Cardiac Output, Low Clinical Trial
Official title:
The Performance of a Capnodynamic Method Compared to Transpulmonary Thermodilution in Open Abdominal Surgery
The capnodynamic method non-invasively calculates effective pulmonary blood flow (EPBF) continuously during surgery. In this study EPBF is compared to cardiac output (CO) measured with Transpulmonary Thermodilution (TPTD) att baseline and during hemodynamic changes in patients scheduled for open abdominal surgery at the Karolinska University Hospital, Solna, Sweden.
The capnodynamic method continuously calculates effective pulmonary blood flow (EPBF) with
the help of a capnodynamic equation:
ELV x (FACO2(n) - FACO2(n-1)) = deltat(n) x EPBF (CvCO2 - CvCO2(n)) - VTCO2
ELV Effective lung volume [L] EPBF Effective pulmonary blood flow [L/min] n current breath
n-1 previous breath FACO2 mean alveolar carbon dioxide fraction CvCO2 mixed venous carbon
dioxide content [Lgas/Lblood] CcCO2n pulmonary end-capillary carbon dioxide content
[Lgas/Lblood] VTCO2n volume [L] of carbon dioxide eliminated by the current, nth, breath
delta t n current breath cycle time [min]
The equation above describes the mole balance between the CO2 delivered to lungs (EPBF), the
volume taking part in the gas exchange (ELV) and CO2 excreted from the lungs (VTCO2).
Normally there is no difference in CO2 between the actual and the preceding breath as the
same amount of CO2 as delivered to the lungs as is excreted. When small changes in CO2
concentration are inserted into the equation obtained with short inspiratory or expiratory
pauses in three out of nine breaths, nine different equations are obtained. The three unknown
variables; ELV, EPBF and CvCO2can be solved with a linear least square optimization, a
well-known numerical mathematical principle. The breathing pattern is automatically
controlled by the ventilator which provides continuous calculations of EPBF where each value
represents the average of the preceding nine breaths and renews with each breath as the
newest replaces the oldest in the equation system.
At the day of surgery, included patients arrive 45 minutes earlier than otherwise at the
surgical unit. After safe surgical checklist, vital signs are measured and epidural catheter
inserted during a light sedation. Patients are anesthetized and muscle relaxed. An
endotracheal tube is inserted in the trachea. Central arterial and vein catheters are
inserted in sterile conditions with ultrasound guidance. Anesthesia is maintained with
propofol in target controlled infusion and a short acting opioid is added as needed.
The protocol in the first 25 patients comprises simultaneous measurements of EPBF and CO at
1) Baseline (three measurments), 2) PEEP 15cmH2O, 3) PEEP 5cmH2O 4) before epidural
activation, 5) 10-15 minutes after epidural activation 6) before volume infusion (when the
patient is considered hypovolemic by the attending anaesthesiologist) and 7) shortly after
volume infusion.
The attending anesthesiologist has the final responsibility on deciding the order of the
protocol, for example if volume infusion should precede PEEP 15 cmH20 if hypovolemia is
suspected.
The succeeding 10 patients (patients 26 to 35) are PEEP optimized before start of protocol
using a standardiazed open lung tool recruitment. During the PEEP elevation (step 2 in the
protocol above) 10cmH2O is added to the optimal PEEP and then released back to the baseline
optimal PEEP (step 3).
Each comparative measurement of CO and EPBF in the protocol includes as an average of three
TPTD measured with PiCCO2 and and the average of EPBF registered in the beginning of the
first TPTD and in the end of the last TPTD.
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