10.2 Basic Hemodynamic Monitoring
Blood pressure is a variable influenced by both cardiac output (CO) and vascular tone; hence, blood pressure can remain within the normal range in the presence of low-flow states, including hypovolemia, as a result of increased peripheral vascular resistance. Similarly, heart rate may fail to reflect the development of hypovolemia under anesthesia [13].
Combining and integrating parameters from various hemodynamic monitoring systems may help improve our understanding of hemodynamic status [14].
Continuous arterial pressure invasive measurement helps identify the rapid fluctuations in arterial pressure that may occur in high-risk patients. Artifacts (over- or under-damping) should be carefully identified and eliminated, especially when systolic-diastolic components and waveform have to be analyzed. Noninvasive techniques for continuous measurement of blood pressure are usually performed in peripheral arteries and may become unreliable in case of vasoconstriction or low peripheral flow.
Changes in central venous pressure (CVP) with concomitant CO variations can give an indication of RV function and potential peripheral venous congestion, an important factor for organ perfusion [15]. In addition, careful checking of the CVP wave may help to diagnose tricuspid regurgitation with a “v” wave during systole. When the CVP is low with a concomitant low CO, there is some degree of hypovolemia, although changes in CVP correlate poorly with changes in CO [16].
10.3 Cardiac Output Monitoring
The perioperative period is characterized by large variations in whole body oxygen consumption (VO2). The main goal in this period is to maintain an adequate DO2 to meet the fluctuating tissue oxygen requirements. Global DO2 is determined by CO and the oxygen content of arterial blood (CaO2) so, after correction of hypoxemia and anemia (topics that will not be treated here), maintenance of an adequate CO is the next logical step to improve DO2. There are various methods available for monitoring CO: calibrated and not calibrated [17–21].
10.3.1 Pulmonary Artery Catheter: The “Classical” One
Although criticized during the recent years for its intrinsic invasiveness and no clear evidence of improved outcomes [22–25], the pulmonary artery catheter (PAC) is the only tool that provides continuous monitoring of pulmonary artery (PA) pressure, right-sided and left-sided filling pressures, and CO and mixed venous oxygen saturation (SvO2). While the PAC can now be replaced by less invasive hemodynamic monitoring techniques in some cases, in some complex clinical situations, for example, cardiac surgery, organ transplant surgery, and surgery associated with major fluid shifts, high risk of respiratory failure, or in patients with compromised right ventricle (RV) function, the PAC still represents a valuable tool when used by physicians adequately trained to correctly interpret and apply the data provided [26, 27]. In such patients, the PAC can be inserted for limited periods of time and removed when no longer necessary.
10.3.2 Other Cardiac Output Monitoring Devices: The “New” Ones
10.3.2.1 Pulse Contour Analysis
Stroke volume (SV) can be estimated continuously by analysis of the arterial pressure waveform, usually derived from an indwelling arterial catheter or by a noninvasive finger pressure cuff. To calculate SV from a pressure trace, the algorithms used by these devices have to compensate for the overall impedance of the system, based on the estimation of compliance and resistance of the cardiovascular tree. In this regard, optimization of the input signal is imperative, and severe distortions of the arterial waveform (e.g., severe arrhythmias, multiple ectopic beats) and inadequate response of fluid-filled transducer systems (i.e., over- and under-damping) [28] can result in unreliable CO measurement.
Calibrated Devices
The PiCCOplusTM/PiCCO2 TM system (Pulsion Medical Systems, Munich, Germany) consists of a thermistor-tipped catheter which is usually placed in the femoral artery, although catheters for radial, axillary, or brachial applications are also available. The PiCCOTM device measures CO by transpulmonary thermodilution, which additionally provides the computation of volumetric preload parameters (global end-diastolic volume [GEDV], intrathoracic blood volume [ITBV]), and extravascular lung water (EVLW). The CO measured by the Stewart-Hamilton principle from the thermodilution curve is used to calibrate a pulse contour algorithm, which measures the area under the systolic pulse pressure curve and calculates the SV in order to provide beat-by-beat CO measurement. The system has to be frequently recalibrated, at least every eight hours in hemodynamically stable patients and more often if changes in vasoactive support are provided [29]. The system has been validated in a variety of clinical settings [30].
The EV1000TM/VolumeViewTM system (Edwards Lifesciences, Irvine, California) has been more recently introduced, but is analogous to the PiCCOTM monitor, using pulse wave analysis to calculate CO. A proprietary thermistor-tipped femoral artery catheter and a separate sensor are the main components of the system. This system requires calibration by transpulmonary thermodilution. It has been validated off-line against the PiCCOTM and transpulmonary thermodilution in critically ill patients [31].
