β1 (at moderate dose)
α1 (at high dose)
Inotropy
Increased SVR
Fundamentally, a decision must be made as to whether the hypotensive patient requires fluid, inotropy, chronotropy or vasoconstriction. Diastolic function is also important and diastolic dysfunction can be an independent cause of heart failure. Ventricular compliance and optimal filling are dependent on efficient myocardial relaxation (lusitropy). Lusitropy is an active process and can be improved by beta-adrenergic stimulation.
Central venous saturations can be monitored in critically ill patients and offer an indication of oxygen delivery (SvO2 = [SaO2 – VO2/CO][1/Hb × 1.34]). A requirement for increased systemic oxygen utilization (VO2) will be compensated for by increased cardiac output (CO). If oxygen demand is not met, elevated oxygen extraction occurs in the peripheral tissues and SvO2 falls. The normal range is 65–75%. Low SvO2 is predictive of a poor outcome. However, a normal or high SvO2 does not guarantee adequate tissue oxygenation, as in the case of shunting or cell death; here, the tissues fail to extract oxygen and SvO2 may be high despite ongoing cellular hypoxia.
When faced with a hypotensive patient, acceptable practice is to administer an initial fluid challenge. Patients may respond with a blood pressure that increases and remains elevated. A systematic approach to hypotension management is key and in many cases more formal cardiac output monitoring is required (see Chapter 10). Most causes of hypotension can be considered as a problem with one of the following: preload, contractility and afterload. Typically, the clinical history may suggest a contribution from all three; therefore, a more objective method of assessment is required. This may include:
Stroke volume variation (SVV) – a high SVV suggests decreased preload. Fluids are most likely to be required here.
Systemic vascular resistance (SVR) – a low SVR suggests vasodilatation +/− vasoplegia. Common causes for this are sepsis and drugs. Many anaesthetic and sedative agents are implicated here, including central neuraxial blockade. Treatment is with a vasopressor such as noradrenaline or vasopressin. Adrenaline also has vasopressor activity, although would not be a first-line agent because of its potent beta activity.
Cardiac index – a low cardiac index implies that the cardiac output is insufficient for the body surface area. This may be due to a low heart rate or stroke volume. Bradycardias are easily diagnosed and treated. If a low stroke volume is not the result of a reduced preload, the treatment is with an inotrope. Examples include adrenaline and dobutamine. Incidentally, an extreme tachycardia can also cause hypotension by means of a reduction in stroke volume (as a result of a low left ventricular end-diastolic volume due to a reduced filling time). Milrinone and enoximone are also inotropes. They increase the stroke volume for a given preload. However, they do not increase the afterload. Dobutamine is a commonly used inotrope in the intensive care unit, which also increases contractility but can also decrease SVR.
Further reading
Sedation in the intensive care unit
Sedation is the reduction of stress, anxiety, irritability or excitement by administration of a sedative agent or drug. A combination of pharmacological and non-pharmacological therapies are commonly adopted on intensive care units (ICUs). Sedation is required to facilitate certain procedures within the unit and maximize successful patient outcome.
Consider the following true/false questions:
a) Benzodiazepines are related to a higher rate of ICU delirium
b) When used as an infusion, alfentanil has been shown to reduce length of stay when compared to fentanyl
c) Remifentanil produces tachyphylaxis
d) Clonidine and dexmedetomidine are both alpha-2 agonists
e) Under-sedation has been shown to produce higher rates of ICU delirium
Answers: TTTTF
Over-sedation is a major problem, and maintenance of lighter sedation in patients who are otherwise stable has been shown to reduce length of stay, whilst decreasing mortality and morbidity. Sedation should be titrated so that the patient receives the minimum effective dose that ensures safety and comfort. ICU delirium has been associated with higher sedation levels.
There are different tools available to measure the depth of sedation on the ICU, and no tool has been shown to be superior. However, it makes sense that units adopt a single scoring tool to avoid confusion between patients and staff. Some of the sedation scores you may encounter are:
Sedation-Agitation Scale (Riker)
The Ramsay Sedation Scale
The Richmond Agitation-Sedation Scale (RASS)
The Motor Activity Assessment Scale.
The RASS has been demonstrated to be simple to perform and reproduce. Patients are scored from −5, which is unresponsive to physical stimulus, through 0, which is alert and calm, to +4, which is combative and an immediate danger to staff. Appropriate levels (when neuromuscular blockade is not required) are −2 to +1. The RASS score makes up part of the Confusion Assessment Method for the ICU (CAM-ICU) delirium screen.
Delirium is defined as an acute confusional state and includes:
Disturbance of consciousness
Change in cognition
Development over a short time period
Evidence there is a pathophysiological cause related to a general medical or surgical condition.
There are three subtypes: hyperactive, hypoactive and mixed, where the state fluctuates between the former two. The incidence is somewhere between 22% and 81% in intensive care patients and it is associated with significantly higher mortality and increased length of stay.
Table 12.3 shows some of the advantages and disadvantages of commonly used drugs in providing sedation and treating delirium.
Drugs commonly used to provide sedation or treat delirium
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