, thiopentone and isoflurane in oxygen, were positioned in dorsal recumbency and were breathing spontaneously. iNO was on average pulsed during the first 20, 30, 43 or 73% of the inspiration in 15 minute steps. The pulse length that corresponded to the highest (peak) partial pressure of arterial oxygen (PaO(2) ) in the individual horses was determined and delivered for a further 1.5 hours. Data measured or calculated included arterial and mixed venous partial pressures of O(2) and CO(2) , heart rate, respiratory rate, expired minute ventilation, pulmonary and systemic arterial mean pressures, cardiac output and venous admixture. Data (mean ± SD) was analysed using anova with p < 0.05 considered significant.

RESULTS

Although the pulse length of iNO that corresponded to peak PaO(2) varied between horses, administration of all pulse lengths of iNO increased PaO(2) compared to baseline. The shortest pulse lengths that resulted in the peak PaO(2) were 30 and 43% of the inspiration. Administration of iNO increased PaO(2) (12.6 ± 4.1 kPa [95 ± 31 mmHg] at baseline to a range of 23.0 ± 8.4 to 25.3 ± 9.0 kPa [173 to 190 mmHg]) and PaCO(2) (8.5 ± 1.2 kPa [64 ± 9 mmHg] to 9.8 ± 1.5 kPa [73 ± 11 mmHg]) and decreased venous admixture from 32 ± 6% to 25 ± 6%. The increase in PaO(2) and decrease in venous admixture was sustained for the entire 2.5 hours of iNO delivery.

CONCLUSIONS

The improvement in arterial oxygenation during pulsed delivery of iNO was significant and sustained throughout 2.5 hours of anaesthesia.

CLINICAL RELEVANCE

Pulsed iNO potentially could be used clinically to counteract hypoxemia in anaesthetized horses.

Administration of nitric oxide into open lung regions: delivery and monitoring

BACKGROUND

Pulsed administration of nitric oxide has proven effective in relieving pulmonary hypertension and in improving oxygenation. With this delivery method the nitric oxide administration to low ventilated lung regions is avoided with subsequent enhancement in oxygenation. This study presents (i) pulsed administration technique for nitric oxide during artificial ventilation, (ii) evaluation of the delivery in an animal model, and (iii) validation of the delivery device in a laboratory setting.

METHODS

Nitric oxide was delivered in four different pulse volumes synchronously with inspiration. The delivery was monitored with a fast responding high sensitivity nitric oxide monitor and nitric oxide uptake was calculated. Pulse delivery dose range, accuracy of the delivered dose, and stability of successive doses were analysed in a laboratory setting.

RESULTS

Uptake of the delivered nitric oxide was 87-92%. Measured nitric oxide pulse concentration was 1.6-fold the delivery concentration, calculated as the ratio of nitric oxide flow to inspiration flow. Dose accuracy and stability were both 5% or 3 nmol in the validated range of 3-1000 nmol.

CONCLUSION

With pulsed administration nitric oxide therapy can be directed to well-ventilated lung regions. Avoiding administration to the anatomic dead space eliminates nitric oxide exhalation effectively, which makes the method optimal for nitric oxide therapy in a rebreathing circuit. The required dose range from paediatric to adult is covered by the delivery device with a single nitric oxide gas supply.

Effect of different pulses of nitric oxide on venous admixture in the anaesthetized horse

BACKGROUND

Dependent atelectatic lung areas open towards the end of inspiration when the lung opening pressure increases, and recollapse during expiration. We hypothesized that inhaled nitric oxide (NO) counteracts hypoxic vasoconstriction in these collapsing lung areas, resulting in increased pulmonary shunt perfusion.

METHODS

We administered NO as a pulse and varied the pulse timing during inspiration in equine anaesthesia, where atelectasis develops regularly. Six spontaneously breathing standard breed trotters were studied under isoflurane anaesthesia in lateral recumbency. NO pulsed into the first 30% of inspiration (group NOp1) was assumed to affect open lung areas. To cover more open lung areas NO was then pulsed into the first 60% of inspiration (group NOp2). In a third group, administration between 50 and 80% of inspiration was aimed at the intermittently opening lung areas (group NOp3).

RESULTS

With NOp1, venous admixture decreased by 8 (2)% (mean (SEM), P=0.045) and with NOp2 by 10 (1)% (P=0.01). With NOp3, venous admixture reduction was insignificant.

CONCLUSIONS

Pulsed administration of NO in early inspiration is optimal in reducing right to left vascular shunt in atelectatic equine lung. This reduction is positively correlated with the magnitude of the initial shunt. With administration in early inspiration, NO is mostly taken up by the lung. This prevents NO accumulation and NO2 formation in rebreathing circuits. These findings may be important in humans when atelectasis occurs increasingly with overweight and age during anaesthesia, but also in postoperative intensive care and in ARDS.