Non-invasive Monitoring of Cardiac Output in Patients with Trauma: A Narrative Review

INTRODUCTION

The chief indicator of tissue oxygen (O₂) delivery is cardiac output (CO), which a wide range of modalities, both invasive and non-invasive can measure. The gold standard method for measuring the same is the thermodilution technique for CO measurement through pulmonary artery catheterization.¹ The invasiveness of the procedure along with the expertise required for successful placement of this catheter and the interpretation of results makes it a cumbersome modality, especially for trauma and acute care setup. The development of a non-invasive, bedside, easy-to-interpret tool is thus required for the efficient care of such patients. Through this review article, we aim to consolidate and summarise the pre-existing literature on non-invasive CO (NICO) monitoring devices.

CO AND ITS CLINICAL RELEVANCE

Tissue O₂ delivery (DO₂) is a direct product of the CO and content of O₂ in arterial blood, as denoted in the formula below.² CO is again a determinant of stroke volume (SV) and heart rate (HR).

DO₂ [mL/min] = CO [litre/min] × arterial O₂ content [mL/dL] × 10 (CO [litre/min] = SV [litre] × HR [min⁻¹])

Systemic inflammatory response triggered during major surgeries, critical illness or polytrauma causes a significant increase in O₂ requirement at the tissue level. The human body tries to meet this requirement by increasing the DO₂, which as mentioned above is dependent on the CO. This leads to an increase in both CO and O₂ extraction ratio. If the heart fails to optimize tissue delivery of O₂, tissue hypoxia and, ultimately, organ dysfunction ensues. Thus, therapeutic interventions such as the administration of fluids and the initiation of inotropic agents with the aim of optimization of CO are necessary.

NICO ASSESSMENT TECHNIQUES

NICO techniques aim at the estimation of CO without the risk associated with invasive techniques (e.g., pulmonary artery catheter), which carry the risk of infection, bleeding, vascular line-related complications, arrhythmias, etc.

NICO techniques rely on various physiological principles such as bioimpedance, pulse wave analysis, and Doppler ultrasonography. The various modalities of CO monitoring are depicted in Figure 1. NICO devices are advantageous for their ability to monitor CO in real-time continuously and are suitable for a wide range of clinical settings, including the intensive care unit (ICU), emergency department (ED) and outpatient care, while being less costly than invasive techniques.

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Figure 1:

Classification of cardiac output monitoring modalities in standard practice. CO: Cardiac output

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ELECTRICAL BIOIMPEDANCE AND BIOREACTANCE

Electric bioimpedance is a non-invasive technique first used for CO monitoring in the late 1960s. It measures resistance to the flow of electric current, which, in turn, helps derive the CO.³

Bioimpedance and bioreactance modalities assess the extravascular intrathoracic fluid changes as observed by the changes in the electrical current read by electrodes placed on the patient’s chest. It assumes that variations in aortic blood volume correlates with changes in impedance. To measure these changes, a set amplitude electrical current at a particular frequency is applied to the chest. The resulting voltage difference is then recorded to quantify the CO.

Commercially available thoracic impedance monitors are now widely available and can act as an adjunctive noninvasive modality to determine the CO. Many developments and modifications were done in this technology, including “electrical velocimetry,” a technology using modified flow and thereby CO.⁴ The “Endotracheal tube cardiac output monitor” is a device which uses an endotracheal tube with electrodes to non-invasively measure CO using this principle.⁵ First described in a swine model in 2000, this device measures changes in electrical impedance in the aortic blood flow, demonstrating improved signal quality by reducing the signal-to-noise ratio due to its proximity to the aorta.

Bioreactance was developed and first used in the early 2000s, with clinical applications becoming more widespread by the mid-2000s. This method measures trans-thoracic voltage shift, which primarily reflects large vessel blood flow being less influenced by changes in lung water. The device employs four electrode patches, each containing two electrodes, and calculates CO separately for both sides of the body. The final CO is the average of these two measurements. Figure 2a and b shows the electrode placement on a patient and a corresponding diagram. Figure 2c illustrates the relationship between thoracic impedance and ventricular, aortic and atrial pressures, as well as aortic flow.

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Figure 2:

(a) Electrode placements for bioreactance non-invasive cardiac output monitoring for perioperative care of an acute trauma victim as shown in a patient and schematic diagram. (b) Transthoracic voltage changes in response to high-frequency current. (c) Depicting the changes and variations of aortic, atrial, ventricular pressures with aortic flow; impedence changes as a function of time.

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Limitations of this technology stem from the fact that the determination of CO is mainly dependent on electrode positioning. The output value can also vary due to electrocautery disturbances, fluid accumulation in the thoracic cavity due to pleural effusions, pericardial tamponade or pulmonary edema. Changes in peripheral vascular resistance, cardiac arrhythmias, motion artifacts and subcutaneous edema, commonly present in critically ill patients, can also affect the measured values.

Bioreactance technology fails to show rapid changes in CO as it provides readings averaged over 60 s.⁶ In a 2020 prospective cohort study by Razzera et al., the authors assessed the accuracy of bioelectrical impedance analysis parameters in predicting nutritional risk, duration of hospital stay, and mortality among critically ill patients.⁶ A STROBE-compliant study done in 2020 by Li et al. found bioimpedance cardiography to have a good correlation with CO changes with passive leg-raising test responders.⁷

Bioimpedance and bioreactance are valuable NICO devices in trauma patients, providing critical insights into their hemodynamic status. These methods utilize electrical signals to assess changes in body composition and fluid distribution, allowing for real-time evaluation of cardiac performance. In trauma cases, where rapid assessment of cardiovascular stability is essential, bioimpedance and bioreactance offer immediate feedback on blood volume and cardiac function without the need for invasive procedures. This is particularly beneficial in emergency settings, as timely detection of compromised circulation can guide fluid resuscitation and other interventions, eventually improving patient outcomes and reducing the risk of complications. By facilitating continuous monitoring, these techniques enable healthcare providers to make informed decisions swiftly, enhancing the overall management of trauma patients.

ELECTRICAL CARDIOMETRY (EC)

EC is a non-invasive modality for determining SV and CO using four surface electrocardiography (ECG) electrodes. This method became commercially available in 2003 with the release of the AESCULON® device by Cardiotonic, Inc., and is approved by the Food and Drug Administration for use in all age groups barring the elderly. CO measurements from EC have been validated against gold standard methods. The procedure requires placing four ECG electrodes – two on the left neck and two on the lower thorax, as illustrated in Figure 3a and b. Through these electrodes, a constant-amplitude alternating current is applied across the thorax near the aortic arch. The ratio between the applied current and the measured voltage yields conductivity (bioimpedance), which is then tracked over time. Changes in red blood cell (RBC) orientation during the cardiac cycle create a steep increase in conductivity, indicating peak aortic blood acceleration, as shown in Figure 3c. EC operates on an electrical velocimetry model, which works on the principle that trans-aortic blood conductivity varies throughout the cardiac cycle.^8,9

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Figure 3:

(a) Electrode placement in a paediatric patient. (b) Electro cardiometry electrodes and wires. (c) Movement of electric current, arrangement of blood flow before and after the current.

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Before the aortic valve opens, RBCs are randomly oriented due to the absence of blood flow in the aorta. When an electric current passes through the electrodes, it circumvents these randomly aligned cells, leading to a higher voltage measurement and, consequently, lower conductivity. Once the aortic valve opens, the pulsatility of blood flow causes the RBCs to align parallel to the direction of blood flow. As a result, the electric current flows more easily through the aorta, producing a lower voltage and, therefore, higher conductivity. This shift creates a distinctive steep rise in conductivity. Studies have demonstrated improved accuracy and reliability of this technology compared to traditional bioimpedance, particularly in sepsis, hemodynamic instability, and fluid management.^10,11 A 2022 feasibility study by Slagt et al. showed the relevance of EC as a quick-to-install and interpret modality in the helicopter emergency transport of trauma victims.¹² The research is, however, limited to the use of this modality in different trauma cohorts.

