Minimally Invasive and Non-invasive Cardiac Output Monitoring
➧ The concept of determining blood flow/time Cardiac Output (CO) by measuring the dilution of a ‘known substance’ in the blood (Fick’s principle) has been applied by pulmonary artery (PA) catheter using the thermodilution technique remains the ‘Gold standard’ approach of CO monitoring. However, it is not without risk.
I. Esophageal Doppler U/S:
Principle:
➧ It measures blood flow velocity in the descending thoracic aorta by using the change in frequency of the U/S beam as it reflects off a moving object (Doppler shift).
➧ If this measurement is combined with an estimate of the cross-sectional area of the aorta (derived value from pt. age, height & weight using nomograms), it allows hemodynamic variables to be calculated [Stroke Volume (SV), CO, Cardiac Index (CI)].
Advantage:
➧ Provides continuous measurements
Limitations:
➧ The following three conditions must be met to guarantee accuracy:
1-The cross-sectional area must be accurate.
2-The US beam must be directed parallel to the flow of blood
3-The beam direction cannot move to any great degree between measurements.
➧ Variations in the above conditions lead to inaccuracies.
Disadvantages:
➧ The main problem with its use as a continuous CO monitor relates to its precision which indicates the reproducibility of a measurement.
➧ It is operator dependent and it is very easy for the position of the probe to change between measurements which will reduce the precision.
➧ The need for frequent repositioning is not well tolerated by an awake pt. and is therefore need sedation.
II. Echocardiography: [Trans-Thoracic & Trans-Esophageal (TEE)]
Principle:
➧ This technique can be used to calculate SV which can then be multiplied by HR to give the CO.
➧ For the assessment of SV; 2 steps are necessary:
1- Calculation of flow velocity from the area under the Doppler velocity wave. This represents the distance RBCs are projected forward in one cardiac cycle.
2- Determination of area through which the flow is pushed forward (calculated from the diameter assuming a circular shape or determined by direct planimetry). Measurements can be performed at the level of [PA, Mitral Valve (MV), or Aortic Valve (AV)].
Advantage:
➧ Good correlation with thermodilution CO measurements providing that the MV is competent.
Limitations:
➧ It is very difficult to measure the diameter of PA.
➧ Measurement at MV is even more difficult because the shape & size of the valve changes during the cardiac cycle
➧ The AV is the third option for Doppler assessment which can be performed using transgastric or deep transgastric views. In the absence of aortic stenosis, this method is the most accurate for CO measurements.
Disadvantages:
➧ TEE cannot be tolerated by an awake patient as a continuous CO monitor.
➧ Esophageal injury by the probe.
➧ Mediastinitis.
III. Thoracic Electrical Bioimpedance:
Principle:
➧ Changes in thoracic volume cause changes in thoracic resistance (bioimpedance) to “low amplitude, high frequency” currents. If thoracic changes in bioimpedance are measured following ventricular depolarization, SV can be continuously determined.
➧ Increasing fluid in the chest results in less electrical bioimpedance.
➧ This noninvasive technique requires 6-electrodes to inject microcurrents & to sense bioimpedance on both sides of the chest.
➧ Mathematical assumptions and correlations are then made to calculate CO from changes in bioimpedance.
Advantage:
➧ Simple, quick, non-invasive with minimal pt. risk.
Limitations:
➧ The accuracy is questionable in several groups of pt., e.g.; those with AV disease, previous heart surgery, or acute changes in thoracic sympathetic nervous function (e.g., those undergoing spinal a.).
Disadvantages:
➧ Electrode susceptibility to electrical interference.
➧ Electrode placement is an important source of error.
➧ Measurements influenced by intrathoracic fluid shifts and changes in Hct.
IV. Thoracic Bioreactance:
Principle:
➧ Because of the limitations of bioimpedance devices, newer methods of processing the impedance signal have been developed. The most promising technology to reach the marketplace is the NICOM device (Cheetah Medical, Portland, OR), which measures the bioreactance or the phase shift in voltage across the thorax.
➧ The human thorax can be described as an electric circuit with a Resistor (R) and a capacitor (C), which together create the thoracic impedance (Zo).
➧ The values of R and C determine the two components of impedance, which are:
(1) Amplitude (a), the magnitude of the impedance (measured in ohms)
(2) Phase (phi), the direction of the impedance (measured in degrees)
➧ The pulsatile ejection of blood from the heart modifies the value of R and the value of C, leading to instantaneous changes in the amplitude and the phase of Zo. Phase shifts can occur only because of pulsatile flow.
➧ The majority of thoracic pulsatile flow comes from the aorta. Therefore, the NICOM signal is correlated almost totally with the aortic flow.
➧ Furthermore, because the underlying level of thoracic fluid is relatively static, neither the underlying levels of thoracic fluids nor their changes induce any phase shifts and do not contribute to the NICOM signal.
➧ The NICOM monitor contains a highly sensitive phase detector that continuously captures thoracic phase shifts, which together result in the NICOM signal.
➧ NICOM is totally non-invasive. This system consists of a high-frequency (75 kHz) Sine wave generator and 4-dual electrode “stickers” that are used to establish electric contact with the body.
➧ Each sticker has two electrodes, one electrode is used by the high-frequency current generator to inject the high-frequency sine wave into the body, whereas the other electrode is used by the voltage input amplifier.
➧ Two stickers are placed on the right side of the body, and two stickers are placed on the left side of the body. The stickers on a given side of the body are paired, so the currents are passed between the outer electrodes of the pair, and voltages are recorded from between the inner electrodes.
➧ Thus, a non-invasive CO measurement signal is determined separately from each side of the body, and the final noninvasive CO measurement signal is obtained by averaging these two signals.
➧ The system’s signal processing unit determines the relative phase shift (∆ɸ) between the input and output signals. The peak rate of change of ɸ (dɸ/dtmax) is proportional to the peak aortic flow during each beat.
➧ The SV is calculated from the following formula: SV = C × VET × dɸ/dtmax, where C is a constant of proportionality and Ventricular Ejection Time (VET) is determined from the NICOM and electrocardiographic signals.
Advantage:
➧ Totally non-invasive.
➧ Unlike bioimpedance, bioreactance-based CO measurements do not use the static impedance (Zo) and do not depend on the distance between the electrodes for the calculations of SV, both factors that reduce the reliability of the result.
➧ NICOM averages the signal over 1 minute, therefore allowing “accurate” determination of CO in patients with atrial and ventricular arrhythmias.
➧ NICOM assessment of the CO can be performed in ventilated and non-ventilated patients alike.
➧ It is very easy to set up with a high degree of acceptability by nursing staff.
➧ NICOM assessment of the CO can be performed in the emergency room, intensive care unit, and operating room.
Limitations & Disadvantages:
➧ Electrocautery interferes with the NICOM signal. However, as long as the device receives a single for at least 20 sec. within a minute, the CO can be determined. When electrocautery is on for more than 40 sec. in a given minute, the CO for that minute is not displayed.
V. Lithium Dilution CO (LiDCO):
Principle:
➧ Depends on the “indicator dilution technique” which is minimally invasive, requiring only venous (central or peripheral) & arterial lines.
➧ The indicator is isotonic lithium chloride (LiCl) which is injected as a very small bolus (0.3 mmol) via the venous line. LiCl is not normally present in the plasma & not metabolized, and is excreted almost entirely in the urine.
➧ LiCl sensitive sensor, attached to the peripheral arterial line, detects the concentration of LiCl ions in the arterial blood.
➧ The LiCl indicator dilution “wash-out” curve provides an accurate absolute CO value.
Advantage:
➧ Simple and Minimally invasive,
➧ As accurate as, or more accurate than bolus thermodilution.
➧ Safe and does not elicit any hemodynamic changes that are sometimes seen with injections of cold saline.
Limitations & Disadvantages:
➧ The clinical margin of safety: Although the amount of LiCl injected is 100 lower than the lowest clinical doses of ‘lithium-treated patients’, it is recommended to administer not more than 10-20 boluses of lithium.
➧ Side-effects of multiple injections over a short time need to be investigated.
VI. Pulse Pressure Analysis Techniques: (Pulse Contour Analysis Devices)
Principle:
➧ Utilize the arterial pressure tracing curve to estimate the CO and other dynamic parameters; [SV, Systemic Vascular Resistance (SVR), and Blood Pressure (BP)].
➧ It measures the area of the systolic portion of the arterial pressure trace from end-diastole to the end of ventricular ejection, together with an individual calibration factor to account for individual vascular compliance.
➧ Some devices use thermo- or Li-dilution for calibration for subsequent measurement.
➧ Some devices (“FloTrac”; Edwards Life Sciences) do not require calibration with another measure and rely upon statistical analysis and algorithm.
Advantage:
➧ Offers ‘beat-to-beat’ Continuous, non-invasive CO measurement.
➧ Reliable, accurate, precise, and comparable to PA-thermodilution.
➧ Frequent recalibration or even no-calibration (“FloTrac”) is not required.
Limitations & Disadvantages:
➧ Cost.
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