84
Marine Mammal Physiology: Requisites for Ocean Living
gas exchange during short surface intervals in addition to the proposed mechanism for alveolar collapse (Fahlman et al. 2015; Kooyman and Cornell 1981; Kooyman and Sinnett 1979).
4.3.3 Measurement of blood flow, gases, and bubble formation
The cardiac output and the blood-flow distribution is, in addition to pulmonary gas
exchange, possibly the most important variable that diving animals are able to alter to
help manage gases. Blood flow measurements are few and difficult to perform. Dye,
coronary angiography, or thermal dilution experiments can be made to determine cardiac output, but are all invasive. Trans-thoracic ultrasound provides a non-invasive way
to determine cardiac frequency and stroke volume and would be a viable method for
most smaller marine mammals (Miedler et al. in press). To study blood-flow distribution, studies have either provided a snapshot of blood flow distribution at one time
point using micro-labeled spheres during forced dives (Zapol et al. 1979) or continuous
measurements using Doppler probes from a single blood vessel (Ponganis et al. 1990).
While the micro-sphere studies provide global distribution of blood flow, these studies
are done during forced-diving, which most likely alters the physiology of the animal
and questions how valid the response may be as compared with natural dives. Future
alternatives may be to use animals trained to dive on command and short-lived radioactive isotopes that can be imaged in a PET-CT, or ultrasound blood flow probes at selected
blood vessels. While these require invasive surgery, technological advances may minimize impact and provide an interesting alternative that can be used on wild animals.
In addition, ultrasound Doppler probes placed in strategic locations could be used to
answer whether diving marine mammals experience intravascular gas bubbles during
natural dives.
The measurement of blood gases (O 2 and N 2 ) has been undertaken in studies on captive animals during forced dives (Kooyman and Sinnett 1982; Kooyman et al. 1972), and
on wild animals using a bespoke device that sampled arterial gas for later processing in
the lab (Falke et al. 1985). One clever study inserted a needle covered in a gas permeable
silicone sleeve into the muscle of dolphins following a dive bout. The needle was attached
to a mass spectrometer which pulled gases out of the muscle according to the partial pressure gradients, enabling the N 2 washout to be measured (Ridgway and Howard 1979). In
another study, blood samples were collected following a dive bout and PN 2 levels measured using a Van Slyke method (Houser et al. 2010). Recent use of intravascular O 2 electrodes has generated some very interesting results in California sea lions and elephant
seals. In the latter, the evidence of extreme hypoxia tolerance was shown where arterial
and venous PO 2 levels of 15 and 3 mmHg, respectively, were observed during extended
dives (Meir et al. 2009). In the sea lion, the arterial data showed evidence of alveolar collapse and the depth of cessation of gas exchange was much deeper than formerly believed
(McDonald and Ponganis 2012). Together these studies continue to expand the knowledge
of physiological plasticity that appears possible in marine mammals.
Beyond blood gas measurements, other studies have shown that, during certain circumstances, blood gas levels may become supersaturated and then result in intravascular bubbles (de Quiros et al. 2012, 2013b; Dennison et al. 2012; Van Bonn et al. 2011, 2013). Whether
and when bubbles are formed is of particular interest in establishing preconditions for diving diseases. Ultrasound (audible Doppler and visual transthoracic echo imaging) has been
used to study intravascular bubbles in humans. Emerging technologies, such as dual frequency ultrasound, should enable the study of extravascular bubbles. Studies in stranded
marine mammals have shown that under certain circumstances animals may experience
Marine Mammal Physiology: Requisites for Ocean Living
gas exchange during short surface intervals in addition to the proposed mechanism for alveolar collapse (Fahlman et al. 2015; Kooyman and Cornell 1981; Kooyman and Sinnett 1979).
4.3.3 Measurement of blood flow, gases, and bubble formation
The cardiac output and the blood-flow distribution is, in addition to pulmonary gas
exchange, possibly the most important variable that diving animals are able to alter to
help manage gases. Blood flow measurements are few and difficult to perform. Dye,
coronary angiography, or thermal dilution experiments can be made to determine cardiac output, but are all invasive. Trans-thoracic ultrasound provides a non-invasive way
to determine cardiac frequency and stroke volume and would be a viable method for
most smaller marine mammals (Miedler et al. in press). To study blood-flow distribution, studies have either provided a snapshot of blood flow distribution at one time
point using micro-labeled spheres during forced dives (Zapol et al. 1979) or continuous
measurements using Doppler probes from a single blood vessel (Ponganis et al. 1990).
While the micro-sphere studies provide global distribution of blood flow, these studies
are done during forced-diving, which most likely alters the physiology of the animal
and questions how valid the response may be as compared with natural dives. Future
alternatives may be to use animals trained to dive on command and short-lived radioactive isotopes that can be imaged in a PET-CT, or ultrasound blood flow probes at selected
blood vessels. While these require invasive surgery, technological advances may minimize impact and provide an interesting alternative that can be used on wild animals.
In addition, ultrasound Doppler probes placed in strategic locations could be used to
answer whether diving marine mammals experience intravascular gas bubbles during
natural dives.
The measurement of blood gases (O 2 and N 2 ) has been undertaken in studies on captive animals during forced dives (Kooyman and Sinnett 1982; Kooyman et al. 1972), and
on wild animals using a bespoke device that sampled arterial gas for later processing in
the lab (Falke et al. 1985). One clever study inserted a needle covered in a gas permeable
silicone sleeve into the muscle of dolphins following a dive bout. The needle was attached
to a mass spectrometer which pulled gases out of the muscle according to the partial pressure gradients, enabling the N 2 washout to be measured (Ridgway and Howard 1979). In
another study, blood samples were collected following a dive bout and PN 2 levels measured using a Van Slyke method (Houser et al. 2010). Recent use of intravascular O 2 electrodes has generated some very interesting results in California sea lions and elephant
seals. In the latter, the evidence of extreme hypoxia tolerance was shown where arterial
and venous PO 2 levels of 15 and 3 mmHg, respectively, were observed during extended
dives (Meir et al. 2009). In the sea lion, the arterial data showed evidence of alveolar collapse and the depth of cessation of gas exchange was much deeper than formerly believed
(McDonald and Ponganis 2012). Together these studies continue to expand the knowledge
of physiological plasticity that appears possible in marine mammals.
Beyond blood gas measurements, other studies have shown that, during certain circumstances, blood gas levels may become supersaturated and then result in intravascular bubbles (de Quiros et al. 2012, 2013b; Dennison et al. 2012; Van Bonn et al. 2011, 2013). Whether
and when bubbles are formed is of particular interest in establishing preconditions for diving diseases. Ultrasound (audible Doppler and visual transthoracic echo imaging) has been
used to study intravascular bubbles in humans. Emerging technologies, such as dual frequency ultrasound, should enable the study of extravascular bubbles. Studies in stranded
marine mammals have shown that under certain circumstances animals may experience