Novel oxygen uptake parameter aids idiopathic PAH evaluation

Jul 15 2014

By Laura Cowen, medwireNews Reporter

Oxygen uptake efficiency plateau (OUEP), measured during the cardiopulmonary exercise test (CPET), offers a novel method for evaluating gas exchange abnormalities in patients with idiopathic pulmonary hypertension (IPAH), Chinese researchers report.

Xingguo Sun (Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing) and colleagues retrospectively calculated the OUEP, defined as the 90 second average of the highest oxygen uptake (VO₂) to ventilator efficiency (VE) ratio, and the oxygen uptake efficiency slope (OUES; the slope of VO₂ plotted against logVE) in 32 patients with IPAH and 16 healthy controls who completed a CPET.

They found that both OUEP and OUES were significantly lower in patients than in controls, with values of 23.4 versus 37.8 mL/L and slopes of 1.08 L/min versus 1.98 L/min.

The team suggests that the lower OUE experienced by the IPAH patients “may be predominantly attributed to the inability to improve ventilation/perfusion match and distribution of blood flow to the metabolizing muscles”

There were also significant differences in OUE between adjacent time periods (rest, warm up, anaerobic threshold [AT] and peak) for both patients and controls, but the researchers noted that the magnitude of the differences were much smaller for the patients than controls.

OUEP correlated significantly with New York Heart Association functional class, total pulmonary vascular resistance, diffusing capacity for carbon monoxide and the lowest ventilation versus carbon dioxide (CO₂) output ratio during exercise, but no such correlations were observed for OUES.

In addition, Sun and co-authors noted that OUEP was easier to visualise, recognise and calculate, and had less variability than OUES. It also did not require maximal exertion, unlike OUES, so “might be more fitted for IPAH patients unable to perform maximal exercise test”, they say.

When the researchers looked at other CPET variables they found that VE of CO₂ elimination (VE/VCO₂) was significantly greater in patients than in controls at all activity levels, with the greatest differences observed at AT and peak.

Furthermore, partial pressure of end tidal CO₂ (P_ETCO₂) values increased with increasing level of activity until AT and then decreased mildly at peak among the controls, but did not increase at all from resting values in the IPAH patients.

Taken together these findings indicate that “at any required metabolic rate (as VO₂ and VCO₂), the ventilation is over driven by lung compensation for a limited heart function, i.e. mismatched Q/VA. The low and unchanged P_ETCO₂ is the evidence of hyperventilation in patients with IPAH”, write Sun et al in PLos One.

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