Influence of tidal volume on pulse pressure variations in hypovolemic ventilated pigs with acute respiratory distress-like syndrome

Effects of positive end-expiratory pressure on the predictability of fluid responsiveness in acute respiratory distress syndrome patients

The prediction accuracy of pulse pressure variation (PPV) for fluid responsiveness was suggested to be unreliable in low tidal volume (VT) ventilation. However, high PEEP can cause ARDS patients relatively hypovolemic and more fluid responsive. We hypothesized that high PEEP 15 cmH₂O can offset the disadvantage of low VT and improve the predictive performance of PPV. We prospectively enrolled 27 hypovolemic ARDS patients ventilated with low VT 6 ml/kg and three levels of PEEP (5, 10, 15 cmH₂O) randomly. Each stage lasted for at least 5 min to allow for equilibration of hemodynamics and pulmonary mechanics. Then, fluid expansion was given with 500 ml hydroxyethyl starch (Voluven 130/70). The hemodynamics and PPV were automatically measured with a PiCCO2 monitor. The PPV values were significantly higher during PEEP15 than those during PEEP5 and PEEP10. PPV during PEEP15 precisely predicts fluid responsiveness with a cutoff value 8.8% and AUC (area under the ROC curve) of ROC (receiver operating characteristic curve) 0.847, higher than the AUC during PEEP5 (0.81) and PEEP10 (0.668). Normalizing PPV with driving pressure (PPV/Driving-P) increased the AUC of PPV to 0.875 during PEEP15. In conclusions, high PEEP 15 cmH₂O can counteract the drawback of low VT and preserve the predicting accuracy of PPV in ARDS patients.

Effects of acute hemorrhage on intrapulmonary shunt in a pig model of acute respiratory distress-like syndrome

BACKGROUND

In acute respiratory distress syndrome (ARDS), gas exchange and respiratory system mechanics (compliance) are severely impaired. Besides ventilatory parameters, the degree of respiratory abnormality can be influenced by the circulatory state. This study investigated the influence of acute hypovolemia on the respiratory system.

METHODS

We performed a secondary analysis of a previous study including 8 pigs with ARDS-like syndrome induced by lung lavage and surfactant depletion method (ARDS group) and 10 mechanically ventilated pigs with no intervention (CTRL group). Animals of both groups were subjected to hemorrhage and retransfusion successively. We reanalyzed the effect of acute blood volume variations on intrapulmonary shunt (shunt), arterial oxygenation (PaO2:FiO2), global oxygen delivery (DO2) and respiratory system compliance (Crs).

RESULTS

In the ARDS group, after hemorrhage, shunt decreased (-28 +/- 3.5 % (p < 0.001)), respiratory system compliance (Crs) increased (+5.1 +/- 1.0 ml/cm H2O (p < 0.001)) moreover, there was a concurrent increase in PaO2:FiO2 (+113 +/- 19.1 mmHg; p < 0.001) but this did not prevent a reduction in DO2 (-317 +/- 49.8 ml/min; p < 0.001). Following retransfusion, shunt and Crs return towards pre-hemorrhage values. Similar changes, but of smaller magnitude were observed in the CTRL group, except that no significant changes in oxygenation occurred.

CONCLUSIONS

The present analysis suggests that an acute decrease in blood volume results in a decrease in shunt with a parallel improvement in arterial oxygenation and an increase in Crs during ARDS-like syndrome. Our results strengthen the importance to integrate the circulatory condition in the analysis of the state of the respiratory system. However, the translation of this physiological model in a clinical perspective is not straightforward because our model of acute and severe hemorrhage is not strictly equivalent to a progressive hypovolemia, as could be obtained in ICU by diuretic. Furthermore, the present model does not consider the impact of blood loss induced decrease of DO2 on other vital organs function.

TRIAL REGISTRATION

'Not applicable'.

Heart lung interactions during mechanical ventilation

PURPOSE OF REVIEW

To survey the recent medical literature examining studies of the hemodynamic effects of mechanical ventilation.

