Precision of transpulmonary thermodilution: how many measurements are necessary?

Cardiac condition during cooling and rewarming periods of therapeutic hypothermia after cardiopulmonary resuscitation

BACKGROUND

Hypothermia has been used in cardiac surgery for many years for neuroprotection. Mild hypothermia (MH) [body temperature (BT) kept at 32-35°C] has been shown to reduce both mortality and poor neurological outcome in patients after cardiopulmonary resuscitation (CPR). This study investigated whether patients who were expected to benefit neurologically from therapeutic hypothermia (TH) also had improved cardiac function.

METHODS

The study included 30 patients who developed in-hospital cardiac arrest between September 17, 2012, and September 20, 2013, and had return of spontaneous circulation (ROSC) following successful CPR. Patient BTs were cooled to 33°C using intravascular heat change. Basal BT, systolic artery pressure (SAP), diastolic artery pressure (DAP), mean arterial pressure (MAP), heart rate, central venous pressure, cardiac output (CO), cardiac index (CI), global end-diastolic volume index (GEDI), extravascular lung water index (ELWI), and systemic vascular resistance index (SVRI) were measured at 36°C, 35°C, 34°C and 33°C during cooling. BT was held at 33°C for 24 hours prior to rewarming. Rewarming was conducted 0.25°C/h. During rewarming, measurements were repeated at 33°C, 34°C, 35°C and 36°C. A final measurement was performed once patients spontaneously returned to basal BT. We compared cooling and rewarming cardiac measurements at the same BTs.

RESULTS

SAP values during rewarming (34°C, 35°C and 36°C) were lower than during cooling (P < 0.05). DAP values during rewarming (basal temperature, 34°C, 35°C and 36°C) were lower than during cooling. MAP values during rewarming (34°C, 35°C and 36°C) were lower than during cooling (P < 0.05). CO and CI values were higher during rewarming than during cooling. GEDI and ELWI did not differ during cooling and rewarming. SVRI values during rewarming (34°C, 35°C, 36°C and basal temperature) were lower than during cooling (P < 0.05).

CONCLUSIONS

To our knowledge, this is the first study comparing cardiac function at the same BTs during cooling and rewarming. In patients experiencing ROSC following CPR, TH may improve cardiac function and promote favorable neurological outcomes.

Non-invasive cardiac output monitoring during catheter interventions in patients with cavopulmonary circulations

INTRODUCTION

Functionally univentricular hearts palliated with superior or total cavopulmonary connection result in circulations in series. The absence of a pre-pulmonary pump means that cardiac output is more difficult to adjust and control. Continuous monitoring of cardiac output is crucial during cardiac catheter interventions and can provide new insights into the complex physiology of these lesions.

MATERIALS AND METHODS

The Icon cardiac output monitor was used to study the changes in cardiac output during catheter interventions in 15 patients (median age: 6.1 years, range: 4.8-15.3 years; median weight: 18.5 kg, range: 15-63 kg) with cavopulmonary circulations. A total of 19 interventions were undertaken in these patients and the observed changes in cardiac output were recorded and analysed.

RESULTS

Cardiac output was increased with creation of stent fenestrations after total cavopulmonary connection (median increase of 22.2, range: 6.7%-28.6%) and also with drainage of significant pleural effusions (16.7% increase). Cardiac output was decreased with complete or partial occlusion of fenestrations (median decrease of 10.6, range: 7.1%-13.4%). There was a consistent increase in cardiac output with stenting of obstructive left pulmonary artery lesions (median increase of 7.7, range: 5%-14.3%, p = 0.007).

CONCLUSIONS

Icon provides a novel technique for the continuous, non-invasive monitoring of cardiac output. It provides a further adjunct for monitoring of physiologically complex patients during catheter interventions. These results are consistent with previously reported series involving manipulation of fenestrations. This is the first report identifying an increase in cardiac output with stenting of obstructive pulmonary arterial lesions.

Transpulmonary thermodilution assessments: precise measurements require a precise procedure

When incorporating the values of a hemodynamic parameter into the care of patients, the precision of the measurement method should always be considered. A prospective analysis in the previous issue of Critical Care showed that the precision of transpulmonary thermodilution (TPTD) allows for reliable mean values if a standardised procedure is used. The present finding has a physiological basis, as TPTD requires a more prolonged transit time, which in turn reduces the effects that airway pressure and arrhythmia have on venous return-cardiac output steady states. Moreover, this result suggests that the current accepted threshold value of a 15% increase in cardiac output to identify a positive response to a fluid challenge could be reduced in the future. Indeed, this value is mainly related to the precision of the pulmonary artery catheter.

Precision of the transpulmonary thermodilution measurements

Introduction

We wanted to determine the number of cold bolus injections that are necessary for achieving an acceptable level of precision for measuring cardiac index (CI), indexed global end-diastolic volume (GEDVi) and indexed extravascular lung water (EVLWi) by transpulmonary thermodilution.

