Rvswi

1.36 (PAP - CVP) SI/100

5-10 g m/m2

BSA, body surface area; CI, cardiac index; CO, cardiac output; CVP, central venous pressure; HR, heart rate; LVSWI, left ventricular stroke work index; MAP, mean arterial pressure; PAP, pulmonary artery pressure; PCWP, pulmonary capillary wedge pressure; PVR, pulmonary vascular resistance; PVRI, pulmonary vascular resistance index; RVSWI, right ventricular stroke work index; SI, stroke index; SV, stroke volume; SVR, systemic vascular resistance; SVRI, systemic vascular resistance index.

Fig. 14. High-pressure flush test. 1, In an optimally damped pressure-monitoring system, the pressure wave returns to baseline after one oscillation. 2, In an overdamped system, the wave returns to baseline without any oscillations. The systolic blood pressure is underestimated and diastolic pressure overestimated. 3, In an underdamped system, the wave oscillates multiple times before returning to baseline. The arterial wave is amplified. The systolic pressure is overestimated and diastolic pressure underestimated. The mean arterial pressure usually is not significantly affected by overdamping or underdamping.

There are multiple types of central venous catheters, ranging from a single lumen to multiple lumen (double, triple, quad lumens) catheters. Typically, multilumen catheters have slower flow rates because of the smaller radii of incorporated lumens. Recall that resistance to flow is proportional to the fourth power of the radius.

After placement of a central venous pressure catheter, all ports must be aspirated and flushed to confirm proper intra-vascular placement. In clinical practice, a chest x-ray is often obtained to confirm proper positioning of such a catheter. If a pneumothorax develops after accidental puncture of a lung, it will also be evident on chest x-ray.

The central venous pressure waveform gives important information about the mechanical events occurring during a cardiac cycle (Fig. 15). An a wave is caused by atrial contrac-

BSA, body surface area; CI, cardiac index; CO, cardiac output; CVP, central venous pressure; HR, heart rate; LVSWI, left ventricular stroke work index; MAP, mean arterial pressure; PAP, pulmonary artery pressure; PCWP, pulmonary capillary wedge pressure; PVR, pulmonary vascular resistance; PVRI, pulmonary vascular resistance index; RVSWI, right ventricular stroke work index; SI, stroke index; SV, stroke volume; SVR, systemic vascular resistance; SVRI, systemic vascular resistance index.

tion that occurs after the P wave on the ECG. The c wave occurs during the start of ventricular systole as the tricuspid valve is pushed up toward the right atrium. The next portion of the waveform is the x descent, which represents the tricuspid valve being pulled down toward the right ventricle in late systole. The v wave correlates with passive filling of the right atrium while the tricuspid valve is closed. The _y descent completes the waveform and represents the opening of the tricuspid valve, passive emptying of the right atrium, and filling of the right ventricle during diastole.

The central venous pressure waveform provides information primarily concerning the right heart. Yet, the same waveform can be observed for the left heart by recording the pulmonary capillary wedge pressure from a pulmonary artery catheter (discussed in Section 10). The central venous pressure waveform is affected by respirations; thus, it should be read at end expiration. The central venous pressure value is typically defined as the mean venous pressure at the end of exhalation during spontaneous or controlled ventilation. At end expiration, the intrathoracic pressure is closest to atmospheric pressure.

There are multiple clinical conditions that will affect the recorded central venous pressure waveform. For example, tricuspid stenosis may result in large ("cannon") a waves

Table 4

Relative Indications for Central Venous Pressure Catheter

• Large fluid shifts

• Vascular access

• Infusion of medication

• Venous blood sampling

• Major trauma and surgery

• Monitoring of intravascular volume status

• Aspiration of venous air embolus

Table 5

Relative Indications for Pulmonary Artery Catheter

• Major organ transplant (liver, heart, lung)

• Cardiopulmonary bypass surgery

• Pulmonary hypertension

• Aortic aneurysm surgery

• Heart failure (right and/or left heart)

• Pulmonary embolus

See ASA Guidelines for more detailed indications and contraindications (35).

(Fig. 16) as the right atrium contracts and pushes blood past a stenotic valve. Abnormal cardiac nodal rhythms, ventricular arrhythmias, or heart block will result in cannon a waves because the atrium and ventricle are not synchronized and the atrium may be contracting against a closed tricuspid valve. Large a waves may also occur when the resistance to right atrium emptying is significantly increased, as in tricuspid and pulmonary valve stenosis, right ventricular hypertrophy, or pulmonary artery hypertension.

Regurgitant valve disorders such as tricuspid regurgitation will result in large v waves (Fig. 17), representing overfilling of the atrium. Specifically, the large v wave occurs as blood volume from the right ventricle, during systole, backflows into the right atrium past the incompetent tricuspid valve. A noncompliant right ventricle, as in ischemia and heart failure, may also result in large v waves. During atrial fibrillation, a waves are absent because of ineffective atrial contractions. Again, similar waveforms for the left heart are seen from a pulmonary capillary wedge pressure waveform. Diagrams of cannon a and v waves are displayed in Figs. 16 and 17.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment