Central Venous Pressure monitoring is a frequently done procedure in many hospitals. There are many conflicting and poorly understood principles and practices surrounding the measurement of Central Venous Pressures. This article is written by a Lecturer in Health Sciences and helps in explaining some of the "not so clear issues". This article is for the benefit of student nurses and registered nurses.
Central Venous Pressure (CVP for short) is defined as the pressure of blood in the thoracic vena cava just before it (the blood) enters the right atrium of the heart. Normal CVP is 5 to 10 cm H2O.
CVP measurements are important in clinical cardiology because the CVP is a major determinant of the filling pressure of the right ventricle of the heart. The filling pressure of the right ventricle determines the stroke volume i.e. the amount of blood pumped with each contraction of the heart.
Background: Central Venous Pressure is an accurate indicator of the amount of blood returning to the heart from the head, body and limbs via the superior and inferior vena cava. If and when there is blood loss then the CVP reading will be altered (will fall) almost immediately as the amount of blood returning to the heart will have decreased. Central Venous Pressure is also an accurate indicator of the ability of the heart (myocardial pump strength) to pump out blood to maintain normal blood pressure and tissue perfusion. Last but not least, the CVP is an accurate indicator of right ventricular end diastolic volume. In most institutions CVP is measured in cm of water (H2O). On this scale the normal value of CVP is 5 to 10 cm H2O. Some, (very few) institutions measure CVP in mm. Hg (millimetres of mercury). On this scale the normal value is approximately 4 to 8 mm Hg.
Central Venous Pressure in measured using a sterile indwelling central venous catheter (CVC). One end of the CVC is attached to a manometer or an electronic transducer, computer and monitor. Ultrasound may be used to guide CVC insertion. In the facility where I used to work experienced practitioners went in blind. Usually they were successful in locating the desired blood vessels without difficulty.
Central Venous Pressure monitoring is more accurate then blood pressure monitoring because changes in circulating volume will be reflected in changes in CVP values as soon as there is blood loss. Nurses and Nursing students will already know that in the first stage of shock, following blood loss, the compensatory mechanisms “adjust the blood pressure to normal levels”. Consequently, blood pressure readings will remain within the normal range after blood loss but not the CVP.
When there is overloading of the circulatory system or there is heart failure the CVP rises. However, when there is dehydration (e.g. diabetes insipidus), fluid loss due to bleeding or shifting of fluids within the body compartments (e.g. shock) then the CVP will fall. We have studied that “fluid challenges” in the early stages of shock can prevent shock and subsequent death of a patient.