Vascular

Critical care management of the vascular patientHeather Morris

carlos c M Gomez

AbstractThis contribution discusses the critical care management of the vascular

patient.

Keywords inflammation; sIrs; acute lung injury; multiple organ

dysfunction syndrome; weaning; tracheostomies

The principle of intensive care is to support organ function while awaiting recovery from insult. The challenge lies in identifying patients who can wait for recovery and those who cannot (and who therefore need prompt intervention).

Critical care involves a delicate balance between achievable and acceptable physiology. This precarious balance is different for each patient; for instance: • optimum intravascular volume may lead to excessive pul­

monary, peripheral and cerebral oedema • a certain perfusion pressure can be achieved only at the expense

of increased cardiac work and peripheral vasoconstriction • acceptable acid–base chemistry requires an increase in

intrathoracic pressure (ventilation), cardiac work and tissue oedema.

Stress response to surgery

Surgery elicits protective neuroendocrine and inflammatory reac­tions. In general, recovery after surgery relates to the degree of tissue damage and inflammatory reaction, but not to the magni­tude of neuroendocrine changes.

The neuroendocrine response is characterized by a rapid increase in catecholamines and a more gradual rise in other cata­bolic hormones. Glycogen, fatty acids and amino acids are mo­bilized. The renin–aldosterone axis is activated, resulting in the retention of sodium and water.

This article was first published in Surgery 2004; 22(11): 303–6.

Heather Morris FRCA is a Specialist Registrar in Anaesthesia and

Intensive Care Medicine at St Mary’s Hospital, London, UK. Conflicts of

interest: none declared.

Carlos C M Gomez FRCA is a Consultant in Intensive Care Medicine and

Anaesthesia at St Mary’s Hospital, London, UK. Conflicts of interest:

none declared.

surGErY 25:8 338

Inflammation results in the activation of complement, coagu­lation and neutrophil–monocyte systems and production of cytotoxic chemicals. Some individuals produce an insufficient response and are overwhelmed by the injurious process; others mount an excessive inflammatory reaction that leads to wide­spread destruction of tissue. The benefits of modulating an exces­sive stress response must be weighed against risks of excessive immune suppression.

Pathophysiology

Many vascular patients have atherosclerosis. Risk factors for atherosclerosis (e.g. obesity, smoking) also contribute to hyper­tension, diabetes mellitus and chronic obstructive pulmonary disease. The collective effect of these factors is exacerbated by: • the inflammatory response to surgery • crossclamping of large vessels • haemorrhage.

Cardiovascular

Atherosclerosis of the aorta is associated with coronary artery disease. Myocardial infarction/ischaemia is the commonest cause of death in patients undergoing vascular surgery. Atherosclerosis should be suspected if cardiovascular instability is marked and persistent; it can be confirmed by a combination of: • electrocardiography • echocardiography • angiography • serial presence of raised markers of myocardial necrosis.

Treatment (e.g. anticoagulation, fibrinolysis) is limited by sur­gery, though immediate coronary angioplasty and stenting are rapidly becoming the first­line treatments in the UK. Supportive management until recovery of cardiac function forms the corner­stone of treatment.

The systemic inflammatory response syndrome (SIRS) is the manifestation of excessive host response and leads to further tis­sue damage and dysfunction. SIRS is characterized by varying degrees of circulatory instability and end­organ insufficiency. In the ICU, SIRS is usually caused by infection.

Blood pressure (Table 1): hypertensive patients with poorly compliant vessels and diabetics with autonomic neuropathy often have labile blood pressures. Hypertension carries the risk of ventricular distension and cardiac failure. Hypotension can reduce perfusion to the heart, brain and kidney.

Respiratory

Infections of the lower respiratory tract in the critically ill are often caused by multi­resistant organisms acquired in the hos­pital. They are particularly common and problematic in venti­lated patients. Incidence and microbiology vary widely, but there are several common features.

Mechanical ventilation makes normal defence mechanisms less efficient. Patients with chronic lung disease are at particu­lar risk. Critically ill patients are generally immunocompromised and cross­infection is a major problem.

© 2007 Published by Elsevier ltd.