The LiDCOTMplus system (LiDCO Ltd, Cambridge, UK) uses pulse power analysis to calculate SV and is therefore not technically a pulse contour device. The algorithm is based on the principle of conservation of mass (power), assuming a linear relationship between the net power change and the net flow in the vascular system. This system requires correction for vascular compliance, with calibration using a transpulmonary lithium indicator dilution technique performed via an indwelling arterial catheter. It has been validated in critically ill patients [32, 33].
Uncalibrated Devices (with No External Calibration)
The PulsioFlexTM system (Pulsion Medical Systems) displays trends of estimated CO by using the patient’s anthropometric and demographic characteristics (necessary for internal calibration), analysis of the arterial pressure tracing, and a proprietary algorithm for data analysis. The system requires a dedicated additional sensor, which can be connected to a regular arterial pressure catheter. Based on the same pulse contour algorithm used by the PiCCOTM, the device can be calibrated by entering a CO obtained from an external source (e.g., Doppler echocardiography) or by the system’s own internal algorithm.
The LiDCOTM rapid (LiDCO Ltd) device uses the same algorithm as the LiDCOTMplus system, but instead of lithium dilution, nomograms based on the patient’s age, weight, and height are used to estimate SV and CO (the so-called “nominal” SV and CO). An externally estimated CO can be used to calibrate the device.
The FloTracTM/VigileoTM system (Edwards Lifesciences) consists of a proprietary transducer (FloTracTM) connected to a standard (radial or femoral) arterial catheter. Individual demographic variables (age, sex, height, and weight) and a database containing CO variables derived using the PAC are used to calculate impedance and a “normal” SV against which the standard deviation of the pulse pressure sampled during a 20-s interval is correlated to estimate CO. Arterial waveform analysis is used to calculate vascular resistance and compliance. The algorithm used by the VigileoTM device has been modified over time, and recent studies evaluating the device in the perioperative setting have shown an improved performance and a significant reduction in the time needed to adapt to vascular dynamics. In the ICU setting, concerns remain regarding the accuracy in situations of acute hemodynamic instability as well as hyperdynamic conditions, although recent software modifications seem to improve the reliability of CO measurements. The FloTracTM/VigileoTM system has been shown to be suitable for integration into perioperative optimization protocols, resulting in improved clinical outcomes [34, 35].
10.3.2.2 Pitfalls in the Interpretation of Cardiac Output
Although CO can be measured with reasonable accuracy and precision with some of these systems, it is difficult to assess the optimal CO for an individual patient. A “normal” or even high CO does not preclude the presence of inadequate regional and microcirculatory flow, and a low CO may be adequate in a context of low metabolic demand, especially during surgery under general anesthesia. Moreover, simple identification of a low CO does not tell us what to do about it. Data acquired can be correctly interpreted by any of the described devices, but we need to combine/integrate several variables to help decide whether the CO/SV is adequate and how it can be optimized in the most effective manner [36–39].
10.3.2.3 How to Select the Best System?
All monitoring systems have unique characteristics in terms of accuracy, precision, validity, stability, and reliability [18]. Not all monitoring devices have been evaluated against the same set of criteria, and uncertainty remains regarding acceptance thresholds for the performance of CO monitors and the used reference techniques [54–57]. Clinicians must consider the technical limitations of each monitoring system and the potential trade-off between more invasive but highly accurate measurements of CO and less invasive but also less accurate modalities.
Many questions can be raised when considering the choice of CO monitoring in the perioperative period [40]:
- 1.
Are we ready to accept a less accurate measurement in order to limit invasiveness? (Fig. 10.1). A less accurate measurement may be acceptable if the trend analysis is reliable. Cost may also be an important issue.
Fig. 10.1
Perioperative HD monitoring
- 2.
Do we need continuous, semicontinuous, or intermittent measurements? Most complications after surgery do not have a sudden onset (except sudden cardiac failure due, e.g., to myocardial infarction or pulmonary embolism) or an obvious cause (e.g., massive bleeding during surgery), but develop slowly; therefore, semicontinuous or intermittent measurements may be acceptable. However, it should be noted that only beat-by-beat measurement of SV allows assessment of the response to preload-modifying maneuvers, such as a fluid challenge or passive leg raising (PLR) test.
- 3.
Are calibrated or uncalibrated systems preferable? Non-calibrated systems are acceptable for the operating room (OR) or the post-anesthesia care unit (PACU) but may not be suitable for more complex cases, especially in the ICU. In unstable patients, there is a necessity to “recalibrate” more often because of frequent changes in vascular tone and also because derived variables (e.g., EVLW, GEDV) need to be recalculated. A practical option may be to use an uncalibrated system in the OR/PACU and replace it with a calibrated system in the ICU.