At its heart, EC is a development of bioimpedance technology. Instead of relying solely on impedance changes, EC uses the concept of RBC orientation as a key determinant of CO, which again minimizes signal dampening due to motion artifacts. This innovation also has reduced sensitivity to extra-thoracic fluid shifts and improved signal stability in the presence of vascular tone changes. It can, thus, be considered an emerging modality for NICO monitoring. Further validation studies can help establish it as a modality of choice in trauma patients.

APPLICATIONS OF EC

1. Modality for non-invasive hemodynamic monitoring

2. Ideal in pediatrics and neonates

3. Administration of goal-directed therapy and fluid management in operation theatre (OT), ICU and ED.

4. For a complete understanding of the patient’s hemodynamics (flow, preload, contractility and afterload) to identify the etiology of shock

5. Management of heart failure and hypertension

6. For pacemaker optimization in the ICU.

LIMITATIONS

Its applications are limited in patients with moderate to severe aortic valve regurgitation, massive pericardial fluid, patent ductus arteriosus, and arrhythmias.

DOPPLER CO MONITORING DEVICES

The principle of using Doppler devices to measure blood flow velocity was first utilized in clinical research in the 1960s. Doppler ultrasound became more widely adopted for CO measurement by the 1980s, particularly with the development of continuous-wave and pulsed-wave Doppler technologies. Ultrasound waves emitted by the probe are reflected by blood cells, resulting in a frequency shift that depends on their velocity in the great vessels. Blood velocity can then be determined using an equation that defines it as the product of the speed of sound, cos theta, transmitted frequency and half of the frequency shift where theta is the angle of incidence between the beam and reflecting blood. The velocity of the RBCs times the ejection time is the derived stroke distance. Stroke distance times the cross-sectional area quantifies the SV that passes through at the level of Doppler interrogation.

Although the Doppler ultrasound is non-invasive, it does have a few limitations. The Doppler device measures blood flow in the descending aorta and extrapolates it to estimate left ventricular outflow, based on the assumption of a consistent flow distribution between the large venous vessels of the arm and the descending aorta. However, this proportion can vary in unstable patients, such as those with trauma.¹³

A comparative study done in 2005 by Tan et al. predicted the efficacy of Doppler ultrasound technology for monitoring CO in cardiac surgical ICU patients. It was validated to be a rapid modality that was also accurate, non-invasive, and cost-effective.¹⁴ A different prospective non-randomized study done in 2006 by Chand et al. for CO measurement with a new Doppler device, ultrasonic CO monitoring, in off-pump bypass surgeries showed excellent results in measuring CO, SV, and cardiac index.¹⁵

Doppler CO monitoring provides a non-invasive or minimally invasive method for real-time, continuous monitoring of hemodynamic parameters, making it useful in perioperative care, ICUs and other critical care settings. It has special utility in trauma settings where quick diagnosis and management are of paramount importance. However, its accuracy can be influenced by patient anatomy, operator skill, and movement artifacts. While Doppler devices are highly portable and offer valuable insights into cardiac function, they may not provide comprehensive hemodynamic data, and their cost of acquisition in low-resource setup can be a limiting factor.

These modalities are useful for monitoring CO in trauma patients, providing a non-invasive and rapid assessment of hemodynamic status. By utilizing ultrasound waves to evaluate blood flow velocity in major vessels, Doppler methods can accurately estimate CO and identify changes in circulation without the need for invasive procedures. This is particularly advantageous in trauma settings, where timely information is crucial for managing potential shock and guiding fluid resuscitation. In addition, Doppler techniques can be employed at the bedside, allowing for continuous monitoring and quick adjustments to management based on tangible, real-time data, ultimately improving patient outcomes.

VASCULAR UNLOADING TECHNIQUE

It is also known as the volume clamp method. It works on the Penaz principle, named after a Czech physiologist Dr. Jan Penaz, who first described the same in 1973. It utilizes a cuff with a photodiode that is applied on an upper extremity digit, which then applies pressure to the finger. The diameter of the digital artery is measured by transmitting infrared light through the finger and detecting the amount absorbed by RBCs using a sensor built into the cuff. The photoplethysmography signal adjusts the finger cuff pressure to maintain a constant blood volume and, therefore, a constant arterial diameter. As the artery’s diameter remains steady, the cuff pressure equals the intra-arterial pressure, allowing the arterial pressure waveform to be directly obtained. A schematic diagram showing the principle of working is represented in Figure 4.