Methods

We included 91 hemodynamically stable patients (age 59 (25% to 75% interquartile range: 39 to 79) years, simplified acute physiologic score (SAPS)II 59 (53 to 65), 56% under norepinephrine) who were monitored by a PiCCO2 device. We performed five successive cold saline (15 mL, 6°C) injections and recorded the measurements of CI, GEDVi and EVLWi.

Results

Considering five boluses, the coefficient of variation (CV, calculated as standard deviation divided by the mean of the five measurements) was 7 (5 to 11)%, 7 (5 to 12)% and 7 (6 to 12)% for CI, GEDVi and EVLWi, respectively. If the results of two bolus injections were averaged, the precision (2 × CV/√ number of boluses) was 10 (7 to 15)%, 10 (7 to 17)% and 8 (7 to 14)% for CI, GEDVi and EVLWi, respectively. If the results of three bolus injections were averaged, the precision dropped below 10%, that is, the cut-off that is generally considered as acceptable (8 (6 to 12)%, 8 (6 to 14)% and 8 (7 to 14)% for CI, GEDVi and EVLWi, respectively). If two injections were performed, the least significant change, that is, the minimal change in value that could be trusted to be significant, was 14 (10 to 21)%, 14 (10 to 24)% and 14 (11 to 23)% for CI, GEDVi and EVLWi, respectively. If three injections were performed, the least significant change was 12 (8 to 17)%, 12 (8 to 19)% and 12 (9 to 19)% for CI, GEDVi and EVLWi, respectively, that is, below the 15% cut-off that is usually considered as clinically relevant.

Conclusions

These results support the injection of at least three cold boluses for obtaining an acceptable precision when transpulmonary thermodilution is used for measuring CI, GEDVi and EVLWi.

Short-term effectiveness of different volume replacement therapies in postoperative hypovolaemic patients

BACKGROUND AND OBJECTIVE

To examine the kinetics of volume loading with crystalloid and colloid infusions in critically ill patients after major surgery, using the pulse contour cardiac output (PiCCO) monitoring technique.

METHODS

This prospective, randomized, multicentre study of 11 ICUs involved 200 mixed postoperative hypovolaemic patients (50 patients per group) in Hungary. Patients received 10 ml kg of lactated Ringer's solution, succinylated gelatin 4% w/v, 130/0.4 hydroxyethyl starch 6% w/v (HES) or human albumin 5% w/v over 30 min. A complete haemodynamic profile was obtained at 30, 45, 60, 90 and 120 min after baseline. The peak haemodynamic effects, the 120 min changes compared with baseline, the area under the curve (AUC) for the haemodynamic parameters over 120 min and the haemodilution effect of the solutions were analysed. The primary outcome was to compare the AUCs and the secondary outcome was to evaluate the haemodynamic changes at 120 min.

RESULTS

There were significant differences in the AUCs of the haemodynamic parameters between colloids and lactated Ringer's solution in the cardiac index and global end-diastolic volume index (GEDVI); human albumin vs. lactated Ringer's solution in stroke volume variation (SVV); and succinylated gelatin, HES vs. lactated Ringer's solution in the oxygen delivery index (DO2I). Colloid infusions (mainly HES and human albumin) at 120 min caused significant changes in central venous pressure, cardiac index, GEDVI, SVV, DO2I and central venous oxygen saturation compared with baseline. The haemodilution effect was significantly greater in colloids vs. lactated Ringer's solution.

CONCLUSION

In postoperative hypovolaemic patients, lactated Ringer's solution can significantly improve haemodynamics at the end of volume loading, but this effect completely disappears at 120 min. Ten millilitres per kilogram of colloid bolus (especially HES) improved the haemodynamics at 120 min; however, this was by only 5-25% compared with baseline. The colloids caused significantly larger AUCs than lactated Ringer's solution, but only in the cardiac index, GEDVI and DO2I, plus human albumin in the SVV.

Cardiac output derived from arterial pressure waveform

PURPOSE OF REVIEW

Cardiac output (CO) and other flow-based hemodynamic variables have become increasingly important to guide treatment of patients undergoing major surgery with expected fluid shifts in the operating room as well as critically ill ICU patients. Established techniques such as pulmonary artery thermodilution, however, might not be justified in all of these patients. As arterial access is commonly available, less-invasive arterial pressure waveform-based CO devices are becoming more and more popular.

RECENT FINDINGS

Many studies dealing with arterial pressure waveform-based CO have emerged in recent years providing additional information with regard to accuracy of the different commercially available devices. Furthermore, methods of comparative CO studies have been recently brought into question.

SUMMARY

Although there are differences in invasiveness and the need for external calibration, all available devices provide parameters for enhanced hemodynamic monitoring. Initial validation studies of the more established techniques such as the pulse contour cardiac output (PiCCO) or LiDCO were recently met with less enthusiasm, whereas the initially disappointing validation studies of the FloTrac/Vigileo device had encouraging results after software updates. The pressure recording analytical method (PRAM) technique has not so far been sufficiently evaluated to be able to come to a conclusion. Further investigation is required with regard to the ability of the arterial pressure waveform-based methods to guide goal-directed therapy.