“Central venous pressure measurement is not a surrogate of intravascular volume or ventricular preload for fluid resuscitation. For this, fluid responsiveness has to be assessed.”
This discussion is for physiological
purpose only.
Invasive pressure
monitoring of central venous pressure, is measurement of intramural pressure
(Pim) of the vessel.
Flow across a
vessel is a function of intramural pressure gradient and resistance to the
flow.
Force driving flow (F) = ∆P/ R
According to Poiseuille equation resistance is inversely
proportional to fourth power of radius.
(ⁿ= viscosity of fluid, L= length of
tube, r= radius of tube)
Radius of a
distensible tube depends on transmural pressure (Ptm), which is the difference
between intramural pressure and surrounding pressure (Psur).
Ptm = Pim – Psur
Surrounding
pressure for intrathoracic vascular structure is intrathoracic pressure, which
is pleural pressure (Ppl).
In a
collapsible tube, if volume is not allowed to change, then change in surrounding
pressure (Psur) will bring about similar change in intramural pressure (Pim),
so that transmural pressure (Ptm) remains unchanged. As volume will remain
unchanged only if transmural pressure remains constant.
Thus in
positive pressure ventilation changes in pleural pressure will bring about
similar change in intramural pressure of central vein, and this intramural
pressure is measured as central venous pressure.
Pleura space
is a potential space between lung and chest wall, lined by visceral and
parietal pleura respectively. Pleural pressure is a function of elastance/
compliance of lung and chest wall, which act in opposite direction.
Compliance and
elastance are inverse of each other.
In positive
pressure ventilation, relationship between applied airway pressure (PEEP) and
pleural pressure is dependent upon the compliance/ elastance of the lung and
chest wall.
If lung compliance is low, the end expiratory lung volume (EELV) achieved by applied airway pressure (PEEP will also be low and therefore the change in pleural pressure will also be low.
So, worse the
lung compliance and greater the chest wall compliance, lower will be the change
in pleural pressure for an applied airway pressure (PEEP). Therefore stiff
lungs with good chest wall compliance (ARDS/ ILD lung in a thin individual)
will have minimal change in pleural pressure with increasing airway pressure.
On the other
hand, in patients with normal compliant lung with stiff chest wall (obese or abdominal
compartment syndrome), pleural pressure will change markedly with applied
airway pressure (PEEP).
Now let us calculate
the changes in pleural pressure with applied airway pressure.
A young,
thin patient with ARDS is being ventilated with a PEEP of 16 cmH2O. In this patient lung compliance is 30 ml/ cmH2O
(markedly decreased) and chest wall compliance is 100 ml/ cmH2O(normal).
At end
expiration the lung volume (end expiratory lung volume- EELV) will be 480 ml.
EELV = Applied pressure (PEEP)
× Lung Compliance
EELV = 16 ×
30 = 480 ml
Resulting
pleural pressure will be the pressure applied by this lung volume in overcoming
the elastance of chest wall.
Plerual pressure = EELV / Chest wall
Compliance
Pleural pressure = 480 / 100 = 4.8 cmH2O
= 3.5 mmHg.
Thus if
measured CVP is 18 mmHG, real CVP will be 15.5 mmHg (18-3.5), an insignificant difference.
Now let us
consider an young, obese patient with pulmonary hypertension, ventilated with a
PEEP of 10 cmH2O. In this patient lung compliance is normal (100 ml/ cmH2O) but chest
wall compliance is low (60 ml/cmH2O) because of obesity.
With similar
calculation the EELV will be 1000 ml and pleural pressure will be 16 cmH2O (12
mmHg).
If measured
CVP is 16 mmHg, real CVP will be 4 mmHg (16-12), a very significant difference.
Therefore
changes in CVP, brought about by applied PEEP is a function of relative compliance
of the lung and chest wall.
Stiff lung ventilated
with high PEEP, will not result in significant alteration in measured CVP.
Whereas
stiff chest wall with normal lung, ventilated with high PEEP, will result in
marked change in measured CVP.
High PEEP is
needed in stiff lung, stiff chest wall (obesity or abdominal compartment syndrome)
and cardiogenic shock (to reduce afterload).
This is very nice blog. I am impressed. Book online to Consult Critical Care Doctors at Connect2mydoctor. For more info: https://www.connect2mydoctor.com/
ReplyDelete