Index
All patients admitted to the intensive care unit (ICU) should be monitored, but monitoring may vary.
Indications for hemodynamic control
Hemodynamically stable patients require nothing more than continuous electrocardiographic (ECG) monitoring, regular noninvasivenoninvasive blood pressure measurement, and peripheral pulse oximetry.
Those who are unstable or at risk of instability should receive an arterial line for continuous invasive blood pressure measurement and regular arterial blood gas analysis.
Any patient receiving vasopressors or inotropic agents requires a central venous line for drug administration and, when indicated, measurement of CVP and central venous oxygen saturation (ScvO 2).
Advanced hemodynamic monitoring will be required to guide medical treatment when initial resuscitation does not improve the patient’s hemodynamic and respiratory status.
Measurement of CO and its components (preload, afterload, and contractility) will indicate whether there is a continuous need for resuscitation with fluids, vasopressors, or inotropic agents.
In addition, it can be used as a diagnostic tool to determine the type of shock (hypovolemic, cardiogenic, obstructive, or distributive) according to the hemodynamic profile.
Also, to guide resuscitation, the post-convalescence phase during which we are often faced with fluid overload (itself a significant adverse prognostic predictor).
The clinical context (emergency room, operating room, or ICU) and the different possible variables provided by the monitoring method will determine which way is used.
However, an important observation should be added when the indications for monitoring are discussed.
So far, trials have not shown a significant reduction in mortality when comparing monitoring with the standard of care, although complications have potential benefits.
Hemodynamic monitoring basics
Measuring CO begins with understanding Fick’s principle, described by Adolf Fick in 1870.
This indicates that blood flow to an organ can be calculated using an indicator and measuring the amount of hand taken up by the organ and its respective concentrations in arterial and venous blood.
Hemodynamic control methods
Several invasive and less invasive methods have been developed over the past decades to measure CO. The first to be used was the PAC, introduced in the 1970s by Swan, Ganz, and Forrester.
It is still the gold standard in the clinical setting that we refer to when comparing different hemodynamic monitoring methods.
These can be classified as calibrated or uncalibrated techniques or according to their level of invasiveness (invasive, less invasive, or noninvasivenoninvasive).
There is a trend to use less invasive and noninvasivenoninvasive techniques to reduce the risks accompanying (less) invasive procedures.
Repeat calibration is done to eliminate or reduce bias in continuous measurements. It refers to evaluating and adjusting the accuracy of the equipment.
The precision of a technique is the degree to which repeated measurements (at the same time) show the same results, and the accuracy is the degree of approximation of the results to the actual true value.
Uncalibrated techniques reduce bias by implementing correction factors based on patient demographics (age, weight, sex, etc.) or calculations.
However, calibration will often be necessary for aortic preload; afterload, contractility, and compliance can vary widely (such as in critical illness).
Invasive techniques
Pulmonary artery catheter (calibrated)
The gold standard (PAC) is a flow-directed catheter placed through an introducer into the jugular, subclavian, or femoral vein. It travels from the right atrium through the right ventricle to the pulmonary artery.
It allows direct simultaneous measurement of proper atrial pressures (CVP), PAP, and PAOP or wedge pressure, which in turn is indicative of left atrial filling pressures.
Sampling blood from the distal port (pulmonary artery) allows measurement of SVO 2, and using fiber optic reflectometry allows continuous monitoring of SVO 2.
CO is measured with thermodilution; a bolus of cold saline must be administered through the opening in the right atrium, with a thermistor that detects the drop in temperature a few centimeters from the tip of the catheter.
Later, a heating coil is incorporated into the design, nullifying the need for boluses of cold fluid (and thus avoiding bias due to different operators).
However, this CO measurement is not accurate for continuous monitoring. It represents the average value of the last 5 minutes, and changes in CO during preload or afterload alterations cannot be instantly appreciated.
It also provides calculated variables, such as pulmonary and systemic vascular resistance, left and right ventricular stroke work, and oxygen extraction ratio.
Although PAC was the most widely used technique in the past, a clear survival benefit has not been demonstrated.
The complexity of the possible variations in the pressure plots obtained has led to significant interobserver variability, together with reports of very common misinterpretations of the actions.
The best indication for PAC remains when there is right ventricular heart failure or pulmonary hypertension.
No other monitoring device can provide a direct measurement of pressures in the right heart and pulmonary circulation.
Less invasive techniques
Transpulmonary Thermodilution: The PiCCO System (Calibrated / Surrogate Gold Standard)
The PiCCO system provides intermittent (for calibration) and continuous CO measurements using a central venous catheter and an arterial line with a thermistor.
Intermittent CO is measured using a transpulmonary thermodilution technique, where a bolus of cold fluid is injected through the central line.
Using the Stewart Hamilton equation, the area under the thermodilution curve is used to calculate CO.
Based on the analysis of the arterial pulse contour, it is possible to monitor the volume of CO and the trace, allowing the evaluation of the beat with variations of volume stroke and CO under changing preload conditions.
SVV and pulse pressure variation (PPV) have been proposed as variables to guide fluid loading in critical care settings.