RECENT FINDINGS

Ventilation-induced dynamic changes in arterial pulse pressure and stroke volume variation (PPV and SVV, respectively) identify volume responsiveness. The cause of PPV and SVV are due to intrathoracic pressure-induced variations in right atrial pressure changing intrathoracic blood volume over the ventilatory cycle. This explains why PPV and SVV are inaccurate with smaller tidal volumes used in acute lung injury, but remain useful in one-lung ventilation and prone positioning. Noninvasive measures of PPV and SVV using finger plethysmography and aortic root ultrasound or estimates of intrathoracic blood volume by thoracic impedance also predict volume responsiveness. Finally, the PPV-to-SVV ratio varies with vasomotor tone and can be used to identify vasopressor need in hypotensive patients. The clinical implications of these findings are starting to be realized in recommended management principles.

SUMMARY

PPV and SVV predict volume responsiveness, but like all monitoring approaches, need to be understood within the framework of their physiologic determinations.

Influence of tidal volume on pulse pressure variation and stroke volume variation during experimental intra-abdominal hypertension

BACKGROUND

Pulse pressure variation (PPV) and stroke volume variation (SVV) are frequently used to assess fluid responsiveness in critically ill patients on mechanical ventilation (MV). There are many factors, in addition to preload that influence the magnitude of these cyclic variations. We sought to investigate the effect of tidal volume (V(T)) on PPV and SVV, and prediction of fluid responsiveness in a model of intra-abdominal hypertension (IAH).

METHODS

Twelve anesthetized and mechanically ventilated piglets on continuous pulse contour cardiac output monitoring. Hypovolemia was ruled out with 2 consecutive fluid boluses after instrumentation. IAH was induced by intraperitoneal instillation of colloid solution with a goal of reducing respiratory system compliance by 50 %. Subjects were classified as fluid responders if stroke volume increased >15 % after each fluid challenge. SVV and PPV were recorded with tidal volumes (VT) of 6, 12 and 18 ml/kg before IAH after IAH induction and after a fluid challenge during IAH.

RESULTS

V(T) influenced PPV and SVV at baseline and during IAH, being significantly larger with higher V(T). These differences were attenuated after fluid administration in both conditions. After IAH induction, there was a significant increase in SVV with the three-tested V(T), but the magnitude of that change was larger with high V(T): with 6 ml/kg from 3 % (3, 4) to 5 % (4, 6.25) (p = 0.05), with 12 ml/kg from 5 % (4, 6) to 11 % (8.75, 17) (p = 0.02) and 18 ml/kg from 5 % (4,7.5) to 15 % (8.75, 19.5) (p = 0.02). Similarly, PPV increased with all the tested VT after IAH induction, being this increase larger with high VT: with 6 ml/kg from 3 % (2, 4.25) to 6 % (4.75, 7) (p = 0.05), with 12 ml/kg from 5 % (4, 6) to 13.5 % (10.25, 15.5) (p = 0.02) and 18 ml/kg from 7 % (5.5, 8.5) to 24 % (13.5, 30.25) (p = 0.02). One third of subjects responded to fluid administration after IAH, but neither SVV nor PPV were able to identify the fluid responders with the tested V(T).

CONCLUSION

IAH induction in non-hypovolemic subjects significantly increased SVV and PPV with the three tested V(T), but the magnitude of that change was higher with larger V(T). This observation reveals the dependence of functional hemodynamic markers on intrathoracic as well intra-abdominal pressures, in addition to volemic status. Also, PPV and SVV were unable to predict fluid responsiveness after IAH induction. Future studies should take into consideration these findings when exploring relationships between dynamic preload indicators and fluid responsiveness during IAH.

Pulse pressure variability during hemorrhage and reinfusion in piglets: effects of age and tidal volume

PURPOSE

The dynamic change in arterial pulse pressure during mechanical ventilation (PPV) predicts fluid responsiveness in adults but may not be applicable to pediatric patients. We compared PPV during hemorrhage and reinfusion in immature vs mature piglets at two clinically relevant tidal volumes (VT).

METHODS

Following Institutional Animal Care Committee approval, we measured hemodynamics and PPV in two groups of piglets, 10-15 kg (immature, n = 9) and 25-30 kg (mature, n = 10), under stable intravenous anesthesia at VT = 8 and 10 mL·kg(-1). Measurements were taken at baseline, with blood withdrawal in 5 mL·kg(-1) steps up to 30 mL·kg(-1), and during stepwise reinfusion. For each age group and VT, we constructed receiver operating characteristic (ROC) curves to determine the threshold value that was predictive of fluid responsiveness.

