Less invasive indicators of changes in thermodilution cardiac output by ventilatory changes after cardiac surgery

The effect of positive-end-expiratory pressure on stroke volume variation: An experimental study in dogs

Stroke volume variation (SVV) may be affected by ventilation settings. However, it is unclear whether positive-end-expiratory pressure (PEEP) affects SVV independently of the effect of driving pressure. We aimed to investigate the effect of driving pressure and PEEP on SVV under various preload conditions using beagle dogs as the animal model. We prepared three preload model, baseline, mild and moderate haemorrhage model. Mild and moderate haemorrhage models were created in nine anaesthetized, mechanically ventilated dogs by sequentially removing 10 mL/kg, and then an additional 10 mL/kg of blood, respectively. We measured cardiac output, stroke volume (SV), SVV, heart rate, central venous pressure, pulmonary capillary wedge pressure and the mean arterial pressure under varying ventilation settings. Peak inspiratory pressure (PIP) was incrementally increased by 4 cmH₂ O, from 9 cmH₂ O to 21 cmH₂ O, under PEEP values of 4, 8, and 12 cmH₂ O. The driving pressure did not significantly decrease SV under each preload condition and PEEP; however, significantly increased SVV. In contrast, the increased PEEP decreased SV and increased SVV under each preload condition and driving pressure, but these associations were not statistically significant. According to multiple regression analysis, an increase in PEEP and decrease in preload significantly decreased SV (P < .05). In addition, an increase in the driving pressure and decrease in preload significantly increased SVV (P < .05). Driving pressure had more influence than PEEP on SVV.

Impact of Positive End-Expiratory Pressure on Thermodilution-Derived Right Ventricular Parameters in Mechanically Ventilated Critically Ill Patients

OBJECTIVES

To examine the effect of positive end-expiratory pressure (PEEP) on right ventricular stroke volume variation (SVV), with possible implications for the number and timing of pulmonary artery catheter thermodilution measurements.

DESIGN

Prospective, clinical pilot study.

SETTING

Academic medical center.

PARTICIPANTS

Patients who underwent volume-controlled mechanical ventilation and had a pulmonary artery catheter.

INTERVENTION

PEEP was increased from 5-to-10 cmH2O and from 10-to-15 cmH2O with 10-minute intervals, with similar decreases in PEEP, from 15-to-10 cmH2O and 10-to-5 cmH2O.

MEASUREMENTS AND MAIN RESULTS

In 15 patients, right ventricular parameters were measured using thermodilution at 10% intervals of the ventilatory cycle at each PEEP level with a rapid-response thermistor. Mean right ventricular stroke volume and end-diastolic volume declined during incremental PEEP and normalized on return to 5 cmH2O PEEP (p = 0.01 and p = 0.001, respectively). Right ventricular SVV remained unaltered by changes in PEEP (p = 0.26), regardless of incremental PEEP (p = 0.15) or decreased PEEP (p = 0.12). The coefficients of variation in the ventilatory cycle of all other thermodilution-derived right ventricular parameters also were unaffected by changes in PEEP.

CONCLUSIONS

This study showed that increases in PEEP did not affect right ventricular SVV in critically ill patients undergoing mechanical ventilation despite reductions in mean right ventricular stroke volume and end-diastolic volume. This could be explained by cyclic counteracting changes in right ventricular preloading and afterloading during the ventilatory cycle, independent of PEEP. Changes in PEEP did not affect the number and timing of pulmonary artery catheter thermodilution measurements.

Systolic time intervals vs invasive predictors of fluid responsiveness after coronary artery bypass surgery

OBJECTIVES

Haemodynamic parameters for predicting fluid responsiveness in intensive care patients are invasive, technically challenging or not universally applicable. We compared the initial systolic time interval (ISTI), a non-invasive measure of the time interval between the electrical and mechanical activities of the heart measured by impedance cardiography, with invasively measured haemodynamic parameters in predicting fluid responsiveness after cardiac surgery.

METHODS

Thirty-two clinically hypovolemic patients admitted to the intensive care unit after coronary artery bypass surgery received 500 ml of gelatine solution in two volume loading steps of 250 ml at an infusion rate of 1000 ml/h. Haemodynamic and biochemical measurements were done at baseline and 15 min after each volume loading step with continuous recording of the impedance cardiogram and electrocardiogram.

