Explain the response of the circulation to situations such as
changes in posture haemorrhage, hypovolaemia, anaemia, intermittent positive pressure ventilation, positive end-expiratory pressure, andthe Valsalva manoeuvre.
A Valsalva is forced expiration against a closed glottis. This can be achieved by increasing PAW to 40mmHg for 15 seconds. This increase in intrathoracic pressure alters many haemodynamic parameters.
A Valsalva manoeuvre consists of four phases:
- PAW is increased to 40cmH2O, with a corresponding increase in PThoracic
- SBP and DBP increase due to:
- Compression of the aorta
- Increased LV preload due to ejection of blood in the pulmonary vasculature
- VR falls due to increased PThoracic
- CO falls due to decreased VR
Baroreceptors are activated by the fall in BP, and SNS outflow increases, causing:
- The Valsalva ceases, and PAW returns to 0cmH2O
- PVR rapidly drops as alveolar vessels re-expand
- SBP and DBP rapidly fall due to:
- Decreased PVR causing decreased LV preload
- Loss of high intrathoracic pressure compressing the aorta
- VR normalises
- CO normalises due to normal VR and PVR
- HR falls (reflex bradycardia)
- BP falls
Abnormal responses occur in cardiac failure and autonomic neuropathy.
In CCF a square-wave patten is produced:
- Increasing PAW resulting in a sustained increase in SBP and DBP
- There is a slight decrease in SBP and DBP for the few seconds in phase III when airway pressure is released
Appears to be due to the increased circulating volume, as this difference resolves in venesected cardiac patients, and is demonstrated in normal individuals who are transfused to a high circulating volume.
Baroreceptor response to the Valsalva is minimal in both phase II and IV:
- In phase II, there is no compensatory increase in sympathetic outflow, so BP continues to fall until PAW returns to 0mmHg
- In phase IV, there is no compensatory increase in vagal tone and so BP returns to normal without overshooting
- Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
- Judson WE, Hatcher JD, Wilkins RW. Blood Pressure Responses to the Valsalva Maneuver in Cardiac Patients with and without Congestive Failure. Circulation. 1955;11:889-899.