Determinants of aortic pressure variation during positive-pressure ventilation in man

Impact of respiratory cycle during mechanical ventilation on beat-to-beat right ventricle stroke volume estimation by pulmonary artery pulse wave analysis

BACKGROUND

The same principle behind pulse wave analysis can be applied on the pulmonary artery (PA) pressure waveform to estimate right ventricle stroke volume (RVSV). However, the PA pressure waveform might be influenced by the direct transmission of the intrathoracic pressure changes throughout the respiratory cycle caused by mechanical ventilation (MV), potentially impacting the reliability of PA pulse wave analysis (PAPWA). We assessed a new method that minimizes the direct effect of the MV on continuous PA pressure measurements and enhances the reliability of PAPWA in tracking beat-to-beat RVSV.

METHODS

Continuous PA pressure and flow were simultaneously measured for 2-3 min in 5 pigs using a high-fidelity micro-tip catheter and a transonic flow sensor around the PA trunk, both pre and post an experimental ARDS model. RVSV was estimated by PAPWA indexes such as pulse pressure (SVPP), systolic area (SVSystAUC) and standard deviation (SVSD) beat-to-beat from both corrected and non-corrected PA signals. The reference RVSV was derived from the PA flow signal (SVref).

RESULTS

The reliability of PAPWA in tracking RVSV on a beat-to-beat basis was enhanced after accounting for the direct impact of intrathoracic pressure changes induced by MV throughout the respiratory cycle. This was evidenced by an increase in the correlation between SVref and RVSV estimated by PAPWA under healthy conditions: rho between SVref and non-corrected SVSD - 0.111 (0.342), corrected SVSD 0.876 (0.130), non-corrected SVSystAUC 0.543 (0.141) and corrected SVSystAUC 0.923 (0.050). Following ARDS, correlations were SVref and non-corrected SVSD - 0.033 (0.262), corrected SVSD 0.839 (0.077), non-corrected SVSystAUC 0.483 (0.114) and corrected SVSystAUC 0.928 (0.026). Correction also led to reduced limits of agreement between SVref and SVSD and SVSystAUC in the two evaluated conditions.

CONCLUSIONS

In our experimental model, we confirmed that correcting for mechanical ventilation induced changes during the respiratory cycle improves the performance of PAPWA for beat-to-beat estimation of RVSV compared to uncorrected measurements. This was demonstrated by a better correlation and agreement between the actual SV and the obtained from PAPWA.

Perioperative Fluid Management and Volume Assessment

Fluid therapy is an integral component of perioperative care and helps maintain or restore effective circulating blood volume. The principal goal of fluid management is to optimize cardiac preload, maximize stroke volume, and maintain adequate organ perfusion. Accurate assessment of volume status and volume responsiveness is necessary for appropriate and judicious utilization of fluid therapy. To accomplish this, static and dynamic indicators of fluid responsiveness have been widely studied. This review discusses the overarching goals of perioperative fluid management, reviews the physiology and parameters used to assess fluid responsiveness, and provides evidence-based recommendations on intraoperative fluid management.

Comparison of the Efficacy of Different Arterial Waveform-derived Variables (Pulse Pressure Variation, Stroke Volume Variation, Systolic Pressure Variation) for Fluid Responsiveness in Hemodynamically Unstable Mechanically Ventilated Critically Ill Patients

Introduction

This study was conducted to assess fluid responsiveness in critically ill patients to avoid various complications of fluid overload.

Material and methods

This study was done in an ICU of a tertiary care hospital after approval from the institute ethical committee over 18 months. A total of 54 consenting adult patients were included in the study. Patients were hemodynamically unstable requiring mechanical ventilation, had acute circulatory failure, or those with at least one clinical sign of inadequate tissue perfusion. All patients were ventilated using tidal volume of 6–8 mL/kg, RR—12–15/minutes, positive end expiratory pressure (PEEP)—5 cm of water, and plateau pressure was kept below 30 cm water. They were sedated throughout the study. The arterial line and the central venous catheter were placed and connected to Vigileo-FloTrac transducer (Edward Lifesciences). Patients were classified into responder and nonresponder groups on the basis of the cardiac index (CI) after fluid challenge of 10 mL/kg of normal saline over 30 minutes. Pulse pressure variation (PPV), stroke volume variation (SVV), and systolic pressure variation (SPV) were assessed and compared at baseline, 30 minutes, and 60 minutes.

Results

In our study we found that PPV and SVV were significantly lower among responders than nonresponders at 30 minutes and insignificant at 60 minutes. Stroke volume variation was 10.28 ± 1.76 in the responder compared to 12.28 ± 4.42 (p = 0.02) at 30 minutes and PPV was 15.28 ± 6.94 in responders while it was 20.03 ± 4.35 in nonresponders (p = 0.01). We found SPV was insignificant at all time periods among both groups.

Conclusion

We can conclude that initial assessment for fluid responsiveness in critically ill mechanically ventilated patients should be based on PPV and SVV to prevent complications of fluid overload and their consequences.

How to cite this article

Kumar N, Malviya D, Nath SS, Rastogi S, Upadhyay V. Comparison of the Efficacy of Different Arterial Waveform-derived Variables (Pulse Pressure Variation, Stroke Volume Variation, Systolic Pressure Variation) for Fluid Responsiveness in Hemodynamically Unstable Mechanically Ventilated Critically Ill Patients. Indian J Crit Care Med 2021;25(1):48–53.

Cardiopulmonary Interactions: Physiologic Basis and Clinical Applications

The hemodynamic effects of ventilation can be grouped into three concepts: 1) Spontaneous ventilation is exercise; 2) changes in lung volume alter autonomic tone and pulmonary vascular resistance and can compress the heart in the cardiac fossa; and 3) spontaneous inspiratory efforts decrease intrathoracic pressure, increasing venous return and impeding left ventricular ejection, whereas positive-pressure ventilation decreases venous return and unloads left ventricular ejection. Spontaneous inspiratory efforts may induce acute left ventricular failure and cardiogenic pulmonary edema. Reversing the associated negative intrathoracic pressure swings by using noninvasive continuous positive airway pressure rapidly reverses acute cardiogenic pulmonary edema and improves survival. Additionally, in congestive heart failure, states increasing intrathoracic pressure may augment left ventricular ejection and improve cardiac output. Using the obligatory changes in venous return induced by positive pressure breathing, one can quantify the magnitude of associated decreases in venous flow and left ventricular ejection using various parameters, including vena caval diameter changes, left ventricular stroke volume variation, and arterial pulse pressure variation. These parameters vary in proportion to the level of cardiac preload reserve present, thus accurately predicting which critically ill patients will increase their cardiac output in response to fluid infusions and which will not. Common parameters include arterial pulse pressure variation and left ventricular stroke volume variation. This functional hemodynamic monitoring approach reflects a practical clinical application of heart-lung interactions.

Validity of Pulse Pressure Variation (PPV) Compared with Stroke Volume Variation (SVV) in Predicting Fluid Responsiveness

OBJECTIVE

Static monitors for assessing the fluid status during major surgeries and in critically ill patients have been gradually replaced by more accurate dynamic monitors in modern-day anaesthesia practice. Pulse pressure variation (PPV) and systolic pressure variation (SPV) are the two commonly used dynamic indices for assessing fluid responsiveness.