Although limited to patients under controlled mechanical ventilation and in the absence of cardiac arrhythmias, they are wholly sedated.
Limits are low respiratory rate ratio, irregular heartbeat, mechanical ventilation with a low pulmonary ventilation volume, increased abdominal pressure, open chest, and spontaneous breathing.
In addition, the PiCCO allows the measurement of global end-diastolic volume (GEDV), intrathoracic blood volume (ITBV), and extravascular lung water (EVLW). English).
Pulmonary blood volume (PBV), pulmonary vascular permeability index (PVPI), global ejection fraction (GEF), contractility, and systemic vascular resistance (SVR) are derived from these values.
These values can be indexed to body surface area and predicted body weight.
This system has several advantages over PAC; it is less invasive, provides valid continuous CO, and readily available measurements that allow evaluation of fluid responsiveness.
Furthermore, it is supported by human literature data showing a good correlation between intermittent and continuous transpulmonary thermodilution CO with PAC as the gold standard.
Its drawbacks are the need for a specialized arterial line, a central venous line (jugular or subclavian vein), and regular calibration (three to four times a day) with boluses of cold fluids (extra fluid load).
Volume measurement is not automatic or continuous. It is less useful in valvular heart disease, abdominal aortic aneurysms, or enlarged atria, and it is not applicable in arrhythmias or intra-aortic balloon counterpulsation.
Transpulmonary Thermodilution: The VolumeView / EV1000 System (Calibrated)
The VolumeView / EV1000 is similar to the PiCCO but differs in measurement from the GEDV.
In this system, an application formula is used at the maximum upward slope and the downward slope time of the thermodilution curve.
Whereas PiCCO uses time constants derived from the mean appearance, mean transit, and downward slope of the thermodilution curve.
Transpulmonary dye: the LiDCO system (calibrated)
Instead of thermal dilution, LiDCO uses lithium as an intravascular indicator injected through a central vein measured into a peripheral artery using a specialized sensor probe attached to the pressure line.
It is coupled to a pulse contour analysis system (LiDCOrapid / PulseCO). The only additional measured variables compared to PAC monitoring are PPV and SVV.
Data is readily available and provides real-time beat-to-beat variations in CO.
Volume quantification, however, is not available, and the technique cannot be used in children or patients weighing less than 40 kg or under the influence of muscle relaxants.
The lithium sensor detects the positively charged quaternary ammonium ion, which affects your measurements.
Little is known about possible toxic effects or build-up with long-term use of lithium. Furthermore, the ion-selective electrode is delicate and expensive, so it must be replaced every three days.
Ultrasound flow dilution: the COstatus system (calibrated)
The status system calculates CO using transpulmonary ultrasound dilution technology to measure changes in ultrasound speed and blood flow after saline injection.
It requires a primed extracorporeal arteriovenous tube assembly (AV loop) connected between the standard arterial in situ catheters and the central venous catheter.
This is where two ultrasound flow dilution sensors are placed at the arterial and venous ends.
During calibration, a small roller pump circulates blood through the AV circuit from the artery to the vein.
Ultrasound sensors provide an ultrasound dilution curve through which CO can be calculated following the Stewart Hamilton principle.
After calibration, a continuous CO can be calculated through the arterial waveform.
It calculates specific volumetric indices, such as total end-diastolic volume (TEDV), central blood volume (CBV), and active circulating volume (CVA), and can detect intracardiac shunts.
It is validated in adult and pediatric patients. Recalibration is necessary for unstable conditions.
Pulse contour and pulse pressure analysis (not calibrated)
Several devices use the technique of pulse pressure analysis to calculate CO.
The difficulty is that information about heart rate and blood pressure is needed to estimate CO from pulse pressure analysis, but an assessment of pressure must also be made.
Most of the techniques used today are based on a three-element model that integrates characteristic aortic impedance, arterial compliance, and systemic vascular resistance.
These models work relatively well in stable patients but lack precision in unstable patients or when vasoactive drugs are used. Several devices use pulse pressure analysis available.
Cardiac output monitoring system: the Nico system (not calibrated).
The Nico uses a partial rebreathing method to measure CO. The system consists of a CO 2 sensor and air flow combined with a pulse oximeter.
We can measure CO 2 production by multiplying the exhaled CO 2 content by the respiratory minute volume. Arterial CO 2 is derived from final CO 2.
A partial rebreathing cycle should be started every three minutes using a rebreathing process, resulting in reduced CO 2 removal.
Assuming CO is stable under both normal and rebreathing conditions, the difference between average and rebreathing ratios is used to calculate CO.
However, as it depends on stable ventilation, it can be used only in fully sedated patients on volume-controlled ventilation.
Significant lung disease (as in ICU patients with acute respiratory distress syndrome, pneumonia, atelectasis, shunt, etc.) can interfere with measurements.
There are insufficient data to support its accuracy, specifically in critically ill patients.
Transesophageal echocardiography (operator dependent)
Transesophageal echocardiography (TEE) is an essential cardiovascular diagnostic tool in perioperative and intensive care medicine.
