Physiological Importance

The heart is essentially linked in series with the pulmonary system. They are exposed to similar intrathoracic pressure changes, compete for intrathoracic space, and have a number of integrated neural and humoral processes. It is not surprising, therefore, that one system has a significant impact on the other.

Heart and Lung Interaction
For example, as the heart fails, either due to systolic or diastolic heart failure, filling pressures rise, left atrial pressure increases, and these pressures are transferred back to the lungs via the pulmonary circulation. There are a number of receptors in the heart and lungs that are sensitive to pressure, and possibly stretch and/or congestion, that would impact breathing. High pressures in the pulmonary vasculature also increase fluid flux from the vessels out into the interstitial space that can further influence ventilation and perfusion matching in the lung and increase diffusion distances across the alveolar membrane. These changes can stiffen the lungs and increase the work and cost of breathing causing subjects to switch from normal to tachypneic breathing patterns that are more rapid and shallow. The supine position may further alter filling pressures and central blood volume, further aggravating these interactions. Further, during exercise, when the work and cost of breathing increase, the heart's ability to respond to the increased metabolic demand may increase pressure within the lungs further altering gas exchange. It is this link between the cardiovascular and pulmonary systems that appears to change in parallel with disease status, allowing for tracking of disease state with non-invasive gas exchange measurements.

Measuring the Effect on Patients
A number of clinical studies have demonstrated the utility of assessing submaximal or resting gas exchange and breathing pattern data in lieu of maximal exercise testing. Although maximal exercise testing provides an index of maximal functional capacity it is rare in everyday life that patients ever reach these capacities. In addition, the derangements in the pulmonary system associated with heart failure are present at rest and may be accentuated with very light activity or with positional changes. Shape Medical believes that such measures, obtained in a systematic way, are as helpful as the more typical measures obtained during maximal exercise testing for following health status of patients.