Haemodynamic assessments in mechanically ventilated patients

Knowledge of heart–lung interactions is key to manage haemodynamics in mechanically ventilated patients (see also Chapter 5). It allows intensivists to understand the meaning of blood and pulse pressure respiratory variations (PPV). Unlike spontaneous breathing, positive pressure ventilation increases blood pressure and pulse pressure during inspiration following by a decrease during expiration. This is called reverse pulsus paradoxus and includes a ‘d-down’ and a ‘d-up’ effect. No variation means no effect of mechanical ventilation on the heart and especially on the right heart. In case of significant PPV, tidal volume usually reduces right ventricular stroke volume by way of reducing preload where systemic venous return is decreased (fluid expansion is useful to restore haemodynamics, when impaired) or increasing afterload (obstruction of pulmonary capillaries due to alveolar inflation and, in this case, fluid expansion is useless or even sometimes deleterious). Clinical examination as well as evaluation of respiratory variations of superior vena cava by echo, helps to distinguish between these two situations. By studying the beat-by-beat changes in echo parameters induced by positive pressure ventilation heartbeat by heartbeat, echocardiography is perfectly suited to study heart–lung interactions and then to propose an appropriate optimization in case of haemodynamic impairment.

Adult congenital heart disease

The prevalence of congenital heart disease (CHD) in adults is increasing and many of these are likely to be admitted to the intensive care unit (ICU). Some of these patients may have undiagnosed CHD, usually relatively simple lesions such as atrial and ventricular septal defects. Occasionally, these may be more complex lesions (e.g. Ebstein’s anomaly) that even if unrecognized earlier in life can still allow survival into adulthood. Whether simple or complex, CHD can complicate the management of the critically ill patient, particularly if shunting or heart failure is present. The critical care echocardiographer is required to both recognize both normal anatomical variation and definite abnormal structural abnormalities in the adult patient. The aim of this chapter is to familiarize the echocardiographer with common anatomical variants, such as remnants of the right valve of the sinus venosus and crista terminalis, and present a careful and systematic approach to echocardiographic examination that may reliably identify relatively simple undiagnosed CHD in the adult.

Heart–lung interactions

Echocardiography is the most clinically practical method of visualizing cardiac structures and directly observing changes of cardiac function during the respiratory cycle. This chapter will review heart–lung interactions and will focus on the effects of intrathoracic pressure variation on cardiac function that can be measured with advanced critical care echocardiography. These measurements are derived from observing respirophasic variation of stroke volume (SV) and help the intensivist to guide management of haemodynamic failure. The heart–lung interactions that occur with changes in intrathoracic pressure variation have utility in identification of preload sensitivity and adverse patient ventilator interaction. Measurement of the systolic velocity envelope with pulsed-wave Doppler is a requisite skill in order to identify SV variation, as is the recognition that the measurements may be difficult with transthoracic echocardiography.

Basic Doppler principles

Basic two-dimensional echocardiography alone is inadequate for advanced diagnosis and treatment monitoring in critically ill patients because of haemodynamic complexities. To cope with such demands, a critical care physician will also need to be competent in Doppler echocardiography, which provides accurate measurements of blood flows. Experienced echocardiographers are able to draw inference about the cardiac function, intracardiac and intravascular pressures, and other abnormalities from the Doppler flow spectra. Doppler echocardiography also provides objective measurements that can be used for bedside diagnosis or for longitudinal monitoring. Learning Doppler echocardiography has a steep learning curve and has several hurdles to overcome. First, one needs to develop high-level transducer navigation skills to make sure a proper insonation angle is obtained for all Doppler measurements. Second, an understanding of medical ultrasound physics is required not only for knobology purposes, but also for appreciation of the pros and cons of various Doppler modalities. Third, recognition of the limitations of Doppler echocardiography is necessary to avoid misapplications and misinterpretations. Fourth, the operator needs to be able to identify Doppler artefacts so as not to mistake artefacts for real findings. Finally, an understanding of haemodynamic principles is important to execute proper interpretations of Doppler measurements. This chapter will cover mainly the second and third aspects of these. Doppler artefacts will be covered in Chapter 2 and haemodynamic principles in Chapter 3.

Accreditation in advanced critical care echocardiography

The development of smaller, mobile, sophisticated ultrasound machines has been central to echocardiography becoming an everyday tool in the intensive care setting. However, the parallel process of ensuring quality studies are obtained from these machines places a focus on training standards for the doctors operating them. In response, credentialing and certification programmes in advanced critical care echocardiography have come into existence around the world, and although they are not identical, the programmes share many of the same features whether it is run in France, Scotland, Australia, the United States, or elsewhere. The challenge of determining the optimum training programme is an ongoing process and will no doubt evolve further over time. Yet the development of the programmes to date demonstrate how far critical care physicians have come over the past two decades in achieving quality care in the use of echocardiography.