METHODS

In this prospective observational study, 50 patients undergoing major surgeries were monitored for PPV and SPV: after the induction of anaesthesia and after the administration of 500 mL of isotonic crystalloid bolus. Following the fluid bolus, patients with a cardiac output increase of more than 15% were classified as responders and those with an increase of less than 15% were classified as non-responders.

RESULTS

There were no significant differences in the heart rate (HR), mean arterial pressure (MAP), PPV, SVV, central venous pressure (CVP) and cardiac index (CI) between responders and non-responders. Before fluid bolus, the stroke volume was significantly lower in responders (p=0.030). After fluid bolus, MAP was significantly higher in responders but there were no significant changes in HR, CVP, CI, PPV and SVV. In both responders and non-responders, PPV strongly correlated with SVV before and after fluid bolus.

CONCLUSION

Both PPV and SVV are useful to predict cardiac response to fluid loading. In both responders and non-responders, PPV has a greater association with fluid responsiveness than SVV.

Predictors to Intravenous Fluid Responsiveness

Management with intravenous fluids can improve cardiac output in some surgical patients. Management with static preload indicators, such as central venous pressure and pulmonary artery occlusion pressure, has not demonstrated a suitable relationship with changes in the cardiac output induced by intravenous fluid therapy. Dynamic indicators, such as the variability of arterial pulse pressure or stroke volume variation, have demonstrated a suitable relationship. Since improvement in cardiac output does not guarantee an adequate perfusion pressure, in patients with hypotension, it is also necessary to know whether arterial pressure will also increase with intravenous fluid therapy. In this regard, the functional assessment of arterial load by dynamic arterial elastance could help to determine which patients will improve not only their cardiac output but also their mean arterial pressure.

Predictors to Intravenous Fluid Responsiveness

Management with intravenous fluids can improve cardiac output in some surgical patients. Management with static preload indicators, such as central venous pressure and pulmonary artery occlusion pressure, has not demonstrated a suitable relationship with changes in the cardiac output induced by intravenous fluid therapy. Dynamic indicators, such as the variability of arterial pulse pressure or stroke volume variation, have demonstrated a suitable relationship. Since improvement in cardiac output does not guarantee an adequate perfusion pressure, in patients with hypotension, it is also necessary to know whether arterial pressure will also increase with intravenous fluid therapy. In this regard, the functional assessment of arterial load by dynamic arterial elastance could help to determine which patients will improve not only their cardiac output but also their mean arterial pressure.

Fluid responsiveness in acute circulatory failure

Although fluid resuscitation of patients having acute circulatory failure is essential, avoiding unnecessary administration of fluids in these patients is also important. Fluid responsiveness (FR) is defined as the ability of the left ventricle to increase its stroke volume (SV) in response to fluid administration. The objective of this review is to provide the recent advances in the detection of FR and simplify the physiological basis, advantages, disadvantages, and cut-off values for each method. This review also highlights the present gaps in literature and provides future thoughts in the field of FR.

Static methods are generally not recommended for the assessment of FR. Dynamic methods for the assessment of FR depend on heart-lung interactions. Pulse pressure variation (PPV) and stroke volume variation (SVV) are the most famous dynamic measures. Less-invasive dynamic parameters include plethysmographic-derived parameters, variation in blood flow in large arteries, and variation in the diameters of central veins. Dynamic methods for the assessment of FR have many limitations; the most important limitation is spontaneous breathing activity.

Fluid challenge techniques were able to overcome most of the limitations of the dynamic methods. Passive leg raising is the most popular fluid challenge method. More simple techniques have been recently introduced such as the mini-fluid challenge and 10-s fluid challenge. The main limitation of fluid challenge techniques is the need to trace the effect of the fluid challenges on SV (or any of its derivatives) using a real-time monitor. More research is needed in the field of FR taking into consideration not only the accuracy of the method but also the ease of implementation, the applicability on a wider range of patients, the time needed to apply each method, and the feasibility of its application by acute care physicians with moderate and low experience.

Intermittent positive pressure ventilation increases diastolic pulmonary arterial pressure in advanced COPD

OBJECTIVES

To measure the impact of intermittent positive pressure ventilation (IPPV) on diastolic pulmonary arterial pressure (dPAP) and pulmonary pulse pressure in patients with advanced COPD.

BACKGROUND

The physiological effects of raised intrathoracic pressures upon the pulmonary circulation have not been fully established.

METHODS

22 subjects with severe COPD receiving IPPV were prospectively assessed with pulmonary and radial arterial catheterization. Changes in dPAP were assessed from end-expiration to early inspiration during low and high tidal volume ventilation.

RESULTS

Inspiration during low tidal volume IPPV increased the median [IQR] dPAP by 3.9 [2.5-4.8] mm Hg (P < 0.001). During high tidal volume, similar changes were observed. The IPPV-associated change in dPAP was correlated with baseline measures of PaO2 (rho = 0.65, P = 0.005), pH (rho = 0.64, P = 0.006) and right atrial pressure (rho = -0.53, P = 0.011).

CONCLUSIONS

In severe COPD, IPPV increases dPAP and reduces pulmonary pulse pressure during inspiration.

Mechanical ventilation-induced intrathoracic pressure distribution and heart-lung interactions*

OBJECTIVE

Mechanical ventilation causes cyclic changes in the heart's preload and afterload, thereby influencing the circulation. However, our understanding of the exact physiology of this cardiopulmonary interaction is limited. We aimed to thoroughly determine airway pressure distribution, how this is influenced by tidal volume and chest compliance, and its interaction with the circulation in humans during mechanical ventilation.

DESIGN

Intervention study.

SETTING

ICU of a university hospital.

PATIENTS

Twenty mechanically ventilated patients following coronary artery bypass grafting surgery.

INTERVENTION

Patients were monitored during controlled mechanical ventilation at tidal volumes of 4, 6, 8, and 10 mL/kg with normal and decreased chest compliance (by elastic binding of the thorax).

MEASUREMENTS AND MAIN RESULTS

Central venous pressure, airway pressure, pericardial pressure, and pleural pressure; pulse pressure variations, systolic pressure variations, and stroke volume variations; and cardiac output were obtained during controlled mechanical ventilation at tidal volume of 4, 6, 8, and 10 mL/kg with normal and decreased chest compliance. With increasing tidal volume (4, 6, 8, and 10 mL/kg), the change in intrathoracic pressures increased linearly with 0.9 ± 0.2, 0.5 ± 0.3, 0.3 ± 0.1, and 0.3 ± 0.1 mm Hg/mL/kg for airway pressure, pleural pressure, pericardial pressure, and central venous pressure, respectively. At 8 mL/kg, a decrease in chest compliance (from 0.12 ± 0.07 to 0.09 ± 0.03 L/cm H2O) resulted in an increase in change in airway pressure, change in pleural pressure, change in pericardial pressure, and change in central venous pressure of 1.1 ± 0.7, 1.1 ± 0.8, 0.7 ± 0.4, and 0.8 ± 0.4 mm Hg, respectively. Furthermore, increased tidal volume and decreased chest compliance decreased stroke volume and increased arterial pressure variations. Transmural pressure of the superior vena cava decreased during inspiration, whereas the transmural pressure of the right atrium did not change.

CONCLUSIONS

Increased tidal volume and decreased chest wall compliance both increase the change in intrathoracic pressures and the value of the dynamic indices during mechanical ventilation. Additionally, the transmural pressure of the vena cava is decreased, whereas the transmural pressure of the right atrium is not changed.

