Can transiently increasing tidal volume to 8 ml/Kg PBW and measuring PPV improve the reliability of PPV in patients receiving low tidal volume ventilation?

BACKGROUND:

The first line of treatment in patientswith acute circulatory failure is volume expansion. Only half the patients withcirculatory failure positively respond to fluid administration. Excessive fluid loadingis associated with increased complications, mortality and length of intensivecare unit stay. Anaccurate prediction of fluid responsiveness may prevent unnecessary fluidloading and detect patients who benefit from volume expansion. Predicting whichpatients will respond to fluid administration by increasing cardiac output is achallenge in critically ill patients.

Traditionally used static hemodynamicmonitoring indicators, like central venous pressure (CVP), pulmonary capillarywedge pressure (PCWP) etc., are of limited value in predicting fluidresponsiveness in critically ill patients.Various dynamic indices like systolic pressure variation (SPV) stroke volumevariation (SVV) and pulse pressure variation (PPV) based on cardiopulmonaryinteractions in ventilated patients have been shown to accurately predict fluidresponsiveness. Ofthese parameters, PPV is supported to be having the highest level of evidence. However, PPV has certainlimitations. It cannot be used, or is not reliable in certain conditions likepatients with spontaneous breathing activity, cardiac arrhythmias, low tidalvolume, high frequency ventilation, high intra abdominal pressure, open chest.

Low TV ventilation (≤ 6 ml/kgIBW) is commonly used in patients with sepsis, acute lung injury/acuterespiratory distress syndrome (ALI/ARDS). During low tidal volume ventilation,PPV may be unreliable because a low TV might be insufficient to produce asignificant change in the intra thoracic pressure. In these patients PPV mayindicate a non-responsive status even in responders. Another important factor may be that respiratoryrate is often high when ventilating with low tidal volume. PPV has beenobserved to be negligible in fluid responders when the ratio of heart rate torespiratory rate was decreased below 3.6.Muller and colleagues recently confirmed this finding.

Previous clinical andexperimental studies have conflicting results regarding the accuracy of PPVmeasured with a TV below 8 ml/ kg.De Backer et al evaluatedthe influence of tidal volume on ability of PPV to predict fluid responsivenessand showed PPV is reliable only if the tidal volume is >8ml/kg since tidalvolume, but not airway pressure, was the main determinant of right ventricularafterload and right ventricular stroke work, two main components of PPV. Lansdorp et al showed that PPV and other such dynamic indices areonly reliable under strict conditions and low tidal volume can substantiallyreduce their predictive value.They proposed correcting PPV for the appliedtidal volume to improve its predictive value at low tidal volume Muller et al showed that in patients ventilated with low tidalvolume, PPV values <13% does not rule out fluid responsiveness, especiallywhen the airway driving pressure is <or =20 cmH(2)O. Lakhal et al also showed the poorperformance of PPV in early ARDS patients ventilated with low tidal volume inpredicting fluid responsiveness.

Huang et al showed that PPV accurately predicts the response ofcardiac output to volume expansion and remains a reliable predicator of fluidresponsiveness for early ARDS patients ventilated with low tidal volume andhigh PEEP and Freitas et al showedthe accuracy of PPV was adequate in patients with septic when they wereventilated with a TV of 6 ml kg and even devised cut-off value of 6.5% todiscriminate between the responders and non-responders .

 To overcome limitations with PPVwhile ventilating patients with low tidal volume, alternative tests have beenproposed to assess fluid responsiveness such as End Expiratory Occlusion Test(EEOT), Passive leg raising test (PLRT) and mini fluid challenge. Of these, the recentlyproposed EEOT is a relatively simple bedside test that can reliably predictfluid responsiveness. Studies that tested EEOT used tidal volumes between 6.3 -8.8 ml/kg predicted body weight (PBW).However, during protective lung ventilation a tidal volume of ≤ 6 ml/kg IBW isrecommended. Whether EEOT would provide adequate volume to challenge the systemat such low tidal volumes, has not been tested.

We propose using PPV itself, to assessfluid responsiveness, in patients ventilated using lower tidal volume, bymeasuring it after transiently increasing the TV from 6 to 8ml/kg IBW and backto 6ml. We hypothesize that transiently increasing TV to 8 ml/kg and measuringPPV may help unmask fluid responders.Though the PPV may increase in bothresponders and non responders, we hypothesise that the change in PPV (     PPV) may not be the same in both.The    PPV may be indicative of fluidresponsiveness.

 A tidalvolume of 6 ml/kg PBW is recommended in patients with ARDS. Wethink transiently increasing tidal volume for one minute to 8 ml/kg PBW shouldbe safe, as it is similar to giving a recruitment maneuver (RM). RMs are oftenused in ventilated patients, a strategy to increase the transpulmonary pressuretransiently with the goal to reopen those alveolar units that are not aeratedor poorly aerated but reopenable and are recommended in patients with hypoxemia due toARDS.

 A fluid challenge of 7 ml/kg actual body weight of saline willbe given over 10   minutes at the end of the study. Our study willinclude only those patients in whom a clinician has planned to give a fluidchallenge. The international Guidelines for Management ofSevere Sepsis and Septic Shock recommends that a fluid challenge techniqueshould be applied wherein fluid administration is continued as long as there ishemodynamic improvement either based on dynamic or static variables.

Patients will be divided into two groups“Responders†and “Non-Responders†based on an increase in Cardiac index >15%after giving the fluid bolus. This will help us determine whether measuring PPVwhile transiently increasing tidal volume will help us unmask true responders.  This test has the potential to be a simplebedside test, not requiring any change in posture or the use of a continuouscardiac output monitor to assess fluid responsiveness.