AMP 11: Central Venous Pressure Monitoring Flashcards
What is central venous pressure primarily determined by?
- venous return
- right heart function
What determines venous return?
- venous tone
- venous wall compliance
- circulating blood volume
What determines right-sided cardiac output?
- heart rate
- preload
- afterload
- contractility
How does a rise in intrathoracic pressure affect the CVP?
Will compress the vena cava and right atrium and raise CVP
List complications associated of central venous catheters use
- thrombosis
- thromboembolism
- carotid arterial puncture
- infection
- phlebitis
- bleeding
- vascular erosion
- cardiac arrhythmias
- cardiac wall perforation
List complications associated with CVP measurements - excluding CVC use itself
- introduction of air - air embolus
- introduction of bacteria
What is the reference range for CVP?
0-5 cm H2O most commonly cited. Up to 10 cm H2O can be normal
What are the 2 units used for CVP measurements and what is the conversion factor for them?
mm Hg and cm H2O
CVP in cm H2O = CVP in mm Hg x 1.36
How different are pressure measurements between the vena cava and the right atrium?
Pressure measurements between the vena cava and the right atrium are generally considered interchangable
untrue if:
- obstruction between vena cava and right atrium
- vasopressors administered
- fluid bolus administered
How well can peripherally measured venous pressure be used to estimate CVP in dogs and cats?
peripheral venous and CVP has shown poor correlation in dogs and cats
can be used in humans as an estimate for CVP
How do high fluid rates through a proximal port of a double lumen central catheter affect CVP measurements at the distal port?
human studies showed that even pressurized high fluid rates (>9,000 mL/hour) do not affect CVP measurements
this can likely be extrapolated to veterinary medicine
what is the normal peak pressure in the right ventricle
peak right ventricular pressures reach 20-30 mm Hg in the normal animal
What are indications that the central catheter may have been advanced into the right ventricle.
- arrhythmias (can also happen in RA though)
- high pressure readings (up to 20-30 mm Hg)
- water manometer: extreme fluid column fluctuations
- continuous electronic monitor: change in waveform
Briefly explain what intermittent CVP measurement entails. What are pros and cons?
- the central catheter is connected to a water manometer.
- the manometer 0 line is positioned at the same height as the right atrium
- a predetermined volume of saline is infused
- fluid is allowed to equillibrate with patient’s CVP
- can read CVP by reading the height of the fluid column
- recorded in cm H2O
pros: cheap, easy to perform
cons: time-consuming if repeated measurements are needed
Briefly explain what continuous CVP measurement entails. What are pros and cons?
- the central catheter is connected to a pressure transducer
- the venous pressure wave is converted into electrical signals
- venous pressure wave form can be displayed on monitor in real time ⇒ in mm Hg
pros: better monitoring through real-time cont. measurements, no need for repeated interventions once set up
cons: expensive, difficult to set up/requires more skills
What is the atmospheric pressure at sea level?
760 mm Hg
Explain zeroing
atmospheric pressure is assumed to excert same pressure on every object (measuring device, RA, veins, etc.) and can therefore be zeroed out for simplification = atmospheric pressure considered as 0 mm Hg
The process of correcting for atmospheric pressure is called zeroing
pressure transducer systems have ports/stopcocks that can be opened to the atmospheric pressure while leveling this port/ or manometer to the zero reference point
zero reference point = the vascular structure in question, i.e., here RA
What is the level for zeroing in animals in either sternal or lateral recumbency?
want to level manometer or stopcock with the RA
- in lateral recumbency: level of the sternum
- in sternal recumbency: line drawn caudal from shoulder from dorsal spinous process to sternum, at 40% of the height of this line
How does inaccurate determination of the zero reference point affect the CVP measurement?
too low will read a falsely elevated CVP
too high will read a falsely decreased CVP
In the spontaneously breathing animal, how does respiration affect intermittend CVP measurements and when should the CVP be read?
- The CVP will decrease during inspiration and increase during expiration
- the CVP should be read at the end of expiration
- if there is pronounced abdominal effort the CVP should be read at the beginning of the expiratory phase (before abdominal effort sets in)
Why do we measure CVP and what can it be an indicator for?
CVP can be an estimate for RA pressure, which is a major determinant for end-diastolic RV pressure (end-diastolic because at this point the tricuspid valve has been opened long enough for pressures between RA and RV to equillibrate)
RV end-diastolic pressure is related to RV end-diastolic volume, which determines end-diastolic myocardial wall stretch, which determines preload
therefore CVP can be an indirect guide for assessing preload and the patient’s volume status
What is the driving pressure?
the difference between peripehral and central venous pressure, which determines venous return, i.e., adequate pressure gradient for blood flow
What is the average peripheral venous blood pressure?