Respiratory variations in the arterial pressure during mechanical ventilation reflect volume status and fluid responsiveness

Optimal fluid management is one of the main challenges in the care of the critically ill. However, the physiological parameters that are commonly monitored and used to guide fluid management are often inadequate and even misleading. From 1987 to 1989 we published four experimental studies which described a method for predicting the response of the cardiac output to fluid administration during mechanical ventilation. The method is based on the analysis of the variations in the arterial pressure in response to a mechanical breath, which serves as a repetitive hemodynamic challenge. Our studies showed that the systolic pressure variation and its components are able to reflect even small changes in the circulating blood volume. Moreover, these dynamic parameters provide information about the slope of the left ventricular function curve, and therefore predict the response to fluid administration better than static preload parameters. Many new dynamic parameters have been introduced since then, including the pulse pressure (PPV) and stroke volume (SVV) variations, and various echocardiographic and other parameters. Though seemingly different, all these parameters are based on measuring the response to a predefined preload-modifying maneuver. The clinical usefulness of these 'dynamic' parameters is limited by many confounding factors, the recognition of which is absolutely necessary for their proper use. With more than 20 years of hindsight we believe that our early studies helped pave the way for the recognition that fluid administration should ideally be preceded by the assessment of "fluid responsiveness". The introduction of dynamic parameters into clinical practice can therefore be viewed as a significant step towards a more rational approach to fluid management.

Pressão expiratória positiva nas vias aéreas não reproduz as respostas de frequência cardíaca à manobra de Valsalva em homens jovens saudáveis

A pressão expiratória positiva nas vias aéreas (EPAP) é um recurso terapêutico que compreende uma inspiração seguida de expiração contra resistência. Sua aplicação promove ajustes no sistema cardiovascular, de maneira similar ao observado durante a manobra de Valsalva (MV). O objetivo deste estudo foi analisar a resposta da frequência cardíaca (FC) à MV e às diferentes formas de aplicação de EPAP a fim de identificar se e em qual condição esta técnica reproduz a resposta da FC observada na MV, em homens jovens aparentemente saudáveis. Foram estudados 10 sujeitos (24±3 anos; 25±3 kg/m²) que realizaram os procedimentos de MV e EPAP, aleatoriamente em dias diferentes. Na MV o esforço expiratório foi sustentado por 15 s (pressão oral de 40 mmHg [53,4 cmH2O]). Empregou-se duas técnicas de EPAP (isolada e terapêutica) contra 3 níveis de pressão (10, 15 e 20 cmH2O), aplicados aleatoriamente. As manobras foram repetidas três vezes com intervalo de cinco minutos. Considerou-se o maior valor de variação da FC (DFC) de cada manobra para análise. Empregou-se o teste Shapiro-Wilk para verificar a distribuição dos dados e ANOVA para medidas repetidas, com post-hoc de Fisher, considerando-se α<0,05. Os valores de DFC observados na MV foram maiores (p<0,05) que os encontrados nas diferentes técnicas de EPAP, independentemente do nível pressórico empregado. A aplicação de EPAP, nos três níveis pressóricos, gera menor sobrecarga cardíaca e não reproduz as respostas da FC observadas na MV.

Effect of tidal volume, intrathoracic pressure, and cardiac contractility on variations in pulse pressure, stroke volume, and intrathoracic blood volume

PURPOSE

We evaluated the impact of increasing tidal volume (V (t)), decreased chest wall compliance, and left ventricular (LV) contractility during intermittent positive-pressure ventilation (IPPV) on the relation between pulse pressure (PP) and LV stroke volume (SV(LV)) variation (PPV and SVV, respectively), and intrathoracic blood volume (ITBV) changes.

METHODS

Sixteen pentobarbital-anesthetized thoracotomized mongrel dogs were studied both before and after propranolol-induced acute ventricular failure (AVF) (n = 4), with and without chest and abdominal pneumatic binders to decrease chest wall compliance (n = 6), and during V (t) of 5, 10, 15, and 25 ml/kg (n = 6). SV(LV) and right ventricular stroke volume (SV(RV)) were derived from electromagnetic flow probes around aortic and pulmonary artery roots. Arterial pressure was measured in the aorta using a fluid-filled catheter. Arterial PPV and SVV were calculated over three breaths as (max - min)/[(max + min)/2]. ITBV changes during ventilation were inferred from the beat-to-beat volume differences between SV(RV) and SV(LV).

RESULTS

Arterial PP and SV(LV) were tightly correlated during IPPV under all conditions (r (2) = 0.85). Both PPV and SVV increased progressively as V (t) increased and with thoraco-abdominal binding, and tended to decrease during AVF. SV(RV) phasically decreased during inspiration, whereas SV(LV) phasically decreased 2-3 beats later, such that ITBV decreased during inspiration and returned to apneic values during expiration. ITBV decrements increased with increasing V (t) or with thoraco-abdominal binding, and decreased during AVF owing to variations in SV(RV), such that both PPV and SVV tightly correlated with inspiration-associated changes in SV(RV) and ITBV.

CONCLUSION

Arterial PP and SV(LV) are tightly correlated during IPPV and their relation is not altered by selective changes in LV contractility, intrathoracic pressure, or V (t). However, contractility, intrathoracic pressure, and V (t) directly alter the magnitude of PPV and SVV primarily by altering the inspiration-associated decreases in SV(RV) and ITBV.

Dynamic preload indicators fail to predict fluid responsiveness in open-chest conditions

OBJECTIVE

Dynamic preload indicators like pulse pressure variation (PPV) and stroke volume variation (SVV) are increasingly being used for optimizing cardiac preload since they have been demonstrated to predict fluid responsiveness in a variety of perioperative settings. However, in open-chest conditions, the value of these indices has not been systematically examined yet. We, therefore, evaluated the ability of PPV and SVV to predict fluid responsiveness under open- and closed-chest conditions.

DESIGN

Prospective, controlled, clinical study.

SETTING

University hospital.

PATIENTS

Twenty-two patients scheduled for elective coronary artery bypass graft surgery.

INTERVENTIONS

Defined volume loads (VL) (10 mL kg-1 hydroxyethyl starch 6%) intra- and postoperatively.

MEASUREMENTS AND MAIN RESULTS

Stroke volume index was measured 1) before and after a VL intraoperatively in open-chest conditions, and 2) under closed-chest conditions within 1 hour after arrival in the intensive care unit. Central venous pressure and global end diastolic volume were assessed as static preload indicators. In addition, PPV and SVV (both obtained with PiCCO system) were recorded. Fluid-responders were defined by an increase in stroke volume index >or=12% subsequent to the VL. Receiver operating characteristic analysis showed that all preload indicators failed to predict fluid responsiveness in open-chest conditions. Under closed-chest conditions, the areas under the receiver operating characteristic curve for PPV and SVV were 0.884 (p = 0.004) and 0.911 (p = 0.003), respectively, whereas the static and volumetric preload parameters failed to predict fluid responsiveness. A PPV of >or=10% identified fluid-responders with a sensitivity of 64% and a specificity of 100%, while a SVV of >8% identified fluid-responders with a sensitivity of 100% and a specificity of 78%.

CONCLUSIONS

Our results suggest that the dynamic preload indicators PPV and SVV are able to predict fluid responsiveness under closed-chest conditions, whereas all static and dynamic preload indicators fail to predict fluid responsiveness under open-chest conditions.

Effect of elevated PEEP on dynamic variables of fluid responsiveness in a pediatric animal model

BACKGROUND

Previous studies have demonstrated that stroke volume variation (SVV), pulse pressure variation (PPV) and global end-diastolic volume (GEDV) can be used to predict the response to fluid administration. Currently, little information is available whether application of different levels of positive end-expiratory pressure (PEEP), especially in infants and neonates, affects their ability to predict fluid responsiveness. The aim of our study was to assess the effect of increasing PEEP levels on the predictive value of SVV, PPV and GEDV with respect to fluid responsiveness.

METHODS

Stroke volume variation and PPV were monitored continously in 22 anesthetized piglets during changing PEEP levels (5 and 10 cmH(2)O) both before and after fluid loading (FL). GEDV was measured by transpulmonary thermodilution; cardiac output and stroke volume (SV) were measured by pulmonary artery thermodilution. A positive response to FL was defined as > or =15% increase in SV.

RESULTS

Fluid loading induced significant changes in all hemodynamic variables except of heart rate and systemic vascular resistance. At PEEP 5 cmH(2)O, SVV, PPV and GEDV significantly correlated with volume induced percentage change in SV, whereas at PEEP 10 cmH(2)O, this correlation was abolished for PPV. As assessed by receiver operating characteristic curve analysis, SVV and GEDV, independent of PEEP level applied, were the best predictors of a positive response to FL [area under the curve: SVV = 0.88; GEDV = 0.80].

CONCLUSIONS

In this pediatric animal model, SVV and GEDV were sensitive and specific predictors of fluid responsiveness during increasing PEEP levels.

Effect of tidal volume, sampling duration, and cardiac contractility on pulse pressure and stroke volume variation during positive-pressure ventilation

INTRODUCTION

Both pulse pressure variation and stroke volume variation during intermittent positive-pressure ventilation predict preload responsiveness. However, because ventilatory and cardiac frequencies are not the same, increasing the number of breaths sampled may increase calculated pulse pressure variation and stroke volume variation because larger (max) and smaller (min) pulse pressure and stroke volume may occur. Tidal volume and contractility may also alter pulse pressure variation and stroke volume variation. We hypothesized that the magnitude of pulse pressure variation would increase with sampling duration, and that both tidal volume and contractility would independently alter pulse pressure variation and stroke volume variation.

METHODS

In seven pentobarbital-anesthetized intact dogs arterial and left ventricular pressure (Millar) and left ventricular volume (Leycom) were measured over 8 intermittent positive-pressure ventilation breaths at tidal volume of 5, 10, 15, and 20 mL/kg (f = 20/min, 40% inspiratory time) under baseline, esmolol (2 mg/min), dobutamine infusions (5 microg/kg/min) and following volume loading (500 mL NaCl). Stroke volume variation was calculated using pulse contour method (PiCCO, Pulsion Medical Systems, Munich, Germany) averaged over 12 secs. Pulse pressure variation was calculated as 100 x (PPmax - PPmin)/PPmean and calculated over 1, 2, 3, 4, 5, 6, 7, or 8 breaths.

RESULTS

Pulse pressure variation increased progressively with increasing sampling duration up to but not exceeding five breaths. The effect on sampling duration was increased by greater tidal volume. Esmolol infusion decreased both pulse pressure variation and stroke volume variation as compared with baseline (p < 0.05) at all tidal volume levels. However, dobutamine did not alter either pulse pressure variation or stroke volume variation.

CONCLUSION

Sampling duration, tidal volume, and beta-adrenergic blockade differentially alters pulse pressure variation and stroke volume variation during intermittent positive-pressure ventilation. Thus, separate validation is required to define threshold pulse pressure variation and stroke volume variation values used to drive resuscitation algorithms.

Respiratory variations in aortic blood flow predict fluid responsiveness in ventilated children

OBJECTIVE

To investigate whether respiratory variations in aortic blood flow velocity (DeltaVpeak ao), systolic arterial pressure (DeltaPS) and pulse pressure (DeltaPP) could accurately predict fluid responsiveness in ventilated children.

DESIGN AND SETTING

Prospective study in a 18-bed pediatric intensive care unit.

PATIENTS

Twenty-six children [median age 28.5 (16-44) months] with preserved left ventricular (LV) function.

INTERVENTION

Standardized volume expansion (VE).

MEASUREMENTS AND MAIN RESULTS

Analysis of aortic blood flow by transthoracic pulsed-Doppler allowed LV stroke volume measurement and on-line DeltaVpeak ao calculation. The VE-induced increase in LV stroke volume was >15% in 18 patients (responders) and <15% in 8 (non-responders). Before VE, the DeltaVpeak ao in responders was higher than that in non-responders [19% (12.1-26.3) vs. 9% (7.3-11.8), p=0.001], whereas DeltaPP and DeltaPS did not significantly differ between groups. The prediction of fluid responsiveness was higher with DeltaVpeak ao [ROC curve area 0.85 (95% IC 0.99-1.8), p=0.001] than with DeltaPS (0.64) or DeltaPP (0.59). The best cut-off for DeltaVpeak ao was 12%, with sensitivity, specificity, and positive and negative predictive values of 81.2%, 85.7%, 93% and 66.6%, respectively. A positive linear correlation was found between baseline DeltaVpeak ao and VE-induced gain in stroke volume (rho=0.68, p=0.001).

CONCLUSIONS

While respiratory variations in aortic blood flow velocity measured by pulsed Doppler before VE accurately predict the effects of VE, DeltaPS and DeltaPP are of little value in ventilated children.

Comparative study of pressure- and volume-controlled ventilation on pulse pressure variation in a model of hypovolaemia in rabbits

BACKGROUND AND OBJECTIVE

Dynamic indices represented by systolic pressure variation and pulse pressure variation have been demonstrated to be more accurate than filling pressures in predicting fluid responsiveness. However, the literature is scarce concerning the impact of different ventilatory modes on these indices. We hypothesized that systolic pressure variation or pulse pressure variation could be affected differently by volume-controlled ventilation and pressure-controlled ventilation in an experimental model, during normovolaemia and hypovolaemia.

METHOD

Thirty-two anaesthetized rabbits were randomly allocated into four groups according to ventilatory modality and volaemic status where G1-ConPCV was the pressure-controlled ventilation control group, G2-HemPCV was associated with haemorrhage, G3-ConVCV was the volume-controlled ventilation control group and G4-HemVCV was associated with haemorrhage. In the haemorrhage groups, blood was removed in two stages: 15% of the estimated blood volume withdrawal at M1, and, 30 min later, an additional 15% at M2. Data were submitted to analysis of variance for repeated measures; a value of P < 0.05 was considered to be statistically significant.

RESULTS

At M0 (baseline), no significant differences were observed among groups. At M1, dynamic parameters differed significantly among the control and hypovolaemic groups (P < 0.05) but not between ventilation modes. However, when 30% of the estimated blood volume was removed (M2), dynamic parameters became significantly higher in animals under volume-controlled ventilation when compared with those under pressure-controlled ventilation.

CONCLUSIONS

Under normovolaemia and moderate haemorrhage, dynamic parameters were not influenced by either ventilatory modalities. However, in the second stage of haemorrhage (30%), animals in volume-controlled ventilation presented higher values of systolic pressure variation and pulse pressure variation when compared with those submitted to pressure-controlled ventilation.

Limitations of arterial pulse pressure variation and left ventricular stroke volume variation in estimating cardiac pre-load during open heart surgery

BACKGROUND

In addition to their well-known ability to predict fluid responsiveness, functional pre-load parameters, such as the left ventricular stroke volume variation (SVV) and pulse pressure variation (PPV), have been proposed to allow real-time monitoring of cardiac pre-load. SVV and PPV result from complex heart-lung interactions during mechanical ventilation. It was hypothesized that, under open-chest conditions, when cyclic changes in pleural pressures during positive-pressure ventilation are less pronounced, functional pre-load indicators may be deceptive in the estimation of ventricular pre-load.

METHODS

Forty-five patients undergoing coronary artery bypass grafting participated in this prospective, observational study. PPV and SVV were assessed by pulse contour analysis. The thermodilution technique was used to measure the stroke volume index and global and right ventricular end-diastolic volume index. Trans-oesophageal echocardiography was used to determine the left ventricular end-diastolic area index. All parameters were assessed before and after sternotomy, and, in addition, after weaning from cardiopulmonary bypass before and after chest closure (pericardium left open). Patients were ventilated with constant tidal volumes (8 +/- 2 ml/kg) throughout the study period using pressure control.

RESULTS

SVV and PPV decreased after sternotomy and increased after chest closure. However, these changes could not be related to concomitant changes in the ventricular pre-load. The stroke volume index was correlated with SVV and PPV in closed-chest conditions only, whereas volumetric indices reflected cardiac pre-load in both closed- and open-chest conditions. SVV and PPV were correlated with left and right ventricular pre-load in closed-chest-closed-pericardium conditions only (with the best correlation found for the right ventricular end-diastolic volume index).

CONCLUSIONS

SVV and PPV may be misleading when estimating cardiac pre-load during open heart surgery.

Pulse pressure variation as a tool to detect hypovolaemia during pneumoperitoneum

BACKGROUND

Pulse pressure variation (DeltaPP) and systolic pressure variation (SPV) induced by mechanical ventilation have been proposed to detect hypovolaemia and guide fluid therapy. During laparoscopic surgery, chest compliance is decreased by pneumoperitoneum. This may affect the value of SPV and DeltaPP as indicators of intravascular volume status. Thereby, we investigated the effects of pneumoperitoneum and hypovolaemia on SPV and DeltaPP.

METHODS

We measured DeltaPP, SPV and the inspiratory (Deltaup) and expiratory (Deltadown) components of SPV, at baseline, during pneumoperitoneum, during pneumoperitoneum and hypovolaemia and after the return to baseline conditions, in 11 mechanically ventilated rabbits. Pneumoperitoneum was induced by inflating the abdomen with carbon dioxide, and hypovolaemia was induced by controlled haemorrhage.

RESULTS

Pneumoperitoneum induced an increase in SPV from 8.5 +/- 1.6 to 13.3 +/- 2.6 mmHg (+56%, P < 0.05) as a result of an increase in Deltaup from 2.0 +/- 1.0 to 6.7 +/- 2.1 mmHg (+236%, P < 0.05), but no significant change in Deltadown, nor in DeltaPP. Haemorrhage induced a significant (P < 0.05) increase in SPV from 13.3 +/- 2.6 to 19.9 +/- 3.7 mmHg (+50%), in Deltadown from 6.6 +/- 3.3 to 14.0 +/- 4.9 mmHg (+112%) and in DeltaPP from 11.1 +/- 4.8 to 24.9 +/- 9.8% (+124%) but no change in Deltaup. All parameters returned to baseline values after blood re-infusion and abdominal deflation.

CONCLUSIONS

SPV is modified by haemorrhage but it is also influenced by pneumoperitoneum. In contrast, DeltaPP is modified by haemorrhage but not by pneumoperitoneum. These findings suggest that DeltaPP should be used preferentially instead of SPV to detect hypovolaemia and guide fluid therapy during laparoscopic surgery.

Haemodynamic monitoring

The monitoring of cardiovascular function is an indispensable element in anaesthesia. A thorough understanding of pathophysiology in various disease states allows optimal balancing of the invasiveness and completeness of haemodynamic monitoring. The prevention of both intraoperative and postoperative complications is therefore a primary goal.

Effects of sternotomy on heart-lung interaction in patients undergoing cardiac surgery receiving pressure-controlled mechanical ventilation

BACKGROUND

The key concept underlying the dynamic indexes of preload dependence is the physiological heart-lung interaction. During sternotomy this interaction undergoes various changes, some of which remain unclear. Our primary aim was to investigate how the interaction changes during sternotomy by evaluating pulse pressure variations (PPV) with the chest closed and after sternotomy in patients ventilated using the pressure-controlled mode.

METHODS

We prospectively studied 25 patients undergoing coronary artery bypass grafting (CABG) receiving pressure-controlled ventilation. Standard hemodynamic data, PPV and tidal volume delivered were recorded before and after sternotomy, and, with the chest open, before and after positive end-expiratory pressure (PEEP) was applied and inspiratory pressure was increased.

RESULTS

Sternotomy left all variables statistically unchanged from values before thoracotomy although in the subgroup of patients with a PPV > 8% (56%) sternotomy significantly reduced PPV (from 14.4 +/- 5.2% to 8.9 +/- 4.5%). With the chest open, when PEEP was applied at 5 cm H(2)O, tidal volume decreased (from 643 +/- 83 to 587 +/- 104 ml) and stroke volume decreased (from 77 +/- 17 to 72 +/- 15 ml) but PPV remained unchanged. When PEEP was discontinued and inspiratory pressure was increased by 5 cm H(2)O, tidal volume increased (from 643 +/- 83 to 814 +/- 89 ml) and PPV increased (from 8.2 +/- 3.9% to 12.3 +/- 6.8%) but stroke volume remained unchanged.

CONCLUSIONS

In patients ventilated in the pressure-controlled mode, except those with a pre-sternotomy PPV > 8% (fluid responders), sternotomy leaves standard hemodynamic data and PPV unchanged. When the chest wall is open, cyclic changes (tidal volume) but not continuous changes (PEEP) in intrathoracic pressure directly influence PPV.

Prediction of fluid responsiveness using respiratory variations in left ventricular stroke area by transoesophageal echocardiographic automated border detection in mechanically ventilated patients

Background

Left ventricular stroke area by transoesophageal echocardiographic automated border detection has been shown to be strongly correlated to left ventricular stroke volume. Respiratory variations in left ventricular stroke volume or its surrogates are good predictors of fluid responsiveness in mechanically ventilated patients. We hypothesised that respiratory variations in left ventricular stroke area (ΔSA) can predict fluid responsiveness.

Methods

Eighteen mechanically ventilated patients undergoing coronary artery bypass grafting were studied immediately after induction of anaesthesia. Stroke area was measured on a beat-to-beat basis using transoesophageal echocardiographic automated border detection. Haemodynamic and echocardiographic data were measured at baseline and after volume expansion induced by a passive leg raising manoeuvre. Responders to passive leg raising manoeuvre were defined as patients presenting a more than 15% increase in cardiac output.

