Question 6
Examine the 2 traces illustrated in the figure below.The top trace is an
ECG whilst the lower one is an arterial pressure waveform.
a) What abnormality is illustrated by the arterial pressure waveform?
b) Give reasons to justify your answer.
c) List 3 causes of the above phenomenon in critically ill patients
College Answer
Pulsus paradoxus
• The systolic blood pressure fluctuates with breathing (note that this is different than pulsus altemans, where the rhythm is regular but the systolic pressure alternates between one strong and one weak stroke volume).
• There is a difference of greater than 15 mmHg between inspiratory and expiratory systolic pressures.
• R- R interval on ECG is regular, ruling out arrhythmia as the cause for the fluctuating systolic pressure.
causes:
1) Pericardial effusion
2)Asthma
3) Hypovolemia in a mechanically ventilated patient
Discussion
In the grainy mobile-phone-snapped image above, the patient has a significant pulse pressure variation. In fact the picture was taken around the time we inserted a PiCCO. The SVV as measured by that device was 28% at that stage. The cause was "hypovolemia in a mechanically ventilated patient".
Apart from true hypovolemia, one can generate a whole list of causes for why this sort of picture might develop:
- Loss of RV compliance:
- Pericardial effusion
- Mediastinal haemorrhage
- Increased intrathoracic pressure
- High positive pressure of mechanical ventilation, high PEEP
- High AutoPEEP:
- Asthma
- COPD
- Anaphylaxis
- Tension pneumothorax
- Decreased preload
- Decreased intravascular volume
- Increased central venous compliance (eg. in sepsis)
More on this topic is available in a through discussion of the hemodynamic effects of a mechanical breath, as well as in this excellent aticle
The hemodynamic changes in cardiac tamponade and pericarditis are also discussed here.
References
Toska, K., and M. Eriksen. "Respiration-synchronous fluctuations in stroke volume, heart rate and arterial pressure in humans." The Journal of physiology472.1 (1993): 501-512.
Shabetai, Ralph, Noble O. Fowler, and Warren G. Guntheroth. "The hemodynamics of cardiac tamponade and constrictive pericarditis." The American journal of cardiology 26.5 (1970): 480-489.