Kazuya Tachibana

Effect of Ventilatory Settings on Accuracy of Cardiac Output Measurement Using Partial CO2 Rebreathing

Background Recently, a new device has been developed to measure cardiac output noninvasively using partial carbon dioxide (CO2) rebreathing. Because this technique uses CO2 rebreathing, the authors suspected that ventilatory settings, such as tidal volume and ventilatory mode, would affect its accuracy: they conducted this study to investigate which parameters affect the accuracy of the measurement. Methods The authors enrolled 25 pharmacologically paralyzed adult post–cardiac surgery patients. They applied six ventilatory settings in random order: (1) volume-controlled ventilation with inspired tidal volume (VT) of 12 ml/kg; (2) volume-controlled ventilation with VT of 6 ml/kg; (3) pressure-controlled ventilation with VT of 12 ml/kg; (4) pressure-controlled ventilation with VT of 6 ml/kg; (5) inspired oxygen fraction of 1.0; and (6) high positive end-expiratory pressure. Then, they changed the maximum or minimum length of rebreathing loop with VT set at 12 ml/kg. After establishing steady-state conditions (15 min), they measured cardiac output using CO2 rebreathing and thermodilution via a pulmonary artery catheter. Finally, they repeated the measurements during pressure support ventilation, when the patients had restored spontaneous breathing. The correlation between two methods was evaluated with linear regression and Bland-Altman analysis. Results When VT was set at 12 ml/kg, cardiac output with the CO2 rebreathing technique correlated moderately with that measured by thermodilution (y = 1.02x, R = 0.63; bias, 0.28 l/min; limits of agreement, −1.78 to +2.34 l/min), regardless of ventilatory mode, oxygen concentration, or positive end-expiratory pressure. However, at a lower VT of 6 ml/kg, the CO2 rebreathing technique underestimated cardiac out-put compared with thermodilution (y = 0.70x; R = 0.70; bias, −1.66 l/min; limits of agreement, −3.90 to +0.58 l/min). When the loop was fully retracted, the CO2 rebreathing technique overestimated cardiac output. Conclusions Although cardiac output was underreported at small VT values, cardiac output measured by the CO2 rebreathing technique correlates fairly with that measured by the thermodilution method.

Noninvasive Cardiac Output Measurement Using Partial Carbon Dioxide Rebreathing Is Less Accurate at Settings of Reduced Minute Ventilation and when Spontaneous Breathing Is Present

Background Although evaluation of cardiac output by the partial carbon dioxide rebreathing technique is as accurate as thermodilution techniques under controlled mechanical ventilation, it is less accurate at low tidal volume. It is not clear whether reduced accuracy is due to low tidal volume or low minute ventilation. The effect of spontaneous breathing on the accuracy of partial carbon dioxide rebreathing measurement has not been fully investigated. The objectives of the current study were to investigate whether tidal volume or minute ventilation is the dominant factor for the accuracy, and the accuracy of the technique when spontaneous breathing effort is present. Methods The authors enrolled 25 post–cardiac surgery patients in two serial protocols. First, the authors applied three settings of controlled mechanical ventilation in random order: large tidal volume (12 ml/kg), the same minute ventilation with a small tidal volume (6 ml/kg), and 50% decreased minute ventilation with a small tidal volume (6 ml/kg). Second, when the patient recovered spontaneous breathing, the authors applied three conditions of partial ventilatory support in random order: synchronized intermittent mandatory ventilation–pressure support ventilation, pressure support ventilation with an appropriately adjusted rebreathing loop, and pressure support ventilation with the shortest available loop. After establishing steady state conditions, the authors measured cardiac output using both partial carbon dioxide rebreathing and thermodilution methods. The correlation between the data yielded by the two methods was determined by Bland-Altman analysis and linear regression. Results Cardiac output with the carbon dioxide rebreathing technique correlated moderately with that measured by thermodilution when minute ventilation was set to maintain normocapnia, regardless of tidal volumes. However, when minute ventilation was set low, the carbon dioxide rebreathing technique underreported cardiac output (y = 0.70x; correlation coefficient, 0.34; bias, −1.73 l/min; precision, 1.27 l/min; limits of agreement, −4.27 to +0.81 l/min). When there was spontaneous breathing, the correlation between the two cardiac output measurements became worse. Carbon dioxide rebreathing increased spontaneous tidal volume and respiratory rate (20% and 30%, respectively, during pressure support ventilation) when the rebreathing loop was adjusted for large tidal volume. Conclusions During controlled mechanical ventilation, minute ventilation rather than tidal volume affected the accuracy of cardiac output measurement using the partial carbon dioxide rebreathing technique. When spontaneous breathing is present, the carbon dioxide rebreathing technique is less accurate and increases spontaneous tidal volume and respiratory rate.