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Figure 4:

Vascular unloading device showing inflatable cuff with infrared plethysmography which analyses blood volume to estimate parameters.

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Since this technology analyzes the arterial pressure waveform pulse contour for CO monitoring, it relies on a good arterial pressure signal measured. Thus, in patients with finger edema, the signal is disrupted. It might also have limited utility for hypotensive patients with decreased preload and also those with high systemic vascular resistance. Hypothermia is a usual occurrence in trauma victims and can cause peripheral vasoconstriction which subsequently leads to poor-quality arterial signals. Hence, utility in hypothermic or in patients with cool clammy skin its clinical utility is limited.

A prospective study was conducted in 2020 by Fischer et al. to evaluate the external validity of one new digital photoplethysmography monitor (Nexfin) for continuous measurement of mean arterial pressure (MAP) and CO. They ratified the safety, convenience and reliability of this device in measuring arterial pressure.¹⁶

The vascular unloading technique offers a non-invasive, continuous and relatively comfortable method for monitoring CO and blood pressure (BP). However, its accuracy can be compromised by factors such as peripheral vasoconstriction, movement artifacts, and vascular abnormalities. Despite these limitations, it remains a useful tool in perioperative and noncritical care settings where invasive methods are not preferred.

The use of a vascular unloading device for CO monitoring in trauma patients offers significant benefits by providing a reliable, non-invasive method to assess hemodynamic status. This technology works by applying controlled external pressure to reduce vascular resistance, which enhances blood flow and facilitates accurate measurement of CO. In trauma situations, where rapid assessment is vital, vascular unloading devices enable clinicians to obtain immediate insights into cardiac performance and volume status, guiding timely interventions such as fluid resuscitation. In addition, their ease of use and ability to continuously monitor changes make them invaluable in critical care settings.

PULSE WAVE TRANSIT TIME (PWTT)

While the PWTT concept research has been around for about a hundred years, as a method to assess vascular health and BP, its application for CO monitoring specifically gained traction in the early 2000s. PWTT is defined as the time interval between the R-wave in the ECG and the rise point of the pulse wave, as assessed by pulse oximetry. The estimated continuous cardiac output technology offers non-invasive, continuous CO readings through the analysis of the ECG, pulse oximeter-derived waveform and arterial pressure. This is the time from the R wave of the ECG to the onset of the peripheral pulse wave. The peripheral pulse uses a photoplethysmography signal obtained from an oxygen saturation probe attached to the finger.¹⁷ In general, the PWTT shortens as BP increases and lengthens as BP decreases. The major limitation of the esCCO system is the requirement of a reference CO value at the start of measurement for calibrating the device.

There are many wired and wireless systems using PWTT technology in use. Among the wired are the-MP36R system and MP160 system, and among the wireless are the-MP160 system and BioNomadix Logger. Furthermore, a magnetic resonance-compatible MP160 system is available. It has been seen that the post-processing of the photoplethysmography signal in the Masimo module of the patient monitors of Philips) can show a sawtooth artefact in both the PPG and PWTT. This makes the derivation of values inadequate while using the Masimo device.¹⁸

PWTT is a non-invasive and promising method for estimating CO, yet it is limited by its sensitivity to vascular tone, BP, HR, and patient movement. It also struggles in dynamic hemodynamic conditions, peripheral vascular disease, and low CO states, making it relatively less reliable in such scenarios. Despite limitations, PWTT may be a promising method for CO monitoring in trauma patients, which builds on the relationship between arterial pressure and blood flow. By utilizing the measurement time it takes for a pressure wave to travel between two arterial sites, it can provide real-time insights into the cardiovascular status and fluid responsiveness. This non-invasive technique is particularly beneficial in trauma settings, where the rapid and accurate assessment of hemodynamic change is crucial for timely intervention. The ability to monitor CO continuously through PWTT helps clinicians detect early signs of shock or instability, facilitating prompt treatment decisions, and improving overall patient outcomes.