3D echocardiography in intensive care

Three-dimensional echocardiography (3DE) is a rapidly expanding modality with great potential. 3DE, as with 2D, is based on basic principles of ultrasound physics. Sequential ultrasound beams are emitted from piezoelectric elements and received echoes are analysed by computers using specialized algorithms, creating pictures on a screen. Current routine utilization and research of 3DE in intensive care is limited. Technical problems, lack of equipment and training among intensive care practitioners are the major limiting factors. This chapter examines the main principles of 3DE, specific terminology, current advantages and limitations, as well as projected future applications. Application areas of three-dimensional transthoracic echocardiography (3D TTE) and three-dimensional transoesophageal echocardiography (3D TOE) in intensive care are outlined.

Contrast echocardiography

Echocardiography in the intensive care unit (ICU) is notorious for being difficult to perform, leading to frustratingly non-diagnostic studies with a lack of confidence in findings. The use of ultrasound contrast to enhance these images has the potential to salvage inconclusive studies and change management in critically ill patients. Ultrasound contrast, once ‘activated’, produces tiny microspheres containing an inert gas with a stabilizing shell with a diameter of approximately 1–5 μ‎m. Injected intravenously they pass through the pulmonary microcirculation into the systemic circulation. They last approximately 3–5 minutes and remain entirely in the vascular space. The gas is released unchanged by the lungs and the stabilizing shell is typically metabolized by the reticuloendothelial system or by fatty acid metabolism. These agents are essentially safe in the critically ill. Minor side effects occur in 1–2% and are alleviated by ceasing administration. There is a 1 in 10 000 chance of an anaphylaxis-type reaction and hence cardiopulmonary monitoring for at least 30 minutes after administration is recommended, along with suitable resuscitation measures to deal with this unlikely occurrence. Contrast-enhanced echocardiography can help to accurately detect the endocardial border, ventricular dysfunction, regional wall motion abnormalities, left ventricle thrombi, abnormal masses, and enhance Doppler signals among other potential benefits. In addition, the use of contrast can prevent further investigations and transfer which may be detrimental to the critically ill patient.

Echocardiography in trauma

Trauma is a major cause of morbidity and mortality across the globe. Patients with trauma may have blunt or penetrating, cardiac, or non-cardiac injuries. Echocardiography has a role to play in the evaluation of a cardiac as well as non-cardiac trauma patient, not only in the trauma resuscitation phase but also during the critical illness period. Often, time is very limited and detailed echocardiography is possible only in the postresuscitation phase. In a resuscitation scenario, focused assessment protocols are used to evaluate for life-threatening cardiac injuries. In the postresuscitation phase, echocardiography may help by unmasking a pre-existing cardiac pathology or trauma related pathology which may influence management. Transoesophageal echocardiography should be considered if transthoracic views are inadequate. Echocardiography may also diagnose a cardiac pathology related to the critical illness like pulmonary embolism or critical care cardiomyopathy. The main emphasis of this chapter is on traumatic cardiac and aortic injuries.

Acute respiratory failure

Acute respiratory failure is a common reason for admission to the intensive care ward and it is frequently accompanied by haemodynamic instability. Obligatory assessments in every patient should include left ventricular function, left atrial and left ventricular filling pressures in addition to an assessment of right ventricular function and the pulmonary circulation. A systematic echo protocol is warranted to judiciously decide on treatment strategy, including optimization of the patient’s preload, contractility, heart rate, and afterload. This allows for a more effective management of the respiratory disequilibrium, which can continue to be monitored by ultrasound examination. Monitoring of lung parenchyma and pleural space adds to the echo derived information and assist the physician in deciding on an optimal ventilation strategy, need for bronchoscopy, pleural drainage, and patient positioning including proning. The appropriateness of prescribed therapy for the acute respiratory failure can then be monitored by echocardiography and lung ultrasonography to optimize pulmonary gas exchange without haemodynamic deterioration and conversely improve the patient’s haemodynamic status without adding an unnecessary burden onto the respiratory system. After respiratory failure responds to treatment, echocardiography can then assist with the weaning and subsequent withdrawal of mechanical ventilatory support. Where respiratory failure does not respond to conventional measures, a rapid assessment with echocardiography and chest ultrasound helps to decide whether to proceed to extracorporeal life support and, if adopted, its optimal configuration.

Dynamic left ventricular outflow tract obstruction

Left ventricular outflow tract (LVOT) obstruction is often clinically unrecognized unless echocardiographic assessment is performed. Its occurrence is favoured by anatomical factors (i.e. concentric or asymmetrical hypertrophy, excess tissue in mitral valve), hypovolemia and adrenergic stimulation and can occur in various conditions including postoperative setting (especially but not exclusively, after cardiac surgery), stress cardiomyopathy, and sepsis. A high flow in a narrow LVOT generates a Venturi effect in the LVOT which results in the attraction of the anterior mitral leaflet towards the interventricular septum causing LVOT obstruction. Not only this generates an intraventricular (left ventricle to LVOT) pressure gradient but can also be accompanied by mitral regurgitation that can sometimes be severe. Prompt echocardiographic assessment is warranted in order to adequately manage the patient. Typical echocardiographic findings include systolic aliased flow in LVOT on colour Doppler and dagger-shaped or double-peak Doppler flow in LVOT. The systolic anterior movement of the anterior leaflet of the mitral valve should be carefully searched. In some cases mitral regurgitation can be observed. Therapy may include fluid administration, weaning of adrenergic agents, and, whenever possible, beta-blockade administration.