It uses ultrasound to provide real-time images of heart structures and blood flow. The transducer is placed next to the heart in the esophagus to produce these images.
It can help define pathophysiologic abnormalities in patients, such as wall motion abnormalities, pericardial effusions, pulmonary hypertension, and valvular disease, along with other invasive monitoring.
However, there is a significant learning curve, TEE is expensive, and continuous monitoring is not an option.
There is a (low) risk of oropharyngeal hemorrhage and endotracheal tube dislocation; its use is relatively contraindicated in esophageal pathologies and severe coagulation abnormalities.
Esophageal Doppler (operator dependent)
Blood flow in the descending aorta is measured using a flexible ultrasound probe to determine stroke volume and CO.
This probe can be left in place for extended periods (barring dislocation) and can provide real-time CO and afterload interpretation data.
Provides many additional measurements and an estimate of preload through corrected flow time.
It is a promising technique, easy to learn, and associated with a reduced hospital stay and better optimization of perioperative volume.
Non-invasive techniques
Transthoracic echocardiography (operator dependent)
CO can be measured with TTE using pulsed-wave Doppler velocity in the left ventricular outflow tract (LVOT).
It can also be measured in the mitral valve annulus, ascending aorta, right ventricular outflow tract (RVOT), and pulmonary artery, but these have been less validated.
Since there is less influence from systemic vascular resistance (SVR), measurements on RVOT can provide accurate CO, but only if there is no interference due to pulmonary arterial hypertension.
NoninvasiveNoninvasive pulse contour systems (uncalibrated)
These systems strive to determine CO based on an arterial pulse pressure curve, which is estimated using a completely noninvasivenoninvasive technique.
Bioimpedance (not calibrated)
Using skin electrodes, a small electrical current is applied. Changes in voltage over the circuit are caused by changes in the impedance and volume of the conductive tissues.
Blood has a relatively low resistivity, and changes in intrathoracic blood volume have a high impact on impedance as a consequence.
With this assumption, we can postulate that changes in thoracic impedance are highly dependent on three components:
- A reference impedance is indirectly proportional to the thoracic fluid content.
- Tidal changes in intrathoracic blood volume are caused by respiration.
- Minor changes are caused by the cardiac cycle.
The latter is mainly due to changes in aortic volume, which can be used to estimate stroke volume and CO 2.
However, it has significant limitations. All changes influence impedance in the composition of the thoracic fluid, such as pulmonary edema and pleural effusions.
Changes in systemic vascular resistance will influence volume changes in the aorta and interfere with CO measurements.
Estimation of continuous cardiac output (not calibrated)
This is a noninvasivenoninvasive device that estimates CO with an algorithm based on patient characteristics and noninvasivenoninvasive heart rate, peripheral oxygen saturation, and blood pressure measurement.
With these measurements, the transit time of the pulse wave is determined and combined with the heart rate to estimate CO. Although it has the advantage of being noninvasivenoninvasive, it is still a mere estimate of CO.
Studies suggest an unacceptable high deviation compared to validated methods.
Ultrasonic Cardiac Output Monitoring or USCOM (not calibrated)
By measuring flow velocity in the pulmonary and aortic outflow tracts, USCOM combines this with previously calculated valve areas to estimate a CO.
It has a short learning curve and few procedural risks.
However, there are many unattainable images; the proposed valve areas may differ significantly from the truth.
This occurs specifically in elderly patients, patients who are critically ill, and patients with structural heart disease, and there can be a significant difference between the estimated result and the calibrated reference value.
Conclusion
Critically ill patients are often hemodynamically unstable (or at risk of becoming unstable).
Advanced hemodynamic monitoring is recommended in complex situations or patients with shock who do not respond to initial fluid resuscitation.
We are offered various techniques ranging from invasive to less invasive and even noninvasivenoninvasive. These techniques can be calibrated or uncalibrated.
Calibrated techniques offer the best precision; values obtained concerning CO, preload, and afterload and other derived values are of great importance in the hemodynamic stabilization of critical patients.
Relying on uncalibrated techniques can be difficult in critically ill patients.
Changing preload, vasomotor tone, and cardiac function conditions can often lead to misleading results, with the risk of inappropriate medical management, insufficient or excessive resuscitation, and subsequent organ dysfunction.
However, they can be valuable in stable conditions, with less invasive or noninvasivenoninvasive techniques that negate the possibility of complications due to more invasive procedures.
Pulse contour analysis with the additional functional variables SVV and PPV may be of significant value in patients with regular sinus rhythm and completely sedated under controlled mechanical ventilation.
In the medical treatment of critically ill patients, we will have to balance the benefits and risks of different techniques to achieve the best possible outcome for our patients.
We recommend using calibrated techniques in critically ill and unstable patients, preferring the less invasive procedures to the more invasive ones.
The PAC, however, can be beneficial in patients with significant cardiac dysfunction, specifically when it comes to right ventricular dysfunction or pulmonary arterial hypertension.
During resuscitation, the monitoring technique should be reassessed (and also when the patient deteriorates again), and noninvasivenoninvasive techniques should be used, whenever possible, rather than (less) invasive techniques.
