Background: Horses with recurrent airway obstruction (heaves) are described as exhibiting “increased abdominal effort,” but it is unknown whether this translates to an effective contribution to ventilation.
Hypothesis: We hypothesized that heaves is characterized by asynchrony between rib cage and abdominal motions, and that the abdominal component is the major contributor to ventilation.
Animals: The rib cage versus abdominal motion in naturally occurring heaves (n = 15) was compared to controls at rest (n = 7) and during hyperpnea because of lobeline treatment, and the effects of histamine-induced bronchoconstriction in controls (n = 10).
Methods: Flow patterns, phase angle (9) between the rib and abdominal compartments, abdominal (Vabd) contribution to tidal volume (VT), and lung mechanics were measured.
Results: Findings unique to the heaves group included the loss of biphasic expiratory flow, severely increased 9 with the abdomen consistently lagging behind the rib cage, and a reduced contribution of the abdomen to ventilation. A subgroup of heaves (n = 5) with abdominal paradox showed a significant (P < .05) reduction in tidal volume, and increased respiratory rate. Bronchodilation reduced 9 in heaves (P= .06), but 9 remained significantly higher after bronchodilation than found in controls.
Conclusions and Clinical Importance: We conclude that breathing pattern in horses with heaves is characterized by severe rib cage/abdominal asynchrony, with the rib cage motion in synchrony with flow, therefore dominating ventilation. In a subset of heaves, the abdominal compartment (diaphragm, abdominal muscles) was completely out of synchrony with flow (“abdominal paradox”) despite the clinical appearance of “increased abdominal effort.”