RESULTS

Pulse pressure variability was significantly lower in immature vs mature pigs and at VT 8 vs VT 10 at every measurement period. The difference in PPV induced by changing VT was less in immature animals. Significant areas under the ROC curve were obtained in immature pigs at both VTs but in mature animals at VT 10 alone. A PPV threshold was calculated to be 8.2% at VT 8 and 10.9% at VT 10 in immature animals vs 15.9% at VT 10 in mature animals, but sensitivity and specificity were only 0.7.

CONCLUSION

Pulse pressure variability values are lower and less sensitive to VT in immature vs mature pigs. Adult PPV thresholds do not apply to pediatric patients, and a single PPV value representing fluid responsiveness should not be assumed.

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.

Predictive value of pulse pressure variation for fluid responsiveness in septic patients using lung-protective ventilation strategies

Background

The applicability of pulse pressure variation (ΔPP) to predict fluid responsiveness using lung-protective ventilation strategies is uncertain in clinical practice. We designed this study to evaluate the accuracy of this parameter in predicting the fluid responsiveness of septic patients ventilated with low tidal volumes (TV) (6 ml kg⁻¹).

Methods

Forty patients after the resuscitation phase of severe sepsis and septic shock who were mechanically ventilated with 6 ml kg⁻¹ were included. The ΔPP was obtained automatically at baseline and after a standardized fluid challenge (7 ml kg⁻¹). Patients whose cardiac output increased by more than 15% were considered fluid responders. The predictive values of ΔPP and static variables [right atrial pressure (RAP) and pulmonary artery occlusion pressure (PAOP)] were evaluated through a receiver operating characteristic (ROC) curve analysis.

Results

Thirty-four patients had characteristics consistent with acute lung injury or acute respiratory distress syndrome and were ventilated with high levels of PEEP [median (inter-quartile range) 10.0 (10.0–13.5)]. Nineteen patients were considered fluid responders. The RAP and PAOP significantly increased, and ΔPP significantly decreased after volume expansion. The ΔPP performance [ROC curve area: 0.91 (0.82–1.0)] was better than that of the RAP [ROC curve area: 0.73 (0.59–0.90)] and pulmonary artery occlusion pressure [ROC curve area: 0.58 (0.40–0.76)]. The ROC curve analysis revealed that the best cut-off for ΔPP was 6.5%, with a sensitivity of 0.89, specificity of 0.90, positive predictive value of 0.89, and negative predictive value of 0.90.

Conclusions

Automatized ΔPP accurately predicted fluid responsiveness in septic patients ventilated with low TV.

Heart-lung interactions with different ventilatory settings during acute lung injury and hypovolaemia: an experimental study

BACKGROUND

The functional haemodynamic variables pulse pressure variation (PPV), stroke volume variation (SVV), and systolic pressure variation (SPV) are widely used to assess haemodynamic status. However, it is not known how these perform during acute lung injury (ALI). This study evaluated the effects of different ventilatory strategies on haemodynamic parameters in pigs with ALI during normovolaemia and hypovolaemia.

METHODS

Eight anaesthetized Agroceres pigs [40 (1.9) kg] were instrumented with pulmonary artery, PiCCO, and arterial catheters and ventilated. Three ventilatory settings were randomly assigned for 10 min each: tidal volume (VT) 15 ml kg(-1) and PEEP 5 cm H(2)O, VT 8 ml kg(-1) and PEEP 13 cm H(2)O, or VT 6 ml kg(-1) and PEEP 13 cm H(2)O. Data were collected at each setting at baseline, after ALI (lung lavage+Tween 1.5%), and ALI with hypovolaemia (haemorrhage to 30% of estimated blood volume).

RESULTS

At baseline, high VT increased PPV, SVV, and SPV (P<0.05 for all). During ALI, high VT significantly increased PPV and SVV [(P = 0.002 and P = 0.008) respectively.]. After ALI with hypovolaemia, ventilation at VT 6 ml kg(-1) and PEEP 13 cm H(2)O decreased the accuracy of functional haemodynamic variables to predict hypovolaemia, with the exception of PPV (area under the curve 0.875). The parameters obtained by PiCCO were less influenced by ventilatory changes.

CONCLUSIONS

VT is the ventilatory parameter which influences functional haemodynamics the most. During ventilation with low VT and high PEEP, most functional variables are less able to accurately predict hypovolaemia secondary to haemorrhage, with the exception of PPV.