Klaus Hopster

I graduated as a Veterinarian in 2006 and completed my Doctoral Thesis about “ventilation with PEEP and alveolar recruitment during inhalation anaesthesia in horses and its influence on the early recovery period” in 2007. From 2007 until 2011 I performed an ECVAA residency at the University of Veterinary Medicine in Hannover, Germany and became a Diplomate in 2012. I am currently working as a lecturer and clinician in equine anaesthesia. My actual research focuses on the peripheral perfusion and oxygenation during anaesthesia in horses.

Special interest group info

Today’s topic: Blood pressure measurement in horses.
During anaesthesia systemic cardiovascular parameters are routinely measured and used to predict perfusion and/or oxygenation of the periphery although it has been shown, that correlation between systemic cardiovascular parameters (e.g. blood pressure) and peripheral perfusion is not always linear.
However, measurement of blood pressure (BP) is an important tool in the monitoring of patients undergoing general anaesthesia. It has become standard practice within medical fields and has been recommended for veterinary anaesthesia.
Arterial BP is measured using the systolic and diastolic pressures and the mean arterial pressure (MAP). BP is the driving force of blood flow and, therefore, tissue perfusion. It refers to the pressure of the blood within the arteries.
Why is blood pressure monitored?
Main reason is that hypotension may increase anaesthesia-related morbidity and mortality. A strong association between hypotension and development of postanaesthetic myopathy further underlines importance of monitoring BP in anaesthetized horses.
Anaesthetic agents and many adjunctive drugs have a direct impact on cardiovascular function. They affect heart, BP, cardiac output and respiratory function. The associated changes are usually dose dependent.
How is hypotension managed?
Hypotension negatively affects tissue perfusion and oxygenation as well as distribution of anaesthetic drugs or other medications.

• It is ideal to use the least amount of inhalant possible to achieve an adequate plane of anaesthesia. Typically, this is accomplished through multimodal anaesthesia/analgesia.
• Aside from decreasing the amount of inhalant used, most common way to prevent hypotension is fluid therapy. An average adult horse should receive approximately 5 ml/kg/h crystalloid fluid. The fluid should be adjusted based on physical status and response to fluid therapy.
• Dobutamine is the standard positive inotropic agent for treating equine hypotension during anaesthesia. It increases cardiac contractility and therefore stroke volume and cardiac output resulting in an increase in blood pressure. Care must been taken when the patient is severely hypovolaemic. Dobutamine has a short half-life and should be given as a CRI. It has a fast onset of action and can be dosed be effect.
• Phenylephrine is a potent vasopressor by stimulation alpha-1-receptors. The drug can increase diastolic pressure and preload by increasing vascular tone. Phenylephrine does not increase cardiac output or peripheral blood flow and can cause severely side effects as vagus mediated reflex bradycardia. It has a rapid onset and a short lasting action.
• Colliods and hypertonic saline should be administered when the patient has a low blood pressure due to severe hypovolaemia.

How is blood pressure monitored?
Invasive blood pressure monitoring is the most accurate and reliable method and arterial catheterization provides the additional advantage of facilitating arterial blood sampling for gas analysis. The direct blood pressure waveform further helps to evaluate cardiac function, specifically relating to left ventricular function. It can also be used to assess inotropic and vasopressor therapy and fluid resuscitation efforts.
Cardiac output, stroke volume, vessel wall compliance, peripheral vascular resistance and heart rate can affect the BP waveform. To form a more complete picture of horse´s cardiovascular status, the waveform should be analyzed along with the BP and the capillary refill time.
In some circumstances a reliable, automated non-invasive blood pressure measurement devices would be useful. They are useful for monitoring trends but become significantly inaccurate if the patient is hypotensive, moving, or has bradycardias or arrhythmias.
Here some helpful and new literature about non-invasive BP measurement in horses:

• Tearney CC, Guedes AG, Brosnan RJ (2015) Equivalence between invasive and oscillometric blood pressures at different anatomic locations in healthy normotensive anaesthetised horses. Equine Vet J. 2015:
  There was only one location (tail) and one cuff width (6 cm; cuff width-to-tail circumference ratio of 0.25) that resulted in equivalence between MAP measured with the oscillometric and the invasive methods (P = 0.8). All other locations (metacarpus, radius/ulna, metatarsus) and cuff widths were not equivalent (P≤0.01).
• Drynan EA, Schier M, Raisis AL (2015) Comparison of invasive and noninvasive blood pressure measurements in anaesthetized horses using the Surgivet V9203. Vet Anaesth Analg. 2015: Systolic, mean and diastolic NIBP measured using the Surgivet V9203 are a clinically acceptable alternative to IBP measurements in anaesthetized horses undergoing routine elective surgeries.
• Hatz LA, Hartnack S, Kümmerle J, Hässig M, Bettschart-Wolfensberger R (2015) A study of measurement of noninvasive blood pressure with the oscillometric device, Sentinel, in isoflurane-anaesthetized horses. Vet Anaesth Analg. 2015:
  According to the high variability of NIBP compared to IBP, NIBP measurements as measured by the Sentinel in the manner described here are not considered as an appropriate alternative to IBP to measure blood pressure in anaesthetized horses.
• Tünsmeyer J, Hopster K, Feige K, Kästner SB (2015) Agreement of high definition oscillometry with direct arterial blood pressure measurement at different blood pressure ranges in horses under general anaesthesia. Vet Anaesth Analg. 2015:
  Good agreement with invasive arterial blood pressures was obtained with HDO at normotensive levels in horses. At high and low pressure ranges HDO was unreliable.
  The main indication for blood pressure measurements is the detection of hypotension, BUT most of the actual literature shows a large bias between non-invasive and invasive BP monitoring, ESPECIALLY when the blood pressure is low.
  According your personal experiences and the literature: Where is the place for and what is the indication of non-invasive blood pressure measurement devices in equine anaesthesia?

The main indication for blood pressure measurements is the detection of hypotension, BUT most of the actual literature shows a large bias between non-invasive and invasive BP monitoring, ESPECIALLY when the blood pressure is low.

According your personal experiences and the literature: Where is the place for and what is the indication of non-invasive blood pressure measurement devices in equine anaesthesia?