Results

Cardiac output increased significantly in response to volume expansion induced by passive leg raising (from 2.16 ± 0.79 litres per minute to 2.78 ± 1.08 litres per minute; p < 0.01). ΔSA decreased significantly in response to volume expansion (from 17% ± 7% to 8% ± 6%; p < 0.01). ΔSA was higher in responders than in non-responders (20% ± 5% versus 10% ± 5%; p < 0.01). A cutoff ΔSA value of 16% allowed fluid responsiveness prediction with a sensitivity of 92% and a specificity of 83%. ΔSA at baseline was related to the percentage increase in cardiac output in response to volume expansion (r = 0.53, p < 0.01).

Conclusion

ΔSA by transoesophageal echocardiographic automated border detection is sensitive to changes in preload, can predict fluid responsiveness, and can quantify the effects of volume expansion on cardiac output. It has potential clinical applications.

Interdépendance cœur–poumons chez le patient ventilé par pression positive

Heart-lung interactions during positive-pressure ventilation are characterized by an extreme sensibility to the patient's intravascular volume status. Indeed, a fall in cardiac output due to decreased ventricular preload can be observed when initiating positive-pressure ventilation. This phenomenon is due to the close anatomic-functional association between heart and lungs, and to the fact that pulmonary volume and intrathoracic pressure variations cyclically modify heart-lung interactions. The present review address the questions of the physiological and physiopathological effects of positive-pressure ventilation on the right and left venous returns, and on pulmonary and systemic vascular resistances.

Increased intra-abdominal pressure affects respiratory variations in arterial pressure in normovolaemic and hypovolaemic mechanically ventilated healthy pigs

OBJECTIVE

To evaluate the effect of increased intra-abdominal pressure (IAP) on the systolic and pulse pressure variations induced by positive pressure ventilation in a porcine model.

DESIGN AND SETTING

Experimental study in a research laboratory.

SUBJECTS

Seven mechanically ventilated and instrumented pigs prone to normovolaemia and hypovolaemia by blood withdrawal.

INTERVENTION

Abdominal banding gradually increased IAP in 5-mmHg steps up to 30 mmHg.

MEASUREMENTS AND MAIN RESULTS

Variations in systolic pressure, pulse pressure, inferior vena cava flow, and pleural and transmural (LVEDPtm) left-ventricular end-diastolic pressure were recorded at each step. Systolic pressure variations were 6.1+/-3.1%, 8.5+/-3.6% and 16.0+/-5.0% at 0, 10, and 30 mmHg IAP in normovolaemic animals (mean+/-SD; p<0.01 for IAP effect). They were 12.7+/-4.6%, 13.4+/-6.7%, and 23.4+/-6.3% in hypovolaemic animals (p<0.01 vs normovolaemic group) for the same IAP. Fluctuations of the inferior vena cava flow disappeared as the IAP increased. Breath cycle did not induce any variations of LVEDPtm for 0 and 30 mmHg IAP.

CONCLUSIONS

In this model, the systolic pressure and pulse pressure variations, and inferior vena cava flow fluctuations were dependent on IAP values which caused changes in pleural pressure swing, and this dependency was more marked during hypovolaemia. The present study suggests that dynamic indices are not exclusively related to volaemia in the presence of increased IAP. However, their fluid responsiveness predictive value could not be ascertained as no fluid challenge was performed.

ASSESSING INTRAVASCULAR VOLUME BY DIFFERENCE IN PULSE PRESSURE IN PIGS SUBMITTED TO GRADED HEMORRHAGE

We assessed changes in intravascular volume monitored by difference in pulse pressure (dPP%) after stepwise hemorrhage in an experimental pig model. Six pigs (23-25 kg) were anesthetized (isoflurane 1.5 vol%) and mechanically ventilated to keep end-tidal CO2 (etCO2) at 35 mmHg. A PA-catheter and an arterial catheter were placed via femoral access. During and after surgery, animals received lactated Ringer's solution as long as they were considered volume responders (dPP>13%). Then animals were allowed to stabilize from the induction of anesthesia and insertion of catheters for 30 min. After stabilization, baseline measurements were taken. Five percent of blood volume was withdrawn, followed by another 5%, and then in 10%-increments until death from exsanguination occurred. After withdrawal of 5% of blood volume, all pigs were considered volume responders (dPP>13%); dPP rose significantly from 6.1+/-3.3% to 19.4+/-4.2%. The regression analysis of stepwise hemorrhage revealed a linear relation between blood loss (hemorrhage in %) and dPP (y=0.99*x+14; R2=0.7764; P<.0001). In addition, dPP was the only parameter that changed significantly between baseline and a blood loss of 5% (P<0.01), whereas cardiac output, stroke volume, heart rate, MAP, central venous pressure, pulmonary artery occlusion pressure, and systemic vascular resistance, respectively, remained unchanged. We conclude that in an experimental hypovolemic pig model, dPP correlates well with blood loss.

Arterial pressure-based technologies: a new trend in cardiac output monitoring

New trends in cardiovascular monitoring use the arterial pulse as a less invasive means of assessing cardiac output. When adopting a new technology into practice, three questions need to be answered: (1) is the method technologically sound?, (2) is it based on physiologic principles?, and (3) are the applications clinically important? This article provides a clinical review on the technology, physiology, and applications of a new arterial pressure-based method of determining cardiac output and stroke volume variation as an additional parameter for fluid status assessment.

The influence of tidal volume on the dynamic variables of fluid responsiveness in critically ill patients

Respiratory-related variabilities in stroke volume and arterial pulse pressure (Delta%Pp) are proposed to predict fluid responsiveness. We investigated the influence of tidal volume (Vt) and adrenergic tone on these variables in mechanically ventilated patients. Cyclic changes in aortic velocity-time integrals (Delta%VTI(Ao), echocardiography) and Delta%Pp (catheter) were measured simultaneously before and after intravascular volume expansion, and Vt was randomly varied below and above its basal value. Intravascular volume expansion was performed by hydroxyethyl starch (100 mL, 60 s). Receiver operating characteristic curves were generated for Delta%VTI(Ao), Delta%Pp and left ventricle cross-sectional end-diastolic area (echocardiography), considering the change in stroke volume after intravascular volume expansion (> or =15%) as the response criterion. Covariance analysis was used to test the influence of Vt on Delta%VTI(Ao) and Delta%Pp. Twenty-one patients were prospectively included; 9 patients (43%) were responders to intravascular volume expansion. Delta%VTI(Ao) and Delta%Pp were higher in responders compared with nonresponders. Predictive values of Delta%VTI(Ao) and Delta%Pp were similar (threshold: 20.4% and 10.0%, respectively) and higher than that of left ventricle cross-sectional end-diastolic area at the appropriate level of Vt. Delta%Pp was slightly correlated with norepinephrine dosage. Delta%Pp increased with the increase in the level of Vt both before and after intravascular volume expansion, contrasting with an unexpected stability of Delta%VTI(Ao). In conclusion, Delta%VTI(Ao) and Delta%Pp are good predictors of intravascular fluid responsiveness but the divergent evolution of these two variables when Vt was increased needs further explanation.

Pre-ejection period variations predict the fluid responsiveness of septic ventilated patients

OBJECTIVES

In septic patients with acute circulatory failure, reliable predictors of fluid responsiveness are needed at the bedside. We hypothesized that the respiratory change in pre-ejection period (DeltaPEP) would allow the prediction of changes in cardiac index following volume administration in mechanically ventilated septic patients.

DESIGN

Prospective clinical investigation.

SETTING

A ten-bed hospital intensive care unit.

