Systolic reserve maintains left ventricular-vascular coupling when challenged by adverse breathing mechanics and hypertension in healthy adults

Augmented negative intrathoracic pressures (nITP) and dynamic hyperinflation (DH) are adverse breathing mechanics (ABM) associated with chronic obstructive pulmonary disease (COPD) that attenuate left ventricular (LV) preload and augment afterload. In COPD, hypertension (elevated systemic arterial load) commonly adds additional afterload to the LV. Combined ABM and hypertension may profoundly challenge ventricular-vascular coupling and attenuate stroke volume (SV), particularly if LV systolic reserve is limited. However, even in the healthy heart, the combined impact of ABM and systemic arterial loading on LV function and ventricular-vascular coupling has not been fully elucidated. Healthy volunteers (10 M/9 F, 24 ± 3 yr old) were challenged with mild (-10 cmH₂O nITP and 25% DH) and severe (-20 cmH₂O nITP and 100% DH) ABM, without and with postexercise ischemia (PEI) at each severity. LV SV, chamber geometry, end-systolic elastance (E_es), arterial elastance (E_a), and ventricular-vascular coupling (E_es:E_a) were quantified using echocardiography. Compared with resting control (58 ± 13 mL), SV decreased during mild ABM (51 ± 13 mL), mild ABM + PEI (51 ± 11 mL), severe ABM (50 ± 12 mL), and severe ABM + PEI (47 ± 11 mL) (P < 0.001); similar trends were observed for LV end-diastolic volume. The end-diastolic radius of septal curvature increased, indicating direct ventricular interaction, during severe ABM and severe ABM + PEI (P < 0.001). Compared with control (1.99 ± 0.41 mmHg/mL), E_a increased progressively with mild ABM (2.21 ± 0.47 mmHg/mL) and severe ABM (2.50 ± 0.56 mmHg/mL); at each severity, E_a was greater with superimposed PEI (P < 0.001). However, well-matched E_es increases occurred, and E_es:E_a was unchanged throughout. ABM pose a challenge to ventricular-vascular coupling that is accentuated by superimposed PEI; however, in healthy younger adults, the LV has substantial systolic reserve to maintain coupling.NEW & NOTEWORTHY In healthy younger adults, combined dynamic hyperinflation (DH) and negative intrathoracic pressures (nITP) attenuate left ventricular filling, but through different mechanisms at different severities. DH and nITP contribute to increased left ventricular afterload through mechanical effects in addition to presumed reflexive regulation, which can be further increased by elevated arterial loading. However, within this demographic, the left ventricle has substantial reserve to increase systolic performance, which matches contractility to afterload to preserve stroke volume.

Attenuation of human hypoxic pulmonary vasoconstriction by acetazolamide and methazolamide

RATIONALE

Acetazolamide, a carbonic anhydrase inhibitor used for preventing altitude illness attenuates hypoxic pulmonary vasoconstriction (HPV) while improving oxygenation. Methazolamide, an analog of acetazolamide, is more lipophilic, has a longer half-life, and activates a major antioxidant transcription factor. However, its influence on the hypoxic pulmonary response in humans is unknown.

OBJECTIVE

To determine if a clinically relevant dosing of methazolamide improves oxygenation, attenuates HPV and augments plasma antioxidant capacity in men exposed to hypoxia when compared to an established dosing of acetazolamide known to suppress HPV.

METHODS

In this double-blind, placebo-controlled, cross-over trial, eleven participants were randomized to treatments with methazolamide (100mg b.i.d.) and acetazolamide (250mg t.i.d.) for two days prior to 60 minutes of hypoxia (F_IO₂≈0.12).

MEASUREMENTS

Pulmonary artery systolic pressure (PASP), alveolar ventilation (V̇_A), blood gases and markers of redox status were measured. Pulmonary vascular sensitivity to hypoxia was determined by indexing PASP to alveolar PO₂.

RESULTS

Acetazolamide caused greater metabolic acidosis compared with methazolamide, but the augmented V̇_A and improved oxygenation with hypoxia were similar. The rise in PASP with hypoxia was lower with methazolamide (9.0 ± 0.9 mmHg) and acetazolamide (8.0 ± 0.7 mmHg) compared with placebo (14.1 ± 1.3 mmHg; P < 0.05). The pulmonary vascular sensitivity to hypoxia (ΔPASP/ΔP_AO₂) was reduced equally by both drugs. Only acetazolamide improved the non-enzymatic plasma antioxidant capacity.

CONCLUSIONS

Although acetazolamide only had plasma antioxidant properties, methazolamide led to similar improvements in oxygenation and reduction in HPV at a dose causing less metabolic acidosis than acetazolamide in humans.