RESULTS

Forty-four percentage (n = 14) of patients showed a stroke volume (SV) index increase >10%. ISTI predicted fluid responsiveness with an optimum threshold of >153 ms (P = 0.023) and a sensitivity of 71% and specificity of 78%. The predictive values of ISTI did not differ from those of arterial pressure or SV at baseline. A decrease of ISTI of ≥8.3 ms predicted fluid responsiveness with the highest positive predictive value (88%, P = 0.004) among the variables, and absence thereof virtually excluded fluid responsiveness (specificity 94%).

CONCLUSIONS

Non-invasively measured ISTI is able to predict and monitor fluid responsiveness after cardiac surgery non-inferiorly to invasively measured haemodynamic indices.

Comparing hemodynamic effects with three different measurement devices, of two methods of external leg compression versus passive leg raising in patients after cardiac surgery

External leg compression (ELC) may increase cardiac output (CO) in fluid-responsive patients like passive leg raising (PLR). We compared the hemodynamic effects of two methods of ELC and PLR measured by thermodilution (COtd), pressure curve analysis Modelflow™ (COmf) and ultra-sound HemoSonic™ (COhs), to evaluate the method with the greatest hemodynamic effect and the most accurate less invasive method to measure that effect. We compared hemodynamic effects of two different ELC methods (circular, A (n = 16), vs. wide, B (n = 13), bandages inflated to 30 cm H2O for 15 min) with PLR prior to each ELC method, in 29 post-operative cardiac surgical patients. Hemodynamic responses were measured with COtd, COmf and COhs. PLR A increased COtd from 6.1 ± 1.7 to 6.3 ± 1.8 L·min(-1) (P = 0.016), and increased COhs from 4.9 ± 1.5 to 5.3 ± 1.6 L·min(-1) (P = 0.001), but did not increase COmf. ELC A increased COtd from 6.4 ± 1.8 to 6.7 ± 1.9 L·min(-1) (P = 0.001) and COmf from 6.9 ± 1.7 to 7.1 ± 1.8 L·min(-1) (P = 0.021), but did not increase COhs. ELC A increased COtd and COmf as in PLR A. PLR B increased COtd from 5.4 ± 1.3 to 5.8 ± 1.4 L·min(-1) (P < 0.001), and COhs from 5.0 ± 1.0 to 5.4 ± 1.0 L·min(-1) (P = 0.013), but not COmf. ELC B increased COtd from 5.2 ± 1.2 to 5.4 ± 1.1 L·min(-1) (P = 0.003), but less than during PLR B (P = 0.012), while COmf and COhs did not change. Bland-Altman and polar plots showed lower limits of agreement with changes in COtd for COmf than for COhs. The circular leg compression increases CO more than bandage compression, and is able to increase CO as in PLR. The less invasive Modelflow™ can detect these changes reasonably well.

Comparison of noninvasive continuous arterial waveform analysis (Nexfin) with transthoracic Doppler echocardiography for monitoring of cardiac output

STUDY OBJECTIVES

To compare the Nexfin cardiac output (CO) with the CO obtained from transthoracic Doppler echocardiography (TTE) during routine cardiac function screening.

DESIGN

Observational clinical study.

SETTING

Echocardiography laboratory.

PATIENTS

40 ASA physical status 1 and 2 patients scheduled for routine TTE examination.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

In 40 patients scheduled for routine TTE examination, we obtained simultaneous CO measurements with Doppler ultrasound and derived from Nexfin blood pressure measurements. Correlation and level of agreement between Nexfin and TTE were analyzed using Pearson correlation coefficient and Bland-Altman plots. The Pearson correlation coefficient for Nexfin versus TTE was 0.68 (CI: 0.46 - 0.82, P < 0.0001). Bland-Altman analysis showed a bias of 0.51 ± 1.1 L/min and limits of agreement of -1.6 to 2.6 L/min, with a percentage error of 39%.

CONCLUSIONS

Considering limits of precision of CO measurements with Doppler echocardiography (± 30%), the agreement between noninvasive CO measurement with the Nexfin and TTE is reasonable.