In patients with shock, hemodynamic monitoring techniques can be used for identifying the type of shock, selecting the therapeutic intervention, and evaluating the patient's response to therapy [1▪▪]. Several techniques are available and differ in terms of invasiveness, and the number and nature of the provided hemodynamic variables . Monitoring of arterial blood pressure (ABP) using an artery catheter is the simplest hemodynamic monitoring system, used in most cases of shock states [1▪▪]. It not only provides measurements of ABP in real time, but also enables continuous monitoring of pulse pressure variation (PPV), which is a dynamic marker of preload responsiveness . A further step is to measure and monitor cardiac output (CO). Finally, some advanced hemodynamic monitoring systems provide the measurements of other variables, which could be helpful in complex cases. In this article, we review the available hemodynamic monitoring techniques in light of recent clinical studies. We also discuss their relative place in patients with circulatory shock.
THE AVAILABLE HEMODYNAMIC MONITORING TECHNIQUES
Invasive uncalibrated arterial pulse contour analysis monitors
The FloTrac/Vigileo system (Edwards LifeSciences, Irvine, California, USA), ProAQT/PulsioFlex system (Pulsion Maquet, Munich, Germany), and LiDCOrapid monitor (LiDCO, Ltd., London, UK) provide real-time CO measurements by deriving ABP waveform recorded from an artery catheter. They use different proprietary algorithms that analyze the characteristics of the ABP waveform along with patient-specific anthropometric and demographic data. These devices can be used either with a radial or a femoral artery catheter. The main advantage of these systems is their simplicity to provide continuous and real-time CO measurements without any calibration. They can be used to assess the short-term CO response to diagnostic tests such as passive leg raising (PLR)  or to therapies such as fluid infusion. However, their reliability has been seriously questioned in cases of acute changes in vascular tone, in particular, during septic shock with use of vasopressors [5,6▪], during cardiac surgery , and during liver transplantation . Moreover, these systems do not provide other hemodynamic variables, such as cardiac filling pressures or volumes, cardiac systolic function indices, and extravascular lung water. This represents an important disadvantage compared with advanced monitoring systems such as the pulmonary artery catheter (PAC) or the transpulmonary thermodilution systems. Nevertheless, uncalibrated CO monitors can calculate and display in real time the value of PPV and/or stroke volume variation (SVV). Thus, these uncalibrated devices could be suitable for predicting fluid responsiveness (PPV, SVV, CO, response to PLR) and for evaluating the response to fluid administration, in particular, in patients without septic shock or not receiving vasopressors.
The MostCare monitor (Vygon Health, Padua, Italy) uses the pressure-recording analytical method (PRAM) to provide real-time CO monitoring from the analysis of the ABP waveform without external calibration. Its proprietary algorithm does not use the area under the systolic part of the ABP curve, but uses the physics theory of perturbation . This device can be used either with a radial or a femoral artery catheter. Although a good agreement between PRAM CO and thermodilution CO was found in patients without shock , contradictory results were found in severe sepsis patients [11,12].
Noninvasive uncalibrated cardiac output monitors
The ClearSight device
The ClearSight (ex-Nexfin) device (Edwards LifeSciences) provides a real-time CO measurement by deriving the finger ABP waveform, which is recorded noninvasively thanks to an inflatable cuff wrapped around the middle phalange of a finger. This technique uses the volume-clamp principle, a transfer correction, and a pulse contour method based on the systolic pressure area and a physiological three-element Windkessel model. It seems to be valuable to track the CO changes [13–16] in the perioperative context, although poor results even for trending ability were reported in cardiac surgery patients receiving vasopressors [17,18]. The reliability of this device is questionable in patients with shock and/or receiving vasopressors [19–21], even for tracking changes in CO induced by fluid infusion [19,20]. One potential advantage of this device is to provide PPV and SVV noninvasively. However, one study in cardiac surgery patients showed that PPV and SVV obtained from this device could not predict fluid responsiveness very well .
Applanation tonometry cardiac output device
The radial applanation tonometry has been recently proposed to monitor CO in real time and noninvasively. The T-line system (Tensys, San Diego, California, USA) uses the ABP waveform continuously recorded by a sensor located in a bracelet placed around the patient's wrist to derive CO using a proprietary autocalibrating algorithm. A clinical study performed in patients after cardiac surgery (40% receiving norepinephrine) showed promising results in terms of trending ability compared to pulmonary artery thermodilution . Additional confirmation studies are required.
