Fluid Responsiveness After CArdiac Surgery Study

This prospective observational study program has a set of objectives within three major domains that will be analysed separately and synergistically:

Peripheral near infra-red spectroscopy (NIRS)

Aims:

1. to describe the effects of non-sanguineous intravascular volume expansion on systemic oxygen delivery
2. to describe how changes in systemic oxygen delivery impact on peripheral oxygen kinetics as measured by tissue oxygen saturation (StO2) in the thenar and forearm regions at rest and using a vascular occlusion test
3. to determine the clinical applicability of using thenar and forearm NIRS to guide haemodynamic optimisation following cardiac surgery
4. to assess the relation between systemic and peripheral oxygen kinetics vs. onset of organ dysfunction and need for extended stay in ICU following cardiac surgery

Hypotheses:

1. Peripheral oxygen kinetics using NIRS offer more specific guidance for haemodynamic optimisation post cardiac surgery compared to using systemic variables only
2. Impaired peripheral oxygen kinetics as assessed using NIRS can be used to identify patients at risk for developing organ dysfunction or needing extended period of ICU support an earlier stage compared to traditional systemic variables

Main measurements:

1. peripheral near infrared spectroscopy (NIRS) measuring tissue oxygen saturation (StO2) at the thenar eminence and forearm during a 3 minute vascular occlusion test at 50mmHg above systolic pressure, following the establishment of a baseline (an StO2 average over 3 minutes with less than 5% variability)
2. StO2 measurements are repeated at 30 minutes intervals during the first 6 hours following admission to ICU.
3. vascular occlusion tests are repeated before and after volume expansion.

Cardiac output and common carotid Doppler sonography

Aims:

1. to apply computerised algorithms to derive advanced haemodynamic information (mean systemic filling pressure, heart efficiency, vascular compliance) in patients requiring fluid boluses following cardiac surgery
2. to correlate changes in cardiac output, cardiac power and estimates of fluid responsiveness using the pulmonary artery catheter with the variables mean systemic filling pressure and heart efficiency
3. to characterise indices of fluid responsiveness by measuring volume efficiency
4. to determine the impact of changes in vascular resistance state on the assessment of changes in volume state in the post-operative cardiac surgery period
5. to assess the relation between fluid responsiveness, fluid management and clinical outcomes in ICU following cardiac surgery
6. to evaluate the potential for carotid Doppler blood flow measurements as a surrogate parameter for cardiac output, including incorporation into computerised algorithms

Hypotheses:

1. the derived measures of mean systemic filling pressure and heart efficiency are superior to identify fluid responsiveness compared to cardiac output measurements alone in patients following cardiac surgery
2. carotid Doppler blood flow measurements can be used as a surrogate parameter for cardiac output and in particular for changes in cardiac output when fluid responsiveness is investigated

Main measurements:

1. Pulmonary arterial thermodilution studies (for cardiac output, stroke volume, cardiac index) by at least triplicate injections within a ± 10% range
2. Computerised algorithms are used to derive an analogue mean systemic filling pressure, heart efficiency, volume efficiency based on cardiac output, central venous pressure and mean arterial pressure
3. Common carotid Doppler sonography to measure vessel diameter, cross sectional area and time average mean velocity to generate blood flow as well as respirophasic peak flow velocity variation

Oxygen and carbon dioxide gradients

Aims:

1. to assess any insufficiency of oxygen delivery to match the oxygen consumption, resulting in anaerobic metabolism, by determining i) arteriovenous concentration gradients for lactate, ii) the venoarterial carbon dioxide tension gradient [P(v-a)CO2] and the arteriovenous oxygen content gradient ([C(a-v)O2]
2. to assess whether central venous blood gas analyses can substitute pulmonary arterial (mixed venous) blood gas analyses in the [P(v-a)CO2] and [C(a-v)O2] gradients and ratio
3. to assess whether arteriovenous O2 and CO2 gradients are more specific and contemporaneous measures of anaerobic metabolism compared to serum lactate

Hypotheses:

1. Changes in [C(a-v)O2] and [P(v-a)CO2] gradients can be used as prognostic markers with persistently abnormal values correlating with the presence and degree of organ failure
2. Changes in [C(a-v)O2] and [P(v-a)CO2] gradients can be used as targets during haemodynamic optimisation
3. Cumulative measurements of [C(a-v)O2] and [P(v-a)CO2] gradients to generate an area under the curve value can be used to assess and target the presence of an oxygen debt

Main measurements

1. Central venous, pulmonary arterial (mixed venous) and arterial blood gas analyses (pO2, pCO2, bicarbonate and lactate concentrations)

A fluid bolus will be administered as 250ml or 500ml of fluid (crystalloid or colloid) at 1500ml/hour. Measurements will be conducted immediately before fluid administration and 10 minutes post fluid cessation.

The following risk scores will be calculated: Sequential Organ Failure Assessment (SOFA), Acute Physiology and Chronic Health Evaluation (APACHE II and III), Simplified Acute Physiology Score (SAPS2), Australian New Zealand Risk of Death (ANZROD), Euroscore 2.

Routine biochemical tests will be recorded including electrolytes, renal function tests, liver function tests.