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Poster presentation

Simple circuits to maintain isocapnia regardless of ventilation

JA Fisher¹, A Vesely¹, H Sasano¹, R Somogyi¹, G Volgyesi¹, L Fedorko¹ and S Iscoe²

¹The University of Toronto Health Network, Toronto, Ontario, Canada

²Department of Physiology, Queen's University, Kingston, Ontario, Canada

corresponding author email

Neural Control of Breathing
Rotorua, New Zealand, 1-4 September 2001

Respiratory Research 2001, 2(Suppl 1):P33doi:10.1186/rr151

Received:	2 August 2001
Published:	17 August 2001

© 2001 BioMed Central Ltd

Poster presentation

Many studies of respiratory control require a constant CO₂ to eliminate it as a confounding variable. Methods for controlling CO₂ generally rely on feedback circuits which are subject to phase lags and instabilities.

In 1998, we introduced a simple circuit ([1]; Fig. 1A) which stabilizes CO₂ levels regardless of increases in ventilation. It passively adds CO₂ to the circuit from an external source at a rate proportional to any increase in ventilation above control. Because the CO₂ in this reserve gas has a PCO₂ equal to that in mixed venous blood, it cannot contribute to pulmonary CO₂ exchange.

Alternatively, CO₂ can be supplied from previously exhaled gas. Banzett and colleagues [2] suggested one version. Our version (Fig. 1B) operates on the same principle but is more compact, relying on a simple bypass valve between the inspiratory and expiratory lines and which opens only when the fresh gas reservoir is exhausted. When ventilation exceeds the fresh gas flow, the 'extra' gas inhaled will be expired gas from the preceding expiration. Pro-portional rebreathing of expired gas with a PCO₂ approximately equal to that of mixed venous blood prevents hypocapnia. Moreover, if the fresh gas flow consists of a hyperoxic gas mixture, such as 100% O₂, partial rebreathing ensures that FIO₂ remains close to that of the administered gas.

Is an external gas source mandatory for maintaining isocapnia regardless of ventilatory level? The circuit in the black box contains no compressed gas or electronics. We challenge you to breathe through it and a) change your expired CO₂ level using any breathing pattern you want, and b) explain how our circuit works.

Possible applications for this "mystery circuit" include preventing hyperventilation-induced hypocapnia in acute mountain sickness and resuscitation of asphyxiated neonates [3].

References

1. Sommer LZ, Iscoe S, Silverman J, Dickstein J, Fink A, Robicsek A, Sommer D, Kruger J, Greenberg A, Volgyesi G, Fisher JA: A simple breathing circuit minimizing changes in alveolar ventilation during hyperpnoea.

   Eur Respir J 1998, 12:698-701. PubMed Abstract | Publisher Full Text

2. Banzett RB, Garcia RT, Moosavi SH: Simple contrivance "clamps" end-tidal PCO₂ and PO₂ despite rapid changes in ventilation.

   J Appl Physiol 2000, 88:1597-1600. PubMed Abstract | Publisher Full Text

3. Saugstad OD: Resuscitation of the asphyxic newborn infant: new insight leads to new therapeutic possibilities.

   Biol Neonate 2001, 79:258-60. PubMed Abstract | Publisher Full Text

4. Takeuchi A, Rucker J, Vesely A, Sommer LZ, Tesler J, Lavene E, Maleck WH, Volgyesi G, Fedorko L, Iscoe S, Fisher JA: A simple 'new' method to accelerate clearance of carbon monoxide from human subjects.

   Am J Respir Crit Care Med 2000, 161:1816-1819. PubMed Abstract | Publisher Full Text

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