The esophageal Doppler continuously calculates the blood flow in the ascending aorta from the aortic blood velocity (Doppler probe), the aortic diameter, and the heart rate. The aortic diameter is either measured (M-mode echo) or estimated from morphologic data. Finally, from the descending aorta blood flow, the esophageal Doppler devices estimate CO, based on the hypothesis that the blood flow in the descending thoracic aorta represents 70% of the systemic blood flow. This technique is more suitable for the operating room than for the ICU because the probe can move easily into the esophagus when the patient is moving . The esophageal Doppler can assess changes in cardiac preload using the flow time corrected for heart rate and changes in contractility using the mean acceleration and the peak of velocity of the systolic aortic blood flow . Aortic blood flow variations can serve as predictors of fluid responsiveness in mechanically ventilated patients .
Bioreactance (Nicom, Cheetah Medical, Boston, Massachusetts, USA) monitors CO in real time and is totally noninvasive. It is based on the frequency and phase modulation of the output voltage in response to a high-frequency electrical current delivered by skin surface electrodes placed on the patient's chest and neck. The output current is recorded by other electrodes on the skin surface, with a time delay called a phase shift, which depends on the stroke volume. The main limits of this technique are overweight patients, increased intrathoracic volume (pulmonary edema, pleural effusion), and cardiac arrhythmias. Validation studies in critically ill patients showed a poor agreement between CO values provided by bioreactance and by thermodilution [26,27▪].
Lithium dilution monitor
The lithium dilution provides intermittent CO measurements after injection of a small amount of lithium in a central venous vein and detection of changes in lithium concentration in a radial artery using a catheter equipped with a lithium-selective sensor (LiDCOplus monitor, LiDCO Ltd., London, UK). This technique has been validated against pulmonary artery thermodilution . It is also used to calibrate a pulse power algorithm of the arterial waveform that provides a continuous estimate of CO. The agreement between the lithium dilution CO and the pulse power CO remains acceptable for up to 4 h after calibration in ICU patients .
At present, two transpulmonary thermodilution systems are available: the PiCCO/PulsioFlex system (Pulsion Maquet) and the VolumeView/EV1000 system (Edwards LifeSciences). The transpulmonary thermodilution technique consists of injecting a bolus of cold saline in a vein of the superior vena cava territory and to measure the resulting changes in temperature in a femoral artery with thermistor-tipped arterial catheter. The mathematical analysis of the thermodilution curve allows CO calculation. The measurement of CO by transpulmonary thermodilution has been demonstrated to be not only accurate but also precise . The reliability is not altered in patients with renal replacement therapy, even at high blood flows . The analysis of the thermodilution curve also provides other hemodynamic variables. The global end-diastolic volume (GEDV) is a marker of cardiac preload. The cardiac function index (CFI) and the global ejection fraction (GEF) are markers of cardiac systolic function. The extravascular lung water (EVLW) is a quantitative measure of pulmonary edema and the pulmonary vascular permeability index (PVPI) is a marker of the permeability of the alveolo-capillary barrier. The EVLW and PVPI are markers of severity in acute respiratory distress syndrome . Transpulmonary thermodilution also serves to calibrate the femoral artery pulse contour analysis, which provides real-time CO, SVV, and/or PPV. Because of a potential drift with time, frequent recalibration is mandatory [1▪▪], in particular, in patients with septic shock, who are receiving vasopressors . Use of these devices is contraindicated in patients with femoral artery occlusive disease and is inefficient in patients with extracorporeal membrane.
Pulmonary artery catheter
The PAC provides intermittent measurements of CO after injection of a cold bolus of saline in the right atrium through the proximal port of the catheter. This method is considered as the gold standard to measure CO [1▪▪]. A modified PAC equipped with a proximal thermal filament provides semicontinuous CO measurements, which, however, cannot track in real time abrupt changes of CO. The PAC provides some other important variables such as pulmonary artery pressure (PAP), pulmonary artery occlusion pressure (PAOP), mixed venous carbon dioxide pressure (PvCO2), and mixed venous oxygen saturation (SvO2) – a variable which can also be monitored in real time when a fiberoptic probe is included in the catheter.
