Effects of the ventilation pattern and pulmonary blood flow on lung heat transfer

Abstract

To investigate the relative role of pulmonary perfusion compared to ventilation on lung heat exchange, we determined the effects of blood flow, tidal volume and frequency of ventilation on the rate of lung heat transfer. In anesthetized dogs and isolated, perfused lungs, we investigated the dependence of the overall lung heat transfer coefficient (HTC) and lung thermal capacitance upon ventilation and pulmonary blood flow. The relationship between the HTC and pulmonary blood flow was strongly dependent on ventilation parameters. A distributed model of non-steady-state heat exchange adequately described these observations and demonstrated that changes in pulmonary blood flow may be considered as changes in the effective conductivity of the bronchial walls as 0.4 (0.1) J s(-1)m(-1) K(-1) per (l/min(-1)) of pulmonary blood flow. Our model describes the complex relationship between HTC, ventilation pattern, and effective thermal conductivity of the bronchial walls, all of which present limitations for the use of lung heat transfer to determine pulmonary blood flow.

Lung heat transfer

“…The results of the study by Serikov et al (2004) are in contrast with highly encouraging findings, reported by the same author in previous animal and human studies , where the s was not influenced by physiologic changes of minute ventilation (ventilation-to-perfusion ratio <5). The author also assumed the effective coefficient of lung thermal conductivity to be a constant and recommended a new non-invasive method for estimating cardiac output by simply measuring s. Serikov found an excellent correlation between cardiac output and the inverse s in 48 patients (r=0.89).…”

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Serikov and colleagues evaluated the effects of the ventilation pattern and pulmonary blood flow on lung heat transfer (Serikov et al 2004). The authors conclude that the pattern of ventilation substantially affects the values of the overall heat transfer coefficient of the lungs determined at the tracheal level, and thus limits the usefulness of measuring lung heat transfer for determining pulmonary blood flow.

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“…The authors conclude that the pattern of ventilation substantially affects the values of the overall heat transfer coefficient of the lungs determined at the tracheal level, and thus limits the usefulness of measuring lung heat transfer for determining pulmonary blood flow. A surrogate measure of overall thermal capacitance was determined with the time constant s of the expiratory temperature decay curve between two steady states (warm humidified inspiratory gas at body temperature followed by dry inspriatory gas at room temperature).The results of the study by Serikov et al (2004) are in contrast with highly encouraging findings, reported by the same author in previous animal and human studies , where the s was not influenced by physiologic changes of minute ventilation (ventilation-to-perfusion ratio <5). The author also assumed the effective coefficient of lung thermal conductivity to be a constant and recommended a new non-invasive method for estimating cardiac output by simply measuring s. Serikov found an excellent correlation between cardiac output and the inverse s in 48 patients (r=0.89).…”

Lung heat transfer: author?s reply

“…Despite the promising correlations, further extension of these studies into patients with mismatched ventilation and perfusion, low tidal volumes, and possible lung, bronchial or vascular pathology demonstrated that more complex relationships exist between simple indexes of lung heat exchange and pulmonary blood flow. To further clarify these relationships, we undertook this present animal study, combined with complex mathematical modeling, to determine whether there are limits to the use of a simple lumped-sum model of heat exchange (Serikov et al 2004). These results clearly indicate that the limitations of our previous simple model are substantial.…”

“…We agree that pulmonary heat transfer is a promising parameter in critical care and pulmonary medicine. It should indeed be investigated and developed further from this biomechanical perspective and our publication (Serikov et al 2004) is intended to serve this goal.…”