Effects of reduced rebreathing time, in spontaneously breathing patients, on respiratory effort and accuracy in cardiac output measurement when using a partial carbon dioxide rebreathing technique: a prospective observational study

IntroductionNew technology using partial carbon dioxide rebreathing has been developed to measure cardiac output. Because rebreathing increases respiratory effort, we investigated whether a newly developed system with 35 s rebreathing causes a lesser increase in respiratory effort under partial ventilatory support than does the conventional system with 50 s rebreathing. We also investigated whether the shorter rebreathing period affects the accuracy of cardiac output measurement.MethodOnce a total of 13 consecutive post-cardiac-surgery patients had recovered spontaneous breathing under pressure support ventilation, we applied a partial carbon dioxide rebreathing technique with rebreathing of 35 s and 50 s in a random order. We measured minute ventilation, and arterial and mixed venous carbon dioxide tension at the end of the normal breathing period and at the end of the rebreathing periods. We then measured cardiac output using the partial carbon dioxide rebreathing technique with the two rebreathing periods and using thermodilution.ResultsWith both rebreathing systems, minute ventilation increased during rebreathing, as did arterial and mixed venous carbon dioxide tensions. The increases in minute ventilation and arterial carbon dioxide tension were less with 35 s rebreathing than with 50 s rebreathing. The cardiac output measures with both systems correlated acceptably with values obtained with thermodilution.ConclusionWhen patients breathe spontaneously the partial carbon dioxide rebreathing technique increases minute ventilation and arterial carbon dioxide tension, but the effect is less with a shorter rebreathing period. The 35 s rebreathing period yielded cardiac output measurements similar in accuracy to those with 50 s rebreathing.

Changes in respiratory pattern during continuous positive airway pressure in infants after cardiac surgery

PurposeSpontaneous breathing trials are commonly used in adults to enable smooth weaning from mechanical ventilation. However, few investigations have examined spontaneous breathing tests in infants. We investigated how respiratory patterns of infants changed during continuous positive airway pressure (CPAP) and whether successful extubation followed CPAP.MethodsFifty-one consecutive post—cardiac surgery infants satisfied the following weaning criteria: stable hemodynamics, pH > 7.30, tidal volume > 5 ml·kg−1, and respiratory rate < 50 breaths·min−1 with pressure control of 10–16 cm H2O. We applied CPAP of 3 cm H2O for 30 min to these 51 infants. During CPAP, tidal volume, respiratory rate, and arterial blood gases were measured. CPAP was terminated if the patient showed a sustained increase or decrease in heart rate or blood pressure (>20%), a decrease in arterial oxygen saturation (>5%), agitation, or diaphoresis. After the completion of CPAP, tracheal extubation was performed. We considered extubation successful if no reintubation was required in the ensuing 48 h.ResultsAlthough hemodynamic and ventilatory variables were unstable for the first 5 min, they stabilized after 10 min of CPAP. Fifty infants completed the CPAP trial safely. Of these, 46 (92%) underwent successful extubation after the CPAP trial. The failure group (4 infants) showed lower pH, higher arterial carbon dioxide tension, and more rapid shallow breathing during CPAP than the success group.ConclusionAfter cardiac surgery, when infants recovered stable hemodynamics and spontaneous breathing, the ventilatory pattern and hemodynamics became stable after 10 min of CPAP. Ninety-two percent of the patients were successfully extubated following a 30-min CPAP trial.

Steroid replacement therapy for severe heart failure after Norwood procedure

A 15-day-old neonate demonstrated severe heart failure and capillary leak syndrome after undergoing a Norwood procedure for hypoplastic left heart syndrome. Because she developed severe subcutaneous edema and baseline blood cortisol was low, we suspected relative adrenal insufficiency. After 18 days of dexamethasone administration, her hemodynamics and respiratory function improved, and she was successfully extubated and discharged from hospital. When hemodynamics are unstable in neonates after major cardiac surgery, relative adrenal insufficiency and steroid replacement should be considered.

Expiratory tidal volume displayed on Bird 8400STi can exceed the preset tidal volume due to cardiogenic oscillation: a lung model study

We noticed that monitored tidal volumes often exceeded preset tidal volumes in patients with large cardiogenic oscillation. To investigate whether triggering modes affect this discrepancy, we simulated cardiogenic oscillation of 90 breaths/min in a lung model, which was ventilated with a Bird 8400STi ventilator (Bird, Palm Springs, CA, USA). The magnitude of cardiogenic oscillation was defined as peak expiratory flow fluctuation at the lung model. Two respiratory rates (5 and 10 breaths/min) and two triggering modes (flow-triggering and pressure-triggering) were applied, while tidal volume was set at 500 ml. We recorded tidal volume on a ventilator monitor and calculated the discrepancy from the set tidal volume. We also measured fluctuation in flow and airway opening pressure created by cardiogenic oscillation. During flow-triggering, larger flow fluctuation and smaller airway pressure fluctuation were observed compared with during pressure-triggering. During flow-triggering, the discrepancy between monitored tidal volume and set value ranged from 0 to +327 ml at 5 breaths/min, and from 0 to +105 ml at 10 breaths/min. There was a linear correlation between the magnitude of cardiogenic oscillation and the overestimation of tidal volume. By contrast, during pressure-triggering, the discrepancy was small. In conclusion, tidal volume is overestimated during flow-triggering but not during pressure-triggering when cardiogenic oscillation is large.