Table 1 lists the main strengths and drawbacks of the main hemodynamic monitoring systems.
Echocardiography is not a hemodynamic monitoring technique, as it cannot provide continuous hemodynamic measurement. Nevertheless, a recent study has shown that a new single-use miniaturized transesophageal echocardiography probe could be left in place for up to 72 h in critically ill patients with a good tolerance and could be useful for hemodynamic management of mechanically ventilated patients with shock . At present, echocardiography is still considered today as the first step of the cardiovascular exploration in patients with shock, in particular, to initially evaluate the type of shock and to sequentially evaluate the cardiac function [1▪▪]. Its two main advantages are noninvasiveness and ability to assess cardiac function far better than any other method. The left ventricular ejection fraction (LVEF) is one of the most important variables provided by echocardiography. Since it depends on both left ventricular contractility and afterload, LVEF must be interpreted in function of the systolic ABP. This is particularly important during shock when left ventricular afterload can change markedly over a short period [1▪▪]. The stroke volume can be estimated by echocardiography from the product of the velocity–time integral (VTI) of the subaortic flow and the area of the left ventricular outflow tract. The VTI represents the distance travelled by red blood cells during one systole and is measured by drawing the outline of the subaortic flow in pulsed Doppler. The area of the left ventricular outflow tract is calculated from its diameter. It is noteworthy that even a small error in the subaortic diameter measurement may result in a marked error in the CO value. Nevertheless, because the aortic annulus is fibrous, the area of the left ventricular outflow tract does not change over a short time. Thus, the relative changes in CO can be estimated by the relative changes in VTI, whose measure is easier and less subject to errors.
Echocardiography can also assess the left heart diastolic function. Combination of tissue Doppler imaging of the mitral annulus and pulsed Doppler of transmitral flow allows a semiquantitative estimation of left ventricular filling pressures . Echocardiography can also predict fluid responsiveness using analysis of the respiratory variations of the subaortic maximal velocity  or of the inferior vena cava diameter , and using the response of the subaortic VTI to PLR . Finally, echocardiography is useful to evaluate the right ventricular function. In particular, it is the gold standard to detect acute cor pulmonale .
Echocardiography has the disadvantage to be operator-dependent and to require a period of training for the operator before being skilled enough, in particular, to deal with complex cardiac diseases or when the transesophageal approach is used. However, acquiring basic critical care transthoracic echocardiography skills requires only a limited period of training .
How to choose a hemodynamic monitoring device in patients with shock?
Hypovolemia, depressed vascular tone, and cardiac dysfunction are the main cardiovascular disorders potentially involved during shock. These disorders can be either isolated or combined in different ways. Evaluation of cardiac function and preload responsiveness is thus essential to identify the main mechanism of shock, to select the adequate therapy, and to assess its efficacy. The choice of the appropriate hemodynamic monitoring may differ depending on the phase of shock, the complexity of shock, and the response to the initial therapy (Fig. 1).
During the initial phase of shock
In patients with shock, it is most often necessary to insert a central venous catheter for drugs and fluid infusion. This also enables us to measure central venous pressure (CVP), central venous oxygen saturation (ScvO2), and central venous carbon dioxide pressure (PcvCO2). The CVP is a poor predictor of fluid responsiveness and can hardly serve to guide fluid therapy . However, CVP reflects the downstream perfusion pressure of most organs, and measuring it can help in choosing the optimal mean ABP to target. The ScvO2 is essential to measure in shock states since a low value (<70%) can identify the patients for whom elevation of oxygen delivery to the tissues, in general, through an increase in CO, should be prioritized . In cases when ScvO2 is normal or high owing to impairment of oxygen extraction capacities (e.g. during septic shock), the difference between PcvCO2 and arterial carbon dioxide pressure can help in identifying patients in whom elevation of CO can still be beneficial . Insertion of an arterial catheter is recommended when the patient is not responsive to the initial therapy and/or when receiving a vasopressor [1▪▪]. It provides accurate measurements of ABP and, in particular, of diastolic ABP. As a reliable indicator of the arterial tone, diastolic arterial pressure helps clinicians to identify patients who are eligible for vasopressor therapy. The arterial catheter also allows calculation of PPV. In addition to clinical assessment and to basic monitoring (central venous and arterial catheters), echocardiography is proposed as a first-line modality of hemodynamic evaluation in every patient with shock [1▪▪,43▪▪]. Echocardiography helps to quickly identify the type of shock and to select an adapted therapeutic strategy [1▪▪,43▪▪].