RADIAL ARTERY APPLANATION TONOMETRY

This non-invasive technique allows the continuous beat-to-beat derivation of the CO. This was initially described in 1963. The T-line system is a commercially available contraption that uses a patented algorithm for the beat-to-beat recording of the arterial waveform.¹⁹ An input sensor is placed over the radial artery, with its placement refined by an electromechanical system. MAP can be measured from the arterial pressure signal obtained at the optimal “applanation” site, where the artery’s transmural pressure is zero.

As waveform analysis forms the basis for assessing CO, the technology behind this contraption relies on the standard of the recorded arterial pressure waveform. Accurate CO estimation requires optimal sensor placement over the radial artery. Rapid arm movements by the patient can lead to disturbed or inaccurate CO readings.

In 2010, Jeleazcov et al. conducted a comparative study to assess the accuracy of a new device, CNAP (CNSystems Medizintechnik AG, Graz, Austria), which provides continuous non-invasive arterial pressure monitoring based on the radial artery tonometry principle. The study compared the readings with arterial pressure measurements.²⁰ It was found that the device offers real-time arterial pressure estimates that are comparable to those obtained from an invasive intra-arterial system.

In a study done in 2017 by Zayat et al., the authors tried to measure CO in cardiac surgery patients. Comparing the radial artery tonometry with Doppler echocardiography-derived parameters, they found the device to be accurate in measuring the CO and SV when compared with the transthoracic echocardiography in pre-operative cardiothoracic patients.²¹

In summary, while radial artery applanation tonometry can provide valuable insights into arterial pressure waveforms and BP, its indirect approach, reliance on assumptions and technical limitations make it less reliable for CO monitoring, especially in dynamic or critically ill patient scenarios.

CO MONITORING USING PARTIAL CO₂-REBREATHING

The CO monitoring using the partial CO₂ rebreathing method relies on derivation of the law of conservation of mass Fick principle, which states that the amount of a substance taken up by tissue per unit time equals the arteriovenous concentration difference of blood flow to the organ. When applied to the respiratory system, it allows measurement of “O₂ consumption (VO₂) and the difference of arterial (CaO₂) and venous (CvO₂) blood O₂ contents, to derive CO. ”

“CO = VO₂/CaO₂–CvO₂”

“CO = VCO₂/CaCO₂–CvCO₂”

The slope of the CO₂ dissociation curve can, thus, be used as a surrogate of CaCO₂. End-tidal CO₂ can be measured in exhaled gas with a sealed facial mask. The CO can be determined by replacing the measured values in the equation above. It is, however, of limited utility in acute respiratory disease states such as pulmonary edema, pneumonia, and chronic obstructive pulmonary disease exacerbation.

In 2004, Rocco et al. conducted a clinical investigation in a university hospital ICU to assess the reliability and clinical utility of the partial non-invasive CO₂ rebreathing method for measuring CO, comparing it with the standard thermodilution method. “The partial CO₂ rebreathing technique compared well and was reliable in measuring CO in these patients affected by diseases causing low levels of pulmonary shunt, but underestimates it in patients with shunt higher than 35%.”²²

To conclude, while partial CO₂ rebreathing is a non-invasive modality, it has limitations in patients with pulmonary disease, hemodynamic instability, abnormal CO₂ levels or those requiring mechanical ventilation adjustments. It is also not suitable for continuous monitoring, reducing its utility in critically ill patients.

CONCLUSION

The use of NICO monitoring offers a feasible and safe method to assess hemodynamics early in the ED, OT, and ICU, considering the severely compromised hemodynamic status of trauma patients and difficult invasive monitoring access and reliability. It allows for rapid identification of shock and quicker application of appropriate therapies, which may improve the outcome. However, their reliability and clinical utility in trauma settings need to be extensively investigated further before we incorporate them into routine clinical practice.