Vascular

Common vasoactive agents

Agent Receptor pharmacology Effects Indications Disadvantages

adrenaline Equipotent α/β Tachycardia resuscitation arrhythmias

raised cardiac output shock acidaemia

raised blood pressure Hyperglycaemia

Noradrenaline α1>β1 raised blood pressure sIrs/sepsis Vasoconstriction

reflex bradycardia

Dobutamine β1, β2 Tachycardia cardiac failure arrhythmias

raised cardiac output sepsis Myocardial ischaemia

Vasodilation

Dopamine Da1, Da2, β1, α1 renal vasodilation circulatory support arrhythmias

Tachycardia Vasoconstriction

raised blood pressure

Dopexamine β2>Da1>β1 Vasodilation circulatory support arrhythmias

renal vasodilation cardiac failure

Tachycardia Gut perfusion

Milrinone Inhibition of phosphodiesterase III;

raised concentrations of cyclic

adenosine monophosphate

cardiac contractility cardiac failure Vasodilation

Vasodilation

Table 1

Acute respiratory distress syndrome and acute lung injury encompass a clinical spectrum of elevated pulmonary alveolo­capillary permeability characterized by: • refractory hypoxaemia • widespread alveolar infiltrate seen on chest radiograph • respiratory distress which is not caused by (but may coexist

with) raised left atrial or pulmonary capillary pressure.Acute repiratory distress syndrome and acute lung injury are

lung manifestations of the endothelial dysfunction and hyper­inflammatory response normally associated with SIRS.

Renal

Renal impairment in vascular patients is common and abnor­mal renal function preoperatively is a significant risk factor for post­operative renal failure. Also, cardiovascular instability, haemorrhage, suprarenal crossclamping and renal artery revas­cularization compromise kidney perfusion and increase the risk of acute renal failure. Death from acute renal failure remains alarmingly high, despite advances in critical care and renal replacement.

Atherosclerosis can cause renovascular disease. Hyperten­sion and diabetes contribute further to renal impairment (as does most of the medication used to optimize cardiac function). The combination of diuretics and preoperative fasting often leads to hypovolaemia.

Haematological

Vascular surgery is often associated with haemorrhage and transfusion of large volumes of blood. Replacement is usually with packed red blood cells that are low in platelets and coagu­lation factors. The relationship between blood loss, quantity

surGErY 25:8 33

and quality of replacement, observed bleeding and measurable coagulation abnormalities is highly variable. There is a com­plex inter­relationship between surgery and activation of the inflammatory and coagulation cascades.

Dilutional and consumption coagulopathy often coexist. Treat­ment focuses on surgical control of the bleeding source (can be difficult) together with transfusion of concentrated clotting pro­ducts (guided by coagulation tests). Other issues related to blood and clotting product transfusions must also be considered: • temperature • electrolyte and oxygen transport deficiencies of stored products • risk of infection • anaphylaxis.

In the absence of acute coronary ischaemia, critically ill patients are unlikely to derive significant benefit from blood transfusion that aims to increase haemoglobin concentration to >8 g/dl.

Multiple organ dysfunction syndrome

Vascular patients may develop multiple organ failure from which recovery may be prolonged. This is characterized by: • a combination of severe sepsis and SIRS • ventilator­associated pneumonia • ileus • inotrope dependence • wasting and malnutrition • renal failure • global central nervous dysfunction.

A well­orchestrated and enthusiastic multidisciplinary approach with an emphasis on organ support is needed to ensure survival until physiological recovery ensues. Often, the main determinant of resolution of illness is freedom from infection.

9 © 2007 Published by Elsevier ltd.

Vascular

Fluid balance

Careful fluid balance is essential because optimum volume for cardiac and respiratory function may have detrimental effects on renal function. Hypovolaemia (particularly in sepsis or haem­orrhage) can be underestimated, with severe consequences for end­organ perfusion. Hypervolaemia (particularly in left­heart dysfunction) can lead to oedema, which can overwhelm com­pensatory mechanisms.

With capillary leak (a feature of sepsis), satisfactory intra­vascular volume replacement may cause serious peripheral and interstitial lung oedema. The balance between deleterious volume depletion and pulmonary oedema is critical and varies between patients.

