The patient is hypotensive:  is this due to  Hypovolemia?

      
       

 

          
       

What is the patient’s Volume Status?

When approaching hemodynamic monitoring, it is important to decide what it is you wish to measure and what you will do with the information. There is inherent risk with the insertion of any type of monitor.

Arterial lines are inserted to measure beat-to-beat variations in blood pressure, and to sample arterial blood gases in order to guide resuscitation and ventilation.

Central venous pressure monitors are inserted to measure systemic blood volume, venous return and right heart filling pressures. This is sufficient to guide fluid therapy in the majority of patients. There is no normal or average CVP number: the measurement is determined by venous return (influenced by intrathoracic pressure) and right ventricular compliance. Complete heart block, atrial fibrillation, tricuspid stenosis and regurgitation will lead to an inaccurate reading: although the diagnosis of these disorders can be made from the CVP waveform. The central venous pressure should be regarded as a trend. It is conventional to volume load an under-resuscitated patient to a target CVP: I use 8 – 10 mmHg if the non-ventilated patient, and 12 – 16 mmHg, if the patient is on positive pressure ventilation. If there is a question of cardiac disease, cardiac hypertrophy or dilatation or if the patient is middle aged or older, I aim higher – 16 mmHg plus. In many young patients, it is often not possible to raise the CVP above 10 mmHg, such is the efficiency of the cardiovascular system.

The method of using CVP monitors is simple: volume load until the patient reaches the target CVP and stays there.

In the diagram above, the patient has been given three boluses of fluid. Bolus 1 does not budge the CVP measurement: the patient remains under-resuscitated. Bolus 2 raises the CVP up to the target range, but it falls down again, due to redistribution. Following bolus 3, the CVP rises to the target range and stays there: fluid loading is complete.

Pulmonary artery catheters are inserted in patients with cardiac dysfunction to measure left sided filling pressures, cardiac output/stroke volume and mixed venous oxygen saturation. Stroke volume can be measure less invasively with esophageal doppler monitoring, lithium dilution or CO2 rebreathing (Fick principle).

Most critically ill patients require an arterial line and a central venous pressure monitor. Unless there is a strong suspicion that right sided filling pressures are unrepresentative of left sided ones, there is no need to insert a pulmonary artery catheter (PAC) during initial resuscitative efforts. Many physicians misinterpret the objectives of right heart catheterization: left sided filling pressures can only be used to gauge the relationship between the pulmonary circulation and the heart. Right atrial pressures are more representative of systemic vascular volume. Indeed with pulmonary hypertension, the use of left sided pressures may seriously overestimate the systemic blood volume. The purpose of PACs is to construct Starling (pressure-volume) curves of the left ventricle, to determine the end diastolic volume pressure relationship that optimizes stroke volume. The left ventricular end diastolic pressure is not measured directly, but through a surrogate – the pulmonary capillary wedge pressure (PCWP). This is obtained by floating a balloon tipped catheter in, through the right heart, to the lower zone of the lung (where the influence of alveolar pressure is minimized), until the balloon occludes a small branch of the pulmonary artery. Beyond this blood will not flow. This column of blood thus acts as a manometer, directly measuring left atrial pressure. From the mean “wedge” pressure, the left ventricular end diastolic volume (LVEDV) can usually be accurately inferred.

If for any reason there is loss of continuity between the pulmonary circulation and  the left ventricle, a misleading result will be obtained. Examples of this are left ventricular inflow obstruction – mitral stenosis or pulmonary embolism. Likewise, a regurgitant mitral valve will overestimate cardiac output/stroke volume. Under these circumstances echocardiographic examination would be more useful.

If the heart is particularly non compliant (diastolic dysfunction), the “wedge” tends to underestimate the LVEDV. Similarly very high intrathoracic or transalveolar pressures may overestimate the “wedge’. The significance of this is probably overvalued, as the PCWP, like the CVP, should be followed as a trend rather than as an absolute number.

Many ICU physicians are less than pragmatic in their use of these devices; efficacy of right heart catheterization has not been established by randomized controlled trials. PACs are not indicated if the patient has a normal heart, nor are they of particular use in guiding fluid therapy in renal failure. The CVP measures systemic blood volume, the arterial line measures blood pressure. If the patient has a blood pressure within their normal autoregulation range and has a generous CVP, there is little to be gained by using a PAC. If cardiac performance information is required, then echocardiography will give you an accurate estimate of ejection fraction. The bottom line is that nobody “needs a Swan”.

 

 

Using stroke volume monitors

There is no doubt that information about stroke volume (SV) is useful in most resuscitation situations: the SV can be used to titrate filling pressures against cardiac performance. In many ICU situations, particularly sepsis, the SV is reduced, and using a CVP line alone is a best guess. This is the reason many physicians use PACs. There are a number of alternative methods of measuring stroke volume available: the esophageal doppler monitor, the NiCO monitor (CO2 rebreathing), lithium dilution and transesophageal echocardiography. It is important to be aware of these options, as they will become more prevalent in future times.

The diagram above plots stroke volume against filling pressure for a given ICU patient. Note that the optimal filling pressure in this patient is that which achieves the target stroke volume. Further filling, in this patient, leads to excessive stretch to the ventricular muscle and reduced stroke volume. Other patients, with normal hearts may be able to tolerate higher stroke volumes, but there is little to be gained from this outcome.

In figure 3 you can see the effect of the volume loading described for CVP in the first diagram. In this case the SV is being measured, and the CVP targeted in the range of 60 to 70ml, which, in this case, is 16cmH20. For the vast majority of cases, it does not matter which pressure is used for filling – CVP, PCWP, LVEDP. However, if there is significant right-left disassociation such as right or left ventricular infarction/contusion, then left sided pressures should be measured.

Which “pressor do I choose”?

  • If you are sure that this is septic shock - use norepinephrine
  • If this is anesthesia related or spinal shock (and the patient does not have bradycardia) – use phenylephrine.
  • If this is cardiogenic shock – use dobutamine norepinephrine or an intra-aortic balloon pump.
  • If this is anaphylactic shock – use epinephrine.
  • If the patient is bradycardic and hypotensive – use epinephrine.
  • If you don’t know what is causing the shock – use epinephrine or dopamine.
  • If the patient has a history of ischemic heart disease, left ventricular hypertrophy, or a myocardial ischemic event, the CVP may not be a reliable indicator of left sided pressures, and a pulmonary artery catheter (PAC) is indicated.
          
                   
       

         
     

       
       

Please note: these tutorials are for personal study purposes only.  They are not currently peer reviewed, and no responsibility will be taken for mistakes or inaccuracies. Reproduction of information is forbidden. All material is copyrighted by the GasWorks Group.