POCUS and Fluid Responsiveness on Venoarterial ECMO

INTRODUCTION

Venoarterial extracorporeal membrane oxygenation (VA ECMO) is an effective rescue therapy providing temporary cardiac and respiratory support for patients with refractory cardiogenic shock. VA ECMO allows organ perfusion and oxygenation while awaiting myocardial recovery, cardiac transplantation, or long-term mechanical circulatory support. Diagnosis of hospital-acquired pneumonia (HAP) is a daily challenge for the clinician managing patients on venoarterial ECMO. Lung ultrasound (US) can be a valuable tool as the initial imaging modality for the diagnosis of pneumonia. Color Doppler intrapulmonary flow and dynamic air Broncho gram appear to be particularly insightful for the diagnosis of HAP.^[1] COVID-19 has inflicted the world for over 2 years. The recent mutant virus strains pose greater challenges to disease prevention and treatment. COVID-19 can cause acute respiratory distress syndrome (ARDS) and extrapulmonary injury. Dynamic monitoring of each patient’s condition is necessary to timely tailor treatments, improve prognosis, and reduce mortality.

Point-of-care US (POCUS) is broadly used in patients with ARDS. POCUS is recommended to be performed regularly in COVID-19 patients for respiratory failure management. In this review, we summarized the US characteristics of COVID-19 patients, mainly focusing on lung US and echocardiography. Furthermore, we also provided the experience of using POCUS to manage COVID-19-related ARDS.^[2] Significant progress in device technology and in the management of critically ill patients have allowed an international expansion of the use of VA ECMO.^[3] However, short-term mortality remains high, with an overall survival rate of 40%, mainly due to the very high incidence of complications, among which infections are predominant.^[4] Early diagnosis of HAP in VA ECMO patients remains a daily challenge.

Point-of-care lung US (LUS) was demonstrated to be an effective tool in case of acute respiratory failure for ICU patients,^[5,6] community-acquired pneumonia,^[7,8] and ventilator-associated pneumonia.^[9,10] The LUS diagnosis of ventilator-associated pneumonia in intensive care units is more challenging in comparison with the diagnosis of community-acquired pneumonia in emergency departments due to the limited access to the mechanically ventilated patients and the high prevalence of atelectasis. This review describes the usefulness of LUS in the early detection of HAP in cardiac critically ill patients under VA ECMO as well as assess its sonographic features.

In a busy, resource-constrained intensive care unit (ICU) or emergency room, POCUS can allow triage of patients such that unnecessary costly and time-consuming investigations and interventions can be avoided. By allowing the intensivists to perform these assessments at the bedside, POCUS reduces the risks associated with transportation of critical patients. It also mitigates the risks inherent with exposure to ionizing radiation. An additional advantage is that these assessments can be repeated periodically and can thus be used to assess the patient’s dynamic response to interventions and therapies and can lead to improved outcomes.^[11-14]

IMPORTANCE OF POCUS AND FLUID RESPONSIVENESS (FR) ON ECMO

Shock is a potentially life-threatening condition and if not treated promptly, it can lead the patient into a rapid downward spiral ending in death.^[15] Intravenous fluids (IVF) are considered the first-line therapy for shock and are routinely used in ICUs and hospitals to restore effective blood volume and maintain organ perfusion.

Patients are transfused fluids with the premise that increasing stressed venous volume and consequently improving stroke volume and cardiac output will result in better tissue oxygenation and organ function. In the early 70s, when the use of the pulmonary artery catheter was in vogue and fluid therapy was being titrated to fixed targets of the central venous pressure (CVP) or pulmonary artery occlusion pressure (PAOP).^[16] The landmark EGDT (early goal directed therapy) study by Rivers et al.^[17] showed that massive fluid administration (30 mL/kg) during the first 6°h of resuscitation of patients with severe sepsis and septic shock improved outcomes. This study heralded an era of large volume resuscitation and rigid targeted protocols.

However, there is a growing body of evidence^[18-23] that has shown that indiscriminate use of fluids without giving due regard to the patient’s hemodynamic status and response to the purported boluses can be just as detrimental to the patient’s health. The harmful effects of a positive fluid balance can present in the form of pulmonary edema, hypoxemia, tissue edema, renal dysfunction, intraabdominal hypertension, delirium, cerebral edema, intestinal dysfunction, impaired wound healing, prolonged ventilator days, and hospital stay, and has been shown to increase mortality as well.^[20-23] Recent literature has in fact shown that maintaining a negative fluid balance can improve the chances of survival in patients with septic shock and acute kidney injury.^[24] It is important; therefore, to determine whether giving more fluids will result in benefit or harm.

When tissue perfusion is threatened, it is prudent to ascertain which of the three appropriate choice is: Optimization of preload status (fluids), ionotropic support and vasopressors, modulation of afterload, or a combination of the above.

Studies have shown that only about half the hemodynamically unstable patients in a critical care unit will be fluid responsive.^[25] To make matters worse, the FENICE investigators^[26] found that majority of the clinicians used fluids in an empirical, liberal, and unstructured manner, without due consideration to response to fluid challenges. The time has come that IVFs are given the same respect as that afforded to any other pharmaceutical preparation and should be given only after due assessment.

A patient whose stroke volume or cardiac output (CO) rises by a fixed percentage (commonly 10–15%), in response to a predetermined volume of fluid challenge (commonly bolus 500 mL, 100 mL in mini-fluid challenge), over a predetermined period of time is defined to be “fluid responsive.” Several validated tools and technologies exist today that allow assessment and continuous monitoring of FR, such as those based analysis on arterial pulse contour,^[27] transpulmonary thermosdilution, and bioreactance. Although quite a few show promise, most of these need invasive lines and expensive monitors that carry with them their own inherent risks such as pneumothorax and central line associated blood stream infections. The need of the hour is a tool that is inexpensive, non-invasive, easily accessible, and fairly accurate, with reproducible results. The European Society of Intensive Care Medicine has in fact issued a consensus statement on circulatory shock, where in it was proposed that bedside echocardiography be used as a first-line modality in the evaluation of patients with shock.

Theoretically speaking, it appears simple to give fluids to patients that lie on the steep part of the Frank Starling curve and to restrict fluids for patients on the flat part of the curve. However, it is not always possible to pinpoint the patient’s position on the curve, especially when the steepness of the curves varies with changes in the left ventricular (LV) systolic function. A static parameter/marker is measured at a given LV function and presumed to reflect preload at a given point on the Frank-Starling curve. It also assumes that a lower value of preload implies FR. Evidence supports discontinuing the use of static markers of preload, such as CVP and PAOP, because one isolated value does not predict FR.^[28] Dynamic indices, on the other hand, involve the delivery of a preload challenge and therefore assess the actual response of the cardiovascular system to the said challenge.^[29] This preload challenge could be external (fluid bolus), internal provoked (end expiratory occlusion or passive leg raising [PLR]), or provoked spontaneously by mechanical ventilation.

In addition, there is a physiological variability in the dynamic parameters secondary to variations in intrathoracic pressure occurring during both spontaneous and mechanical breaths. Positive pressure ventilation by increasing the intrathoracic pressure during inspiration, decreases the right ventricular (RV) preload and consequently decreases the RV stroke volume (as described by the Frank-Starling relationship). These phenomena are transmitted to the LV pressures after pulmonary transit time. This manifests during expiration as a decrease in LV preload and LV stroke volume.^[30] These changes in stroke volume caused due to heart-lung interactions are monitored before and after a preload challenge. The change is more pronounced when the patient is preload dependent; greater the volume deficit, the larger is the change in the dynamic parameters. The magnitude of the changes indicates the patient’s position the Frank-Starling curve. Heart-lung interactions therefore form the basis of most tests for FR. This forms the basis of the multitude of stroke volume variation (SVV) monitoring systems and can also be assessed by calculating the flow through valves, vessels, or outflow tracts using Doppler echocardiography. There are certain prerequisites to be fulfilled for SVV and its derivatives to be good predictors of FR. These limitations hold true for all continuous cardiac output monitors as well. Few echocardiographic indices can overcome these limitations.

There are several POCUS-guided indices that help the clinician ascertain the state of FR. Some of these are explained below and others are described [Tables 1-5 and Figures 1-4].

Close

Figure 1:

Venoarterial extracorporeal membrane oxygenation circuit.

Export to PPT

Close

Figure 2:

(a) Diagram showing probe positions for focused abdominal ultrasound (US). (Probe positions; (1): Subcostal, (2): Right upper quadrant, (3): left upper quadrant, (4) Pelvic; (b) Blue Points. (1) Just below clavicle (upper BLUE point). (2) Close to the nipple (lower BLUE point). (3) Junction of the horizontal line from the lower BLUE point and the posterior axillary line (posterolateral alveolar and/or pleural syndrome [PLAPS] point).

Export to PPT

Close

Figure 3:

(a) Point-of-care ultrasound (POCUS) showing the relations of right lung, liver, and right kidney. A small right-sided pleural effusion. (b) POCUS showing large hepatic collection and a right-sided pleural effusion. (c) Confluent multiple B-lines suggesting pneumothorax on extracorporeal membrane oxygenation.

Export to PPT

Close

Figure 4:

Abdominal Ultrasonography showing liver with perihepatic fluid.

Export to PPT

Close

Figure 5:

Compressible femoral vein indicates absence of deep venous thrombosis.

Export to PPT

CONCLUSION

Over the past decade, there has been an increasing emphasis on patient safety and evidence-based medicine. Protocolized patient care has been shown to decrease errors, standardize patient care, and improve outcomes. In recent years, the scope and usage of US have expanded to the extent that POCUS has been considered by some as the modern stethoscope. If used judiciously, US-based protocols that incorporate screening of multiple organ systems can impact the accuracy of the patient’s diagnosis and also hasten the management of critically ill patients.