5-10 mm Hg greater than the pressure within the central veins
The venous blood acts as a blood reservoir, what percentage of blood is in the venous circulation?
~ 65 %
what part of the circulation acts as capacitance vessels and how does it respond to hypovolemia?
the splanchnic circulation acts as capacitance vessels and are capable of significant adjustments of wall compliance to adjust for changes in circulating blood volume
hypovolemia leads to constriction of the splanchnic veins which increases the pool of circulating blood and increases venous return
What does a rising CVP with concurrent evidence of impaired tissue perfusion indicate?
cardiac dysfunction
In people and pigs, how has the CVP shown to change with PEEP?
people: PEEP of 10 cm H2O
pigs: PEEP of 15 cm H2O
> increase of 3 mmHg
Explain transmural pressure and how it is different to CVP
- transmural pressure is the physiologic variable that will determine the distension of the vessel
- the CVP in contrast is the pressure exerted on the vascular wall from the inside
- besides the CVP, transmural pressure is also determined by pressures exerted from the outside, i.e., pleural pressure
- transmural pressure = net difference between intravascular (CVP) and extravascular (pleural) pressure
When measuring the CVP during a fluid challenge in a euvolemic animal, what is the typical response?
initial increase in CVP by 2-4 mmHg
fast return to previous CVP within 15 min
When measuring the CVP during a fluid challenge in a hypovolemic animal, what is the typical response?
rises minimally and/or rapidly returns to original CVP (5-15 min) due to fast redistribution of volume
When measuring the CVP during a fluid challenge in a hypervolemic animal, what is the typical response?
marked elevation in CVP after fluid bolus with delayed return to original CVP (> 30min)
If the CVP takes > 30 min to get back to baseline after a fluid bolus, what is this indicative of?
- fluid overload
- decreased cardiac function
- restrictive pericardial disease
What generates the a wave on CVP measurement waveform and when does it appear in relation to the ECG
atrial contraction generates the a wave and it appears shortly after the P wave
What generates the x decent on the CVP waveform?
Atrial relaxation, which will decrease the pressure in the atrium and cranial vena cava
What generates the c wave on CVP measurement waveform and what part of the ECG does it correspond to?
the c wave does not always appear but is caused by bowing of the tricuspid valve into the right atrium during early right ventricular systole. The c wave will appear within the RT interval
What generates the v wave on CVP measurement waveform and when does it appear in relation to the ECG
The v wave is generated by atrial diastolic filling and will appear soon after the T wave on the ECG
What generates the y descent on the CVP measurement waveform
right atrial emptying into the right ventricle
How do you calculate the CVP from the continuous waveform measurements?
- identify the a wave on the CVP waveform by comparing it to the ECG reading. the a wave should begin within the PR interval
- read the pressure on top of the a wave and at the bottom of the x descent
- CVP estimate = (a wave peak + x descent base) / 2
alternative: locate end of QRS complex and draw down perpendicular line = true CVP measurement
How does the breathing cycle of a spontaneously breathing animal affect CVP measurements and when should you obtain the most accurate/reflective CVP measurement
- during inspiration > decrease in pleural pressure > decrease in transmural pressure > causes small decrease in CVP > will return to baseline after passive/normal expiration
- measure CVP at the end of expiration to correspond best to right ventricular end-diastolic filling
How can increased expiratory effort affect CVP measurements? How should you adjust when to measure CVP in these animals?
dyspneic or vocalizing animals > will overestimate CVP if measured at end-expiration
these animals > measure CVP during early expiration, before active abdominal effort/muscle contractions start
Does possitve pressure ventilation increase or decrease CVP measurements? When should you measure CVP in these animals?
increased CVP
measure at the end of expiration
Describe this CVP waveform and what arrhythmias is this typical for
Atrial fibrillation
lack of atrial contraction > lack of a wave
overfilling of the right atrium due to lack of contractions > prominent c wave
Describe this CVP waveform and what arrhythmias is this typical for
VPCs
large “cannon” wave due to AV dissociation > atrial contraction while the tricuspid valve is closed > transient increase in atrial pressure
Describe this CVP waveform and what arrhythmias is this typical for
pericardial effusion
inhibited diastolic filling of the heart > increase in CVP
greater equilization of the pressures within ventricles and atria > flattening of the waveform
rapid reduction in atrial pressure during ventricular systole > prominent x descent, small or absent y descent
Describe this CVP waveform and what arrhythmias is this typical for
tricuspid regurgitation
backflow of blood from the RV through the insufficient tricuspid valve into the RA
obliteration of the x descent, pronounced and merged or close c and v waves
Describe this CVP waveform and what arrhythmias is this typical for
right ventricular pressure overload, e.g., pulmonic stenosis or pulmonary hypertension