Accurate estimation of intravascular filling is very important, and can often be achieved by clinical examination and measure­ments of central venous pressure. Central venous pressure may not reflect left ventricular filling accurately in right­heart dys­function or pulmonary hypertension, so a pulmonary artery cath­eter can be used.

A pulmonary artery catheter measures pulmonary artery pressures, including pulmonary artery wedge pressure. Other haemodynamic indices can be measured (cardiac output) or calculated (vascular resistance, oxygen transport). Pulmonary artery wedge pressure often reflects left atrial pressure, which in turn is proportional to left ventricular end­diastolic pressure and filling. Pulmonary artery wedge pressure may not reflect true left ventricular preload in mitral valve disease or left­heart disease.

Pulmonary artery catheters can have serious complica­tions. Less invasive monitors have gained wide acceptance, particularly if measurement of pulmonary artery pressure is not crucial.

Cardiovascular support

Basic principlesThe heart perfuses itself and has a formidable cardiac output reserve (about five­fold). Extraction of coronary blood oxygen is always near maximal, so increased consumption of oxygen can be met only by an increased supply of oxygen.

The aim of cardiovascular support is to maintain perfusion of the organs and tissues. The balance between the need to maintain end­organ perfusion and its consequent (but oppos­ing) increase in myocardial work and oxygen consumption is very delicate.

The intra-aortic balloon pump is introduced into the descend­ing aorta via the femoral artery. It inflates during diastole, aug­menting diastolic pressure, thereby improving coronary flow. The intra­aortic balloon pump deflates just before end­diastole, so afterload is reduced. The intra­aortic balloon pump improves coronary perfusion, though the perfusion of organs distal to the balloon may be affected.

Ventricular assist devices are used in selected patients with severe refractory primary heart failure as temporary support until recovery ensues or a heart transplant becomes necessary.

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Ventilation

Basic principles (Table 2)Mechanical (positive­pressure) ventilation is predominantly inspiratory support together with some form of expiratory sup­port. The objective of inspiratory support is to inflate the lungs to allow gas exchange without causing trauma to the airway or lung. Expiratory support aims to reduce end­expiratory airway closure without causing impaired exhalation and air­trapping.

In spontaneous and in positive­pressure ventilation, expira­tion is passive and depends on elastic lung recoil and airway resistance. Elastic lung recoil is difficult to manipulate, whereas air resistance can be altered pharmacologically.

Mechanical ventilation should be beneficial when treating conditions associated with inspiratory abnormalities. Mechani­cal ventilation would not be expected to be nearly as useful in predominantly expiratory disease because it cannot alter elastic recoil.

Basic definitions

airway resistance relates pressure gradient required to generate

a certain flow; increased in asthma.

compliance relates change in volume to change in pressure

(dV/dP). compliance is decreased in parenchymal or interstitial

diseases (e,g. infection, oedema, airway collapse, fibrosis), but

is increased in airway disease (e.g. emphysema, bronchitis).

shunt occurs when poorly ventilated alveoli are adequately

perfused; causes hypoxaemia, the treatment for which

necessitates recruitment of underventilated areas. Typical in

pneumonia and lung collapse.

Deadspace occurs when adequately ventilated alveoli are

insufficiently perfused; causes effective fall in alveolar

ventilation with consequent hypoxaemia and hypercapnia.

Positive end-expiratory pressure (PEEP) is applied to maintain

alveoli open at the end of every breath. It increases intrathoracic

pressure and can adversely reduce venous return and cardiac

output.

continuous positive airway pressure (cPaP) is the equivalent

term to PEEP when applied to spontaneous ventilation.

Pressure support refers to ventilator-applied inspiratory pressure

bursts triggered by a spontaneous effort. The tidal volume

delivered depends on lung compliance.

Pressure control is the equivalent term to pressure support

when applied to mandatory or controlled ventilation. Tidal

volume also depends on compliance.

Volume support and volume control are equivalent terms to

pressure support and pressure control, but in these cases the

tidal volume is fixed and the pressure achieved is a function of

compliance.

Table 2

0 © 2007 Published by Elsevier ltd.

Vascular

WeaningThere are many weaning strategies, but no single strategy is better than another.

Respiratory failure: to understand the principle of weaning, one must understand respiratory failure, of which there are three forms.