In patients responding to the initial therapy
Current guidelines do not recommend advanced hemodynamic monitoring in patients with shock, who quickly respond to the selected initial therapy [1▪▪]. Echocardiography can be used for the sequential evaluation of cardiac function and preload responsiveness.
In complex conditions and/or in patients not responding to the initial therapy
In these cases, it may be difficult to determine with certainty the degree of each cardiovascular disorder and to choose an adequate therapy (fluids, vasopressors, and inotropes) without further hemodynamic assessment. Use of transpulmonary thermodilution monitors or PAC is recommended in such situations, in particular, in cases of associated acute respiratory distress syndrome [1▪▪]. The two advantages of transpulmonary thermodilution systems in these complex cases are: to allow a valuable assessment of preload responsiveness (PPV, SVV, pulse contour CO response to PLR) and to provide values of EVLW and PVPI, which can serve as safety parameters for fluid administration . Thus, the benefit–risk ratio of fluid therapy can be well assessed by these monitoring systems. In cases of increased EVLW and preload responsiveness, a therapeutic conflict may exist and the therapeutic strategy should prioritize the predominant organ failure. For example, when hemodynamic and renal failures are predominant, fluid administration should be prioritized even when EVLW is increased. On the contrary, when respiratory failure predominates, vasoactive drugs infusion should be prioritized, even in case of preload responsiveness. The advantage of using PAC is to measure PAP and PAOP. This latter parameter as a measure of pulmonary venous pressure should ideally reflect both the upstream hydrostatic pulmonary capillary pressure and the downstream left atrial pressure. However, PAOP is not correlated to the amount of pulmonary edema , especially in cases of increased capillary permeability, and as a static marker of preload cannot assess preload responsiveness reliably [3,42].
In patients who are not responding to initial therapy, it is recommended to measure CO to evaluate the response to fluids or inotropes [1▪▪]. In patients with refractory shock associated with right ventricular dysfunction diagnosed with echocardiography, a PAC catheter can be inserted in order to measure PAP in addition to CO [1▪▪].
It must be underlined that minimal or noninvasive CO monitoring systems such as uncalibrated pulse contour analysis devices are not recommended in patients with shock, especially those receiving vasopressors [1▪▪]. However, some of these devices might be used to assess the short-term effects of fluid challenge [6▪]. The place of esophageal Doppler and bioreactance is limited in ICU patients.
The particular case of high-risk surgical patients
Compared to patients with shock, high-risk surgical patients most often are far less sick, have less lung injury, and have less abrupt changes in vascular tone at the time of surgery. Therefore, advanced hemodynamic monitoring systems are not mandatory in most of them, and monitoring CO and PPV (or SVV) should be sufficient. Furthermore, in most cases, rather than to obtain an accurate measurement of CO, it is more important to adequately track any change in CO in the operating room. Thus, less invasive hemodynamic monitoring techniques such as esophageal Doppler, bioreactance, or noninvasive uncalibrated CO monitors should find their place in the operating room in high-risk patients. In cases of high-risk surgery, invasive uncalibrated arterial pulse contour analysis monitors could also be indicated. Whatever the CO monitoring system, it could be successfully integrated in a goal-directed therapy approach during and after surgery to reduce the mortality rate and the number of complications [1▪▪,45,46,47▪▪].
Over the past years, hemodynamic monitoring techniques have continuously evolved toward less invasiveness and real-time measurements of variables. Noninvasive hemodynamic monitoring should be reserved for high-risk surgical patients during and after surgery. Echocardiography is currently the first-line evaluation modality in patients with shock. Advanced hemodynamic monitoring systems such as transpulmonary thermodilution or PAC are recommended in patients who do not respond to the initial therapy and/or with associated acute respiratory distress syndrome.
Financial support and sponsorship
Conflicts of interest
X.M. and J.L.T. are members of Medical Advisory board of Pulsion/Maquet. M.J. has no conflict of interests to declare.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
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