When the body’s oxygen balance is such that oxygen demand exceeds oxygen supply, cells become hypoxic and convert to anaerobic metabolism. Lactic acid (lactate) is a by-product of anaerobic metabolism and can be measured in the blood. Elevated lactate levels are associated with tissue hypoperfusion and poor oxygenation. Although other factors can affect lactate levels, the presence of elevated lactate can therefore be used as an indirect marker of poor tissue perfusion and shock.

When a central venous catheter is in place, blood can be drawn from the superior vena cava (distal port) and sent to the lab for measurement of central venous blood oxygen saturation. Central venous blood oxygen saturation (ScvO₂) correlates well with mixed venous blood oxygen saturation (SvO₂) in most circumstances, and can be used to reflect tissue oxygenation. Normal ScvO₂ is approximately 70 % (compared to normal SvO₂ of approximately 65 %). Lower than normal ScvO₂ or SvO₂ is an indication of poor tissue oxygenation and the need for improved oxygen delivery and perfusion. The advantage of using ScvO₂ is that it does not require a pulmonary artery catheter (versus SvO₂ which is drawn from the pulmonary artery).

The goals of hemodynamic monitoring in the critically ill patient are to optimize perfusion and oxygen delivery to tissues, ensure rapid detection of changes in clinical status, and monitor for response to treatment. Although noninvasive monitors (such as a blood pressure cuff) are associated with less risks and complications, it is often necessary to use invasive monitoring techniques to achieve these goals.

A common cause of admission to the intensive care unit is hypotension, which may be due to any number of etiologies (see “Shock” section below). Blood pressure monitoring with a noninvasive cuff may be adequate, but if the blood pressure is significantly low, it may be undetectable or inaccurate by a cuff. In addition to being the most accurate form of blood pressure monitoring, arterial cannulation allows continuous beat-to-beat monitoring. It also serves as a site for obtaining lab measurements of oxygenation, ventilation, and acid–base status. The most common sites for arterial cannulation are radial or femoral arteries, but other arteries may be used if necessary (see Chap. 15, IV, Arterial & Central Line and Gastric Tube Placement Techniques).

Complications associated with arterial cannulation and precautions to decrease the incidence of complications are listed in Table 29.1.

Volume status can be assessed by evaluating the arterial pressure height during controlled mechanical ventilation . Positive pressure ventilation will lead to significant systolic variation (>10 mmHg) of the blood pressure in patients who are hypovolemic (Fig. 29.2).

Recall that global oxygen delivery (DO₂) to the tissues is dependent on the oxygen content of blood (CaO₂) as well as cardiac output (CO) . Cardiac output is equal to the product of heart rate (HR) and stroke volume (SV):
The variables that affect stroke volume include preload, afterload, and contractility. Preload is an estimate of left ventricular volume at the end of diastole. The Frank-Starling curve shows the relationship between preload and stroke volume (Fig. 29.3). In general, increases in preload lead to greater stroke volume. However, a point on the Frank-Starling curve is eventually reached where further increases in preload do not increase stroke volume and may instead lead to decreased stroke volume (as in congestive heart failure). Because it is difficult to measure ventricular volume, ventricular pressure is commonly used to estimate volume and thus preload. Use of a central venous catheter enables monitoring of right atrial pressure or central venous pressure (CVP), which is an estimate of right ventricular preload. In a patient without significant pulmonary hypertension or valvular disease, it can be assumed that right ventricular preload correlates with left ventricular preload because the same blood volume that enters the right heart will traverse to enter the left heart. By way of this assumption, CVP is often used as an estimate of left ventricular preload.

Afterload refers to the myocardial wall tension that is required to overcome the opposing resistance to blood ejection. Right ventricular afterload is clinically represented by the pulmonary vascular resistance (PVR) and left ventricular afterload is clinically represented by the systemic vascular resistance (SVR). SVR may be calculated from the following equation when cardiac output measurements are obtained:
where MAP = mean arterial pressure.

Contractility refers to the ability of the myocardium to contract and eject blood from the ventricle. Contractility depends on preload and afterload so these variables should be optimized first in order to improve contractility. Contractility can be directly measured with the use of echocardiography to estimate ejection fraction. However, once preload and afterload are optimized, contractility is often indirectly represented by cardiac output. If cardiac output remains low despite improvements in preload and afterload, the use of inotropic pharmacologic agents may be initiated to improve contractility.

As described above, invasive CVP monitoring allows continuous measurement of right heart pressures, which can be used to reflect preload. Normal CVP during positive pressure ventilation ranges from 6 to 12 mmHg. A low CVP with hypotension and tachycardia most often corresponds to hypovolemia. Persistent hypotension following a fluid challenge and higher than normal CVP indicates cardiac congestion (as may occur with cardiac tamponade, tension pneumothorax, or myocardial ischemia).

Cannulation sites for CVP placement include subclavian, internal jugular, and femoral veins (also see Chap. 15, IV, Arterial & Central Line and Gastric Tube Placement Techniques). Complications associated with the placement of a central line are presented in Table 29.2. To reduce the number of cannulation attempts and the risk of inadvertent arterial puncture, direct visualization with ultrasound guidance should be used when possible during placement of a central line.

As described above, left heart pressures may be estimated from right heart pressures in most circumstances and CVP may be used to approximate pulmonary capillary wedge pressure (PCWP). However, when left ventricular function is impaired, or significant valvular disease or pulmonary hypertension is present, the use of a pulmonary artery catheter (PAC) may be indicated for more accurate estimations of left heart pressures. Use of a PAC allows continuous monitoring of pulmonary artery pressures, intermittent monitoring of PCWP, and thermodilution for estimation of cardiac output and calculation of systemic vascular resistance. PCWP is used as the best estimation of left ventricular end-diastolic volume (preload), analogous to CVP estimation for the right ventricle.

The PAC can also be used to obtain blood samples for mixed venous oxygen saturation (SvO₂) in order to evaluate oxygen balance. Risks associated with PAC placement include those associated with central line placement as well as additional risks (Table 29.2). Table 29.4 in the next section shows how the use of a PAC can help in the determination of common hemodynamic disturbances in shock.