Postoperative respiratory failure is characterized by a com­bination of: • pain • basal atelectasis • altered intravascular and extracellular volume • hypothermia • drowsiness • capillary leak leading to pulmonary and peripheral oedema.

Correction of these abnormalities and/or physiological support until recovery or compensation is the mainstay of management.

Obstructive respiratory failure is commonly seen in chronic obstructive airway disease and asthma (i.e. diseases of the air­ways characterized by abnormal expiration caused by airway narrowing and/or excessive secretions). When significant, ex­piratory disease leads also to inspiratory weakness. Mechanical ventilation in obstructive respiratory failure provides inspiratory assistance to reduce the work of breathing while the expiratory pathology is reversed and general condition, muscle weakness and nutrition improve. Ventilation parameters for obstructive respiratory failure are: • a low inspiratory pressure (because compliance and inspira­

tion are not normally affected) • low expiratory pressure (sufficient to prevent airway collapse

and to trigger inspiratory effort, but not so high as to cause air­trapping).Weaning is often slow and difficult because mechanical

ventilation does not treat the principal abnormality in obstruc­tive respiratory failure.

Restrictive respiratory failure is the physiological ‘hallmark’ in diseases of the chest wall and neuromuscular disorders marked by predominantly inspiratory difficulties. Parenchymal infection (e.g. pneumonia) is a form of restrictive lung disease in that lung consolidation leads to: • reduced compliance and airway collapse • difficulty in drawing air into the lung • fatigue.

Mechanical ventilation has an important role in the setting of pneumonia. Ventilation parameters due to pneumonia are a: • high mean or plateau airway pressure (but sufficiently low to

prevent trauma) • high expiratory pressure to prevent airway collapse and facili­

tate gas exchange.Cautious ventilation and acceptance of certain physiologi­cal derangement (permissive hypercapnia) is favoured to avoid excessively high pressure and volume (with consequent iatrogenic lung damage).

surGErY 25:8 34

Conditions: there is a spectrum of conditions with varying degrees of inspiratory and expiratory abnormalities. For example, patients with asthma or chronic obstructive airway disease can develop serious pneumonia; those with kyphoscoliosis can also have asthma.

Acute respiratory distress syndrome poses particular chal­lenges for ventilation and weaning. Capillary leak leads to life­threatening non­cardiogenic pulmonary oedema with increased lung water. This leads to decreased compliance and an altered alveolar–capillary membrane (which cause reduced alveolar ventilation and ventilation/perfusion match, leading to shunt and hypoxaemia). Ventilation strategy is as for restrictive res­piratory failure and pneumonia.

TracheostomiesTracheostomies are increasingly being done early in patients likely to require ventilation for more than several days. The ben­efits of less sedation (improved care of the mouth and secretions) and less deadspace outweigh risks (early bleeding, pneumo­thorax, late development of tracheal infection and stenosis).

Advantages of percutaneous tracheostomies at the bedside (e.g. logistics, reduction in tracheal infections) must be weighed against advantages of the traditional surgical technique (e.g. safety and access, particularly in unfavourable anatomy of the neck and airway). Irrespective of technique, the experience of the operator and timing of the procedure are the most important factors related to complications, which can be catastrophic.

Renal failure

The key to prevention of renal failure is: • maintenance of circulating volume and perfusion pressure • early treatment of deteriorating renal function with an em­

phasis on identification and treatment of cause • avoidance of nephrotoxins.

Renal failure is associated with fluid, electrolyte and acid–base imbalance, hence renal replacement therapy may be needed. A range of techniques are available, from peritoneal dialysis to haemofiltration. Most common in the ICU is continuous veno–venous haemodiafiltration. Filtration techniques are associated with less cardiovascular instability than dialysis. ◆

FuRTHER READIng

crane J, cheshire N. recent developments in vascular surgery.

BMJ 2003; 327: 911–15.

Hillman K, Bishop G. clinical intensive care, 2nd edn. cambridge:

cambridge university Press, 2004.

rumbak MJ, Newton M, Truncale T, et al. a prospective, randomized

study comparing early percutaneous dilated tracheotomy to

prolonged translaryngeal intubation (delayed tracheotomy) in

critically ill medical patients. Crit Care Med 2004; 32: 1689–94.

1 © 2007 Published by Elsevier ltd.