PATIENTS

Patients admitted after septic shock equipped with an arterial catheter.

INTERVENTIONS

Pre-ejection period (PEP)--defined as the time interval between the beginning of the R wave on the electrocardiogram and the upstroke of the radial arterial pressure curve (PEPKT) or the pulse plethysmographic waveforms (PEPPLET)--and cardiac index (transthoracic echocardiography-Doppler) were determined before and after volume infusion of colloid (8 mL x kg). DeltaPEP (%) was defined as the difference between expiratory and inspiratory PEP divided by the mean of expiratory and inspiratory values. Respiratory changes in pulse pressure (DeltaPP) was also measured.

MEASUREMENTS AND MAIN RESULTS

: Twenty-two volume challenges were done in 20 deeply sedated patients. DeltaPEPKT, DeltaPEPPLET, and DeltaPP (measured in all patients) before volume expansion were correlated with cardiac index change after fluid challenge (r = .73, r = .67, and r = .70, respectively, p < .0001). Patients with a cardiac index increase induced by volume expansion > or = 15% and <15% were classified as responders and nonresponders, respectively. Receiver operating characteristic curves showed that the threshold DeltaPP value of 17% allowed discrimination between responder/nonresponder patients with a sensitivity of 85% and a specificity of 100%. For both DeltaPEPKT and DeltaPEPPLET, the best threshold value was 4% with a sensitivity-specificity of 92%-89% and 100%-67%, respectively.

CONCLUSIONS

The present study found DeltaPEPKT and DeltaPEPPLET to be as accurate as DeltaPP in the prediction of fluid responsiveness in mechanically ventilated septic patients.

Clinical review: interpretation of arterial pressure wave in shock states

In critically ill patients monitored with an arterial catheter, the arterial pressure signal provides two types of information that may help the clinician to interpret haemodynamic status better: the mean values of systolic, diastolic, mean and pulse pressures; and the magnitude of the respiratory variation in arterial pressure in patients undergoing mechanical ventilation. In this review we briefly discuss the physiological mechanisms responsible for arterial pressure generation, with special focus on resistance, compliance and pulse wave amplification phenomena. We also emphasize the utility of taking into consideration the overall arterial pressure set (systolic, diastolic, mean and pulse pressures) in order to define haemodynamic status better. Finally, we review recent studies showing that quantification of respiratory variation in pulse and systolic arterial pressures can allow one to identify the mechanically ventilated patients who may benefit from volume resuscitation.

Stroke volume and pulse pressure variation for prediction of fluid responsiveness in patients undergoing off-pump coronary artery bypass grafting

STUDY OBJECTIVES

Stroke volume variation (SVV) and pulse pressure variation (PPV) determined by the PiCCOplus system (Pulsion Medical Systems; Munich, Germany) may be useful dynamic variables in guiding fluid therapy in patients receiving mechanical ventilation. However, with respect to the prediction of volume responsiveness, conflicting results for SVV have been published in cardiac surgery patients. The goal of this study was to reevaluate SVV in predicting volume responsiveness and to compare it with PPV.

DESIGN

Prospective nonrandomized clinical investigation.

SETTING

University-based cardiac surgery.

PATIENTS

Forty patients with preserved left ventricular function undergoing elective off-pump coronary artery bypass grafting.

INTERVENTIONS

Volume replacement therapy before surgery.

MEASUREMENTS AND RESULTS

Following induction of anesthesia, before and after volume replacement (6% hydroxyethyl starch solution, 10 mL/kg ideal body weight), hemodynamic measurements of stroke volume index (SVI), SVV, PPV, global end-diastolic volume index (GEDVI), central venous pressure (CVP) and pulmonary capillary wedge pressure (PCWP) were obtained. Also, left ventricular end-diastolic area index (LVEDAI) was assessed by transesophageal echocardiography. Prediction of ventricular performance was tested by calculating the area under the receiver operating characteristic (ROC) curves and by linear regression analysis; p < 0.05 was considered significant. All measured hemodynamic variables except heart rate changed significantly after fluid loading. GEDVI, CVP, PCWP, and LVEDAI increased, whereas SVV and PPV decreased. The best area under the ROC curve (AUC) was found for SVV (AUC = 0.823) and PPV (AUC = 0.808); the AUC for other preload indexes ranged from 0.493 to 0.636. A significant correlation with changes of SVI was observed for SVV (r = 0.606, p < 0.001) and PPV (r = 0.612, p < 0.001) only. SVV and PPV were closely related (r = 0.861, p < 0.001).

CONCLUSIONS

In contrast to standard preload indexes, SVV and PPV, comparably, showed a good performance in predicting fluid responsiveness in patients before off-pump coronary artery bypass grafting.

Pulse pressure variations to predict fluid responsiveness: influence of tidal volume

OBJECTIVE

To evaluate the influence of tidal volume on the capacity of pulse pressure variation (DeltaPP) to predict fluid responsiveness.

DESIGN

Prospective interventional study.

SETTING

A 31-bed university hospital medico-surgical ICU.

PATIENTS AND PARTICIPANTS

Sixty mechanically ventilated critically ill patients requiring fluid challenge, separated according to their tidal volume.

INTERVENTION

Fluid challenge with either 1,000 ml crystalloids or 500 ml colloids.

MEASUREMENTS AND RESULTS

Complete hemodynamic measurements including DeltaPP were obtained before and after fluid challenge. Tidal volume was lower than 7 ml/kg in 26 patients, between 7-8 ml/kg in 9 patients, and greater than 8 ml/kg in 27 patients. ROC curve analysis was used to evaluate the predictive value of DeltaPP at different tidal volume thresholds, and 8 ml/kg best identified different behaviors. Overall, the cardiac index increased from 2.66 (2.00-3.47) to 3.04 (2.44-3.96) l/min m(2) ( P <0.001). It increased by more than 15% in 33 patients (fluid responders). Pulmonary artery occluded pressure was lower and DeltaPP higher in responders than in non-responders, but fluid responsiveness was better predicted with DeltaPP (ROC curve area 0.76+/-0.06) than with pulmonary artery occluded pressure (0.71+/-0.07) and right atrial (0.56+/-0.08) pressures. Despite similar response to fluid challenge in low (<8 ml/kg) and high tidal volume groups, the percent of correct classification of a 12% DeltaPP was 51% in the low tidal volume group and 88% in the high tidal volume group.

CONCLUSIONS

DeltaPP is a reliable predictor of fluid responsiveness in mechanically ventilated patients only when tidal volume is at least 8 ml/kg.

Pulse Pressure Variation Predicts Fluid Responsiveness Following Coronary Artery Bypass Surgery

STUDY OBJECTIVE

To determine whether the degree of pulse pressure variation (PPV) and systolic pressure variation (SPV) predict an increase in cardiac output (CO) in response to volume challenge in postoperative patients who have undergone coronary artery bypass grafting (CABG), and to determine whether PPV is superior to SPV in this setting.

DESIGN AND SETTING

This was a prospective clinical study conducted in the cardiovascular ICU of a university hospital.

PATIENTS

Twenty-one patients were studied immediately after arrival in the ICU following CABG.

INTERVENTION

A fluid bolus was administered to all patients.

