Effects of ventilation mode and blood flow on arterial oxygenation during pulse-delivered inhaled nitric oxide in anesthetized horses

Abstract

OBJECTIVE To determine the impact of mechanical ventilation (MV) and perfusion conditions on the efficacy of pulse-delivered inhaled nitric oxide (PiNO) in anesthetized horses. ANIMALS 27 healthy adult horses. PROCEDURES Anesthetized horses were allocated into 4 groups: spontaneous breathing (SB) with low (< 70 mm Hg) mean arterial blood pressure (MAP; group SB-L; n = 7), SB with physiologically normal (≥ 70 mm Hg) MAP (group SB-N; 8), MV with low MAP (group MV-L; 6), and MV with physiologically normal MAP (group MV-N; 6). Dobutamine was used to maintain MAP > 70 mm Hg. Data were collected after a 60-minute equilibration period and at 15 and 30 minutes during PiNO administration. Variables included Pao2, arterial oxygen saturation and content, oxygen delivery, and physiologic dead space-to-tidal volume ratio. Data were analyzed with Shapiro-Wilk, Mann-Whitney U, and Friedman ANOVA tests. RESULTS Pao2, arterial oxygen saturation, arterial oxygen content, and oxygen delivery increased significantly with PiNO in the SB-L, SB-N, and MV-N groups; were significantly lower in group MV-L than in group MV-N; and were lower in MV-N than in both SB groups during PiNO. Physiologic dead space-to-tidal volume ratio was highest in the MV-L group. CONCLUSIONS AND CLINICAL RELEVANCE Pulmonary perfusion impacted PiNO efficacy during MV but not during SB. Use of PiNO failed to increase oxygenation in the MV-L group, likely because of profound ventilation-perfusion mismatching. During SB, PiNO improved oxygenation irrespective of the magnitude of blood flow, but hypoventilation and hypercarbia persisted. Use of PiNO was most effective in horses with adequate perfusion.

Effects of pulsed inhaled nitric oxide delivery on the distribution of pulmonary perfusion in spontaneously breathing and mechanically ventilated anesthetized ponies

“…17 The favorable respiratory mechanics of spontaneous breathing and the superior effect of PiNO during spontaneous breathing in anesthetized horses has been described in earlier work. 33 During mechanical ventilation, Pao 2 values were significantly greater at baseline, compared with values measured during spontaneous breathing, owing to improved ventilation and a lower alveolararterial difference in partial pressure of oxygen. Consequently, the effect of PiNO during mechanical ventilation may appear somewhat blunted as a result.…”

Cardiac output affects the response to pulsed inhaled nitric oxide in mechanically ventilated anesthetized ponies determined by CT angiography of the lung

“…30 In the ponies supported with dobutamine in this study, the resulting increase in  Qt minimized the negative cardiovascular effects of MV on the response to PiNO delivery and agrees with a previous study. 20 However, in the latter study, regional pulmonary perfusion was not measured. In this present study, we were able to demonstrate that when PiNO was delivered to ponies with dobutamine administered concurrently, pulmonary blood was redistributed to well-ventilated regions of the lung, leading to improvements in oxygenation.…”

“…[14][15][16][17][18][19] However, this poor response to PiNO agrees with a previous study in anesthetized horses. 20 In anesthetized humans with nitroprusside-induced hypotension, gas exchange is adversely affected:  Qs/  Qt Statistical comparisons were performed for T1 and T2. A significant interaction was seen between the effect of dobutamine and concurrent pulsed inhaled nitric oxide, *P < .001.…”

“…[14][15][16][17][18][19] However, if PiNO is administered to mechanically ventilated horses exhibiting concurrent hypotension, PiNO is ineffective. 20 This is in contrast to hypotensive, spontaneously breathing horses, presumably because of differing respiratory mechanics between the 2 modes of ventilation. Moreover, the administration of dobutamine to improve cardiac output and pulmonary perfusion in combination with nitric oxide (NO) during MV, appears to optimize the effect of NO.…”