Fig. 3. Agreement between cardiac output measurements obtained by carbon dioxide rebreathing (CO^NI) and those obtained by thermodilution technique (CO^TD) when spontaneous breathing is present. (A ) Synchronized intermittent mandatory ventilation (SIMV, respiratory rate of 5 breaths/min) plus pressure support ventilation (PSV, 6 cm H²O). (B ) Pressure support ventilation (10 cm H²O). (C ) Pressure support ventilation (10 cm H²O) with the shortest rebreathing loop. Equations and result curves for linear regression analysis are also shown. In A  and B , the rebreathing loop was size adjusted according to the manufacturer's instructions recommended for tidal volume of 12 ml/kg. In C , the loop was fully retracted.