Central venous pressure (CVP) is an estimate of the blood pressure in the right atrium by measuring the pressure in the cranial (or caudal) vena cava (Moses and Curran, 2007). It evaluates three things: the heart's ability to function as a pump; blood volume in relation to volume capacity; and vasomotor tone (indirectly) (Hughes, 2005). CVP measurement is indicated when trying to determine fluid resuscitation end points, as a way of assessing fluid overload, or when heart failure is suspected. CVP measurements can be taken intermittently with a water manometer or continuously with a pressure transducer and patient monitor.
A catheter placed in the cranial or caudal vena cava via the jugular or lateral/medial saphenous vein respectively is required for measurement of CVP. Peripheral venous pressure bears no correlation to CVP. Care must be taken when measuring CVP as consistency is important. For this reason, ideally the same person should take the measurements during the shift; if the patient cooperates all measurements should be taken in the same position.
In addition to administration of fluids and medications these catheters can also be used for frequent blood sampling and administration of parenteral nutrition and potentially irritant solutions.
When the catheter tip is in the intrathoracic venous vasculature, changes in intrathoracic pressure may also affect the CVP reading so should be interpreted with caution in dyspnoeic patients or those on positive pressure ventilation. As the venous circulation is a low pressure system with minimal autoreg-ulation, CVP is generally a better indicator of intravascular volume status than arterial blood pressure (Hughes, 2005). Patients with hypoperfusion secondary to hypovolaemia are likely to have a low CVP and changes in CVP can be used to help guide therapy.
Indications
Central venous pressure (CVP) is very useful for monitoring the effects of fluid therapy in critical patients. It is used to detect hypovolaemia and hypervolaemia, which is particularly important when dealing with shocked patients and patients with cardiac and/or renal disease. CVP is especially useful in those patients who are at risk of volume overload, notably patients with anuric renal failure, heart disease or parenchymal lung disease (Waddell, 2010).
The measurement of CVP is a relatively simple technique that consists of placing a central catheter into the jugular vein, this catheter can also be used for fluid therapy, medication administration, blood sampling etc.
Placement of central catheter
The jugular vein always lies along a line drawn between the angle of the mandible and the thoracic inlet. It is important to extend the neck and to minimize motion to avoid moving the anatomical landmarks once the course of the vein has been identified. The jugular vein is distended by having an assistant occlude the vein at the thoracic inlet. A small skin incision is especially helpful when attempting jugular venous access due to the thickness of the skin in the neck region (facilitative incision). Catheter types available for central venous access include through-the-needle catheters, long, over-the-needle catheters, and single or multi-lumen catheters (Figure 1) placed using the Seldinger technique. Central venous catheters can be placed via percutaneous, facilitated percutaneous (where by a facilitative incision is made through the dermis of the skin), or surgical cut down techniques (Figure 2). The jugular vein is used most often, although alternative sites include the femoral vein, maxillary vein, and medial saphenous vein. Central venous access can also be established via peripheral veins using long intravenous catheters (termed peripherally inserted central venous catheters (PICC)).
Equipment
See the Step-by step guide for placement technique.
Asepsis
Central catheters should always be placed under aseptic conditions. The operator should be surgically prepped, wearing surgical gloves and ideally a gown. The proposed catheter placement site should be clipped and prepped aseptically as for surgery, and the field should be draped. When disconnecting central catheters for connection to a CVP manometer or multiparameter monitor, again asepsis should be maintained. The operator should ideally don surgical gloves but certainly examination gloves to prevent iatrogenic contamination of the central line.
Continuous measurement
Once the central venous catheter is in place the catheter is attached to an electronic transducer which converts pressure into an electric impulse which can be displayed in the form of a continuous electronic waveform in a patient monitoring device (Figure 3). The pressure transducer is placed at the level of the right atrium and zeroed. Placement of the transducer above or below the level of the right atrium can result in an inaccurately low or high CVP respectively.
Normal CVP ranges from 0–5 cmH2O. Most monitors give values in mmHg, so a conversion factor of 1.36 (x mmHg/1.36 = y cmH2O) is used. Most consider CVP to be of most value when used for ‘trending’ (Steele, 2006). Trending looks at an overall change in CVP over time, and because there can be instantaneous changes in CVP, using continuous readings may provide a more accurate picture. The CVP waveform provides a graphical display of five distinct events occurring within the heart. It is important to assess the waveform for the peaks and descents associated with these events, as a waveform without these components may indicate poor placement of the catheter, and therefore inaccurate results. In general the three waves a, c, and v, represent increased pressure within the heart as blood fills the chambers (Figure 4). The descents, x and y, represent pressure decreases within the heart during the cardiac cycle (Steele, 2006).
Normal values
The normal CVP range is 0–10 cm H2O (Hopper, 2006). A CVP less than zero may be due to vasodilation (increased volume capacitance) or hypovolaemia. A CVP in a normal range but in the face of signs consistent with vasoconstriction may be due to hypovolaemia. A CVP greater than 10 may be due to the heart's inability to function as an effective/efficient pump or fluid over-load, vasoconstriction (decreased volume capacitance), pericardial effusion and positive pressure ventilation.
Interpretation of CVP
CVP is frequently used as a tool to guide volume resuscitation. This can be accomplished by measuring CVP before and after a fluid bolus. Typically, an increase in CVP of 2–4 cmH2O then returning to baseline within 15 minutes indicates a normovolaemic patient. A minimal or absent increase in CVP indicates a severely hypovolaemic patient requiring further resuscitation. An increase in CVP with a rapid decline indicates continuing hypovolaemia requiring further resuscitation. A high baseline CVP (often greater than 16 cmH2O) can indicate severe hypervolaemia (as a result of volume overload), cardiac tamponade, restrictive pericarditis and right-sided heart failure (Steele, 2006).
Contraindications
Prior to placement of a central venous catheter, the haemostatic status of the patient should be assessed. Patients with coagulopathy, thrombocytopenia, or thrombocytopathia can experience potentially life-threatening haemorrhage following jugular venous catheterization. Due to the distensible nature of the subcutaneous tissues of the neck and the direct communication with the cranial mediastinum, it can be extremely difficult to control bleeding in this area without surgical intervention. Ideally, the prothrombin time, activated partial thromboplastin time, platelet count and buccal mucosal bleeding time should be assessed. An activated clotting time and a blood smear to estimate platelet numbers may suffice in the emergency situation. A buccal mucosal bleeding time should be performed in patients at higher risk for thrombocytopathia, e.g. in breeds with a high incidence of von Willebrand's disease, in animals receiving non-steroidal anti-inflammatory medications, or in azotaemic patients (Hughes, 2005). In contrast, patients with disease processes associated with hypercoagulability, such as haemolytic anaemia, Cushing's disease, Parvoviral enteritis, or protein-losing nephropathy, may be at an increased risk for developing local thrombosis and the complications that may cause pulmonary thromboembolism following central venous catheterization (Hughes, 2005).
Conclusion
CVP measurement can be used in a variety of situations to assist in diagnosis and optimal fluid therapy management. It is important to obtain CVP measurements in as technically precise a manner as possible, and to obtain consecutive measurements with the patient in the same position each time to ensure consistency. As with any monitoring tool, CVP measurements must be interpreted in light of other diagnostic findings.