- 4.
What alarms do we need? A major problem for patient surveillance by telemetric monitoring is artifact robustness. Any system with too many false alarms is prone to failure as personnel become desensitized.
- 5.
What kind of monitoring for what kind of patient? This decision is not a “one size fits all”; rather, the optimal monitoring technique for each patient will vary depending on the degree of risk and the extent of the surgical procedure (Fig. 10.1).
10.4 Echocardiography
Although difficult to use as a continuous monitor of CO with conventional probes, transthoracic (TTE) or transesophageal (TEE) echocardiography can provide immediate point-of-care assessment of acute hemodynamic changes in selected patients. Echo techniques can also help to visualize the lungs, but this is beyond the scope of this review. Obviously, it is not possible to use TEE in all types of surgery. In addition to the estimation of CO (usually easier with TEE than with TTE), Doppler echocardiographic examination can provide an indication of cardiac function, because it allows visualization of the cardiac chambers, valves, and pericardium [20]. It also allows measurement of the ejected stroke volume (SV) and derived left ventricular (LV) function parameters.
TEE provides several views, including:
The LV short-axis view, which can be used to evaluate LV function. Calculation of the LV fractional area contraction, or the simpler “eyeballing method,” informs about the kinetic (contractile) state and the shape (volume) of the heart. Poor contractility may indicate that inotropic support could help, and “kissing” of the papillary muscle may indicate the need for fluids if the right heart is functioning normally. The short-axis view may also be used to identify septal dyskinesia. The finding of a right ventricle D-shape may suggest the presence of RV dysfunction/failure, indicating a non-adaptation to an acute increase in RV afterload (pulmonary embolism) or RV myocardial ischemia.
The four-chamber view, which can help in assessing LV and RV function by evaluation of the right-to-left size ratio (normal < 0.6).
In more advanced echocardiographic evaluation, fluid status and fluid responsiveness can also be assessed in mechanically ventilated patients by means of the superior vena cava collapsibility index (TEE bicaval view) or inferior vena cava distensibility index (TTE subcostal view). In addition, echocardiography allows the rapid and reliable estimation of SV. Finally, there are particular and specific conditions in which diagnosis and treatment are strictly related to the echocardiographic examination (e.g., pericardial effusion, valve disruptions, aortic dissection, and systolic anterior motion of the mitral valve).
A miniaturized, disposable monoplane TEE probe that can be left in place for up to 72 h (ClariTEETM, ImaCor Inc., Garden City, NY) has recently been introduced and has the potential to provide ongoing qualitative cardiac assessment.
We believe that where expert echocardiography skills are not available, then training programs should be developed to ensure that clinicians taking care of the high-risk patient are familiar with at least the basic applications of TTE and TEE.
Echocardiography has become an indispensable tool in the evaluation of medical and surgical patients. As ultrasound (US) machines have become more widely available and significantly more compact, there has been an exponential growth in the use of transthoracic echocardiography (TTE), transesophageal echocardiography (TEE), and other devices in the perioperative setting. Here, we review recent findings relevant to the use of perioperative US, with a special focus on the hemodynamic management of the surgical patient.
In an attempt to make hemodynamic monitoring less invasive and to acquire additional relevant information not obtained with other monitoring approaches, ultrasound (US) devices are increasingly being used in perioperative medicine [1]. The field is rapidly evolving as technology advances. Here, we describe the basic principles of ultrasonography and how it can be used for hemodynamic monitoring in the perioperative setting.
TTE and TEE allow the differentiation between noncardiac and cardiac causes of hemodynamic instability. Valvular pathologies and abnormalities in ventricular function can be assessed. During noncardiac surgery, the American Heart Association (AHA) and the American College of Cardiology (ACC) recommend the use of echocardiography in the “evaluation of acute, persistent and life-threatening haemodynamic disturbances in which ventricular function and its determinants are uncertain and have not responded to treatment” [41].
10.4.1 Ventricular Function
Global, systolic LV function can be visually estimated. According to current SCA recommendations, this basic qualitative assessment is not precise, but sufficient for the identification of patients who might benefit from inotropic therapy [12]. The SCA recommends using the transgastric (TG) mid-papillary short-axis (SAX) view, as well as the mid-esophageal (ME) four-chamber, the ME two-chamber, and the MOE long-axis (LAX) views for the monitoring of LV function.
10.4.2 Intravascular Volume Status
Hypovolemia is a common cause of cardiocirculatory instability in the operating theater and the intensive care unit. A central concept in the care of critically ill patients and patients undergoing surgery is to predict fluid responsiveness: Will a patient’s hemodynamic situation improve (i.e., increase in SV and CI) with fluid administration or not? If certain preconditions are met (closed chest, controlled ventilation with sufficiently high tidal volumes, regular heart rhythm, and normal intra-abdominal pressure), systolic pressure variation (SPV), arterial pulse pressure variation (PPV), and stroke volume variation (SVV) represent “dynamic” parameters that more reliably predict fluid responsiveness. CVP and LV end-diastolic area (EDA) do not predict fluid responsiveness, as they are static parameters that are dependent not only on volume status. Other variables impacting CVP and LV-EDA include cardiac compliance (i.e., diastolic ventricular function) as well as intrathoracic pressure. Consequently, LV-EDAI (LV-EDA indexed to the body surface area) does not correlate with fluid responsiveness. Other studies confirmed the inferiority of LV-EDA in predicting fluid responsiveness in comparison to dynamic parameters. Different systematic reviews also concluded that LV-EDA is inferior compared to dynamic parameters such as PPV.
10.4.3 Valvular Function
For a basic assessment of valvular regurgitation, visual inspection of the regurgitant jet area, vena contracta width, as well as flow reversal in receiving or originating cardiovascular chambers can be used among other criteria. Stenotic lesions can be grossly evaluated by continuous-wave Doppler using an imaging plane parallel to blood flow (see the Doppler section above). An orienting assessment of valvular function should be part of every basic echocardiographic examination.
10.4.4 Pulmonary Embolism, Pericardial Effusion, and Thoracic Trauma
Hemodynamically relevant pulmonary embolism (PE) is one reason of cardiocirculatory compromise. In the intraoperative or emergency setting, TOE might be the only feasible yet reliable tool to detect the presence of hemodynamically relevant emboli. Signs of RV failure and motion abnormalities of the RV free wall permit the diagnosis of PE in patients with hypotension or shock.
The modern approach to the hemodynamic evaluation including the TTE/TEE evaluation is a part of the anesthetist skill [42].
10.5 Fluid Management and Functional Hemodynamic Monitoring
Inadequate fluid management may lead to reduced CO and DO2 to injured tissues, which is associated with an increased incidence of postoperative complications [43]. Moreover, the systemic inflammatory response associated with tissue injury results in capillary leak and tissue edema (Fig. 10.2). Fluid restriction and diuresis may decrease edema in patients with poor ventricular function but may also increase the incidence of acute kidney injury. Meanwhile, excessive fluid administration may lead to a range of adverse effects including coagulopathies and edema of the lungs, gut, and peripheral tissues (Fig. 10.2). Retention of sodium and water following surgery may reduce requirements for fluids. Once the patient is stabilized, additional amounts of fluids should only be given to correct deficit or continuing losses. Unfortunately, estimates of fluid deficit based on traditional physiological parameters, such as heart rate, blood pressure, and cardiac filling pressures, are not sufficient.
Fig. 10.2
Periop fluid and HD management. HSR high risk surgery, Pts patients, HD hemodynamic, Hb hemoglobin
10.5.1 Static Indicators of Preload
CVP: Many high-risk surgical patients have a CVC in place and a CVC is a requirement for some devices needing calibration by thermodilution. Despite its limitations (vide supra), changes in CVP over time may be helpful to guide fluid therapy, especially when it is low and associated with low flow. A CVP >8 mmHg might also be considered as an “alarm” for potential venous congestion associated or not with fluid overload [15].
GEDV/ITBV and EVLW: These are volumetric parameters derived from transpulmonary thermodilution and are integrated into the PiCCOTM plus, PiCCO2 TM, and EV1000TM monitors. EVLW can help in the identification of (cardiogenic or non-cardiogenic) pulmonary edema and has the potential to increase the safety of fluid therapy in patients with structural lung disease, ARDS, or congestive heart failure.
The end-diastolic area of the left ventricle may be the most reliable static parameter of preload, but is largely dependent on LV diastolic compliance. Its ability to accurately predict fluid responsiveness is limited.
10.5.2 Functional Hemodynamic Parameters
Positive pressure ventilation induces cyclical changes in intrathoracic pressure, which affect preload by decreasing venous return to the right heart and increasing venous return to the left ventricle. The degree of the resulting changes in LV SV (SVV) and pulse pressure (PPV) better predict fluid responsiveness than do static parameters, when RV function is not a limitation and for a fixed tidal volume. Most devices using pulse contour analysis, including the current version of the noninvasive ClearSight monitor, display SVV and PPV. Despite the numerous validity criteria required to interpret such variations, these variables may help predict fluid responsiveness at different thresholds and have been integrated into hemodynamic optimization protocols [44].