MEASUREMENTS

Hemodynamic measurements, including central venous pressure (CVP), pulmonary artery occlusion pressure (PAOP), CO (thermodilution), percentage of SPV (%SPV), and percentage of PPV (%PPV), were performed shortly after patient arrival in the ICU. Patients were given a rapid 500-mL fluid challenge, after which hemodynamic measurements were repeated. Patients whose CO increased by >/= 12% were considered to be fluid responders. The ability of different parameters to distinguish between responders and nonresponders was compared.

RESULTS

In response to the volume challenge, 6 patients were responders and 15 were nonresponders. Baseline CVP and PAOP were no different between these two groups. In contrast, the %SPV and the %PPV were significantly higher in responders than in nonresponders. Receiver operating characteristic curve analysis suggested that the %PPV was the best predictor of fluid responsiveness. The ideal %PPV threshold for distinguishing responders from nonresponders was found to be 11. A PPV value of >/= 11% predicted an increase in CO with 100% sensitivity and 93% specificity.

CONCLUSION

PPV and SPV can be used to predict whether or not volume expansion will increase CO in postoperative CABG patients. PPV was superior to SPV at predicting fluid responsiveness. Both of these measures were far superior to CVP and PAOP.

Cyclic changes in arterial pulse during respiratory support revisited by Doppler echocardiography

It has long been known that there are cyclic changes in arterial pressure during mechanical ventilation. They are caused by cyclic changes in both the right and left ventricular stroke output, occurring in opposite phases. As a result, arterial pulse pressure is increased during inspiration and decreased during expiration. A cyclic improvement in left ventricular systolic function could thus be expected during mechanical lung inflation. We tested this hypothesis in 31 septic patients who were mechanically ventilated in controlled mode by combining left ventricular measurements by transesophageal echocardiography with invasive arterial pressure recordings and Doppler analysis of pulmonary venous flow and right and left ventricular stroke volume. Lung inflation by tidal ventilation significantly improved left ventricular stroke volume (26 +/- 0.4 cm3/m2 [mean +/- SEM] vs. 22.3 +/- 0.4 cm3/m2 at end deflation). Beat-to-beat analysis of pulmonary venous flow velocity illustrated the boosting effect of lung inflation on pulmonary venous return. The beneficial effect of inspiration thus appeared directly related to a significant increase in left ventricular diastolic volume (60.3 +/- 1.5 cm3/m2 vs. 53.3 +/- 1.4 cm3/m2 at end-expiration) and to a lesser extent to an improved left ventricular ejection fraction. We concluded that the transient beneficial hemodynamic effect of tidal ventilation on the left ventricular pump is essentially mediated by an improved left ventricular filling.

Fluid responsiveness in mechanically ventilated patients: a review of indices used in intensive care

OBJECTIVE

In mechanically ventilated patients the indices which assess preload are used with increasing frequency to predict the hemodynamic response to volume expansion. We discuss the clinical utility and accuracy of some indices which were tested as bedside indicators of preload reserve and fluid responsiveness in hypotensive patients under positive pressure ventilation.

RESULTS AND CONCLUSIONS

Although preload assessment can be obtained with fair accuracy, the clinical utility of volume responsiveness-guided fluid therapy still needs to be demonstrated. Indeed, it is still not clear whether any form of monitoring-guided fluid therapy improves survival.

Predicting fluid responsiveness in ICU patients: a critical analysis of the evidence

STUDY OBJECTIVE

To identify and critically review the published peer-reviewed, English-language studies investigating predictive factors of fluid responsiveness in ICU patients.

DESIGN

Studies were collected by doing a search in MEDLINE (from 1966) and scanning the reference lists of the articles. Studies were selected according to the following criteria: volume expansion performed in critically ill patients, patients classified in two groups (responders and nonresponders) according to the effects of volume expansion on stroke volume or on cardiac output, and comparison of responder and nonresponder patients' characteristics before volume expansion.

RESULTS

Twelve studies were analyzed in which the parameters tested were as follows: (1) static indicators of cardiac preload (right atrial pressure [RAP], pulmonary artery occlusion pressure [PAOP], right ventricular end-diastolic volume [RVEDV], and left ventricular end-diastolic area [LVEDA]); and (2) dynamic parameters (inspiratory decrease in RAP [Delta RAP], expiratory decrease in arterial systolic pressure [Delta down], respiratory changes in pulse pressure [Delta PP], and respiratory changes in aortic blood velocity [Delta Vpeak]). Before fluid infusion, RAP, PAOP, RVEDV, and LVEDA were not significantly lower in responders than in nonresponders in three of five studies, in seven of nine studies, in four of six studies, and in one of three studies, respectively. When a significant difference was found, no threshold value could discriminate responders and nonresponders. Before fluid infusion, Delta RAP, Delta down, Delta PP, and Delta Vpeak were significantly higher in responders, and a threshold value predicted fluid responsiveness with high positive (77 to 95%) and negative (81 to 100%) predictive values.

CONCLUSION

Dynamic parameters should be used preferentially to static parameters to predict fluid responsiveness in ICU patients.

Mitral Doppler indices are superior to two-dimensional echocardiographic and hemodynamic variables in predicting responsiveness of cardiac output to a rapid intravenous infusion of colloid

We hypothesized that mitral flow (MF) Doppler measurements could be used to predict cardiac output (CO) responsiveness to fluid challenge. Fourteen patients with normal systolic and diastolic function, scheduled for coronary artery bypass graft surgery, were evaluated as part of a pilot study in which preload was varied immediately before the beginning of cardiopulmonary bypass. A Validation group of 36 patients with different levels of systolic and diastolic function received a rapid infusion of 500 mL of 10% pentastarch. By use of transesophageal echocardiography, we measured left ventricular end-diastolic area, pulsed Doppler indices of the MF and pulmonary venous flow, and standard hemodynamic variables during acute volemic variations. A baseline measurement was first recorded, followed by measurements taken after a decrease (211 +/- 87 mL) and then an increase (176 +/- 149 mL) in preload (pilot study) and before and after 500 mL of pentastarch (validation study). In the pilot study, we found that a low velocity/time integral (VTI) E wave/A wave (E/A) ratio was associated with a larger increase in CO secondary to an increase in preload (r = 0.64, P < 0.05). Stepwise linear regression identified Doppler measurements of the mitral VTI E/A ratio as the most important variable to predict the increase in CO after fluid infusion. In the validation study, a mitral E/A ratio <1.26 before fluid infusion best predicted a 20% increase in stroke volume (receiver operating characteristic curve, 71%; P < 0.05), whereas no other hemodynamic or echocardiographic variable predicted preload responsiveness. We conclude that the MF Doppler filling pattern is an important factor to predict the increase in CO after intravascular fluid challenge in patients undergoing coronary artery bypass grafting.

IMPLICATIONS

In the presence of low cardiac output, the clinician's ability to identify which patients are more likely to benefit from volume administration to improve hemodynamic status while avoiding fluid overload is important. The analysis of Doppler measurement of the mitral flow as an indirect indicator of the individual diastolic pressure/volume relationship may be useful to predict the intravascular volume responsiveness in patients undergoing coronary artery bypass graft surgery.

Recent advances in the clinical application of heart-lung interactions

Clinical applications of heart-lung interactions have centered on the impact of ventilation on regional blood flow and the measures of cardiovascular responsiveness to both positive end-expiratory pressure and fluid resuscitation. These new and exciting applications of established physiology provide new therapeutic options for the caregiver with reduced risk for complications in the patient. This review illustrates several of these studies within the context of known cardiopulmonary physiology.