Respiratory variations in the pulse oximeter plethysmographic waveform (∆POP) have been shown to predict fluid responsiveness in mechanically ventilated patients, similar to changes in the arterial pressure waveform [45]. The MasimoTM (Masimo Corp., Irvine, California, USA) device provides automated calculation of the pleth variability index (PVI) by measuring changes in perfusion index over a time interval including at least one complete respiratory cycle. The PVI has been shown to predict fluid responsiveness in various perioperative settings and has been integrated into fluid optimization algorithms. However, the PVI has the same limitations as the other dynamic parameters and has limited accuracy in the presence of vasoconstriction with or without the use of vasopressors [46–48].
Today, we can recommend that dynamic parameters be used as an integral part of GDT protocols. The limitations of each dynamic index must be taken into consideration as well as the concept of a gray zone. Dynamic parameters neither provide a measure of fluid bolus effectiveness nor should they be used as an indication to give fluids. The final decision to administer fluids must be supported by the apparent need for hemodynamic improvement, the presence of fluid responsiveness, and by the lack of associated risk.
We recommend crystalloid solutions for routine surgery of short duration. However, in major surgery, the use of a goal-directed fluid regimen containing colloid and balanced salt solutions is recommended. Though a black box warning for the use of starch solutions exists within the United States, there is limited data relative to their harm in the perioperative space. Careful consideration should occur in patients with known renal dysfunction and/or sepsis prior to administering starch solutions [43].
10.5.3 Limitations
It is important to note that all the dynamic variables have significant confounding factors [44]. The reliability of these indices is affected by spontaneous breathing activity, arrhythmias, right heart failure, decreased chest wall compliance, and increased intra-abdominal pressure, although most of these limitations are uncommon in the OR. Nevertheless, in the ICU a relatively small proportion of patients present suitable criteria for these indices [49]. Another major limitation of dynamic parameters is that they are dependent on the size of the tidal volume. Some authors have suggested that they require a tidal volume of at least 8 ml/kg body weight [50], although they have been successfully used with tidal volumes of 6–8 ml/kg body weight [47, 48]. A recent study and meta-analysis have indicated a decreased rate of postoperative complications when low tidal volumes are applied during anesthesia [51, 52], and increased use of protective ventilation (lower tidal volumes) in the OR may reduce the usefulness of dynamic parameters or at least require new interpretation rules. Finally, within a range of PPV values of 9–13 %, fluid responsiveness cannot always be reliably predicted; there is a “gray zone” in which prediction of fluid responsiveness is difficult. One study [53] indicated that fluid responsiveness could not be reliably predicted using dynamic measures in as many as 25 % of anesthetized patients.
A passive leg raising (PLR) test has been suggested to overcome some of these limitations in dynamic evaluation, but should be performed rigorously with simultaneous analysis of continuous CO monitoring. It is obviously impractical during most operative conditions [54]. In addition, the blood volume shift from the leg to the central compartment is non-predictable. In a hypovolemic state, it is reasonable to consider a volume shift less than that generated in “normal” volemic conditions.
Despite these limitations and confounding factors, whenever possible, one is advised to assess fluid responsiveness using the available functional hemodynamic parameters before attempting to increase CO with fluid administration. This approach can indicate if and when CO can be further increased by fluids, and identify when the flat portion of the cardiac function curve has been reached, thus preventing unnecessary fluid loading [44]. It is also important to remember that, generally speaking, fluid responsiveness is not an (absolute) indication to give fluids. Decisions about fluid administration should not be based only on dynamic parameters but also on the likely risk associated with fluid administration. During surgery, systematic fluid administration in the presence of fluid responsiveness may improve postoperative outcomes [55].
10.6 Venous Oxygen Saturation
Changes in SvO2 may reflect important pathophysiological changes in the relationship between DO2 and VO2, both of which may fluctuate significantly during the perioperative period.
Reorganization of the Fick equation shows that
![$$ {\mathrm{SvO}}_{{}_2} = {\mathrm{SaO}}_{{}_2}-\left({\mathrm{VO}}_{{}_2}/\left[\mathrm{CO}\kern0.5em \times \kern0.5em \mathrm{Hb}\kern0.5em \times \kern0.5em \mathrm{C}\right]\right) $$](https://i0.wp.com/thoracickey.com/wp-content/uploads/2017/06/A332115_1_En_10_Chapter